Fractional Proximal Reabsorption Research Articles

Acute effect of Russell’s viper (Daboia siamensis) venom on renal tubular handling of sodium in isolated rabbit kidney

Abstract Background: The common complication in cases of poisoning by Russell’s viper (Daboia siamensis) venom (RVV) is acute renal failure, but the pathogenesis involved in the alteration of kidney function is still not well understood. Objective: To clarify the role of RVV in the pathogenesis of renal damage, the present study examines the functional short-term alterations acutely induced by RVV in isolated perfused rabbit kidney. Methods: Effects of RVV on renal tubular handling of sodium including mean perfusion pressure (PP), the renalvascular resistance (RVR), the glomerular filtration rate (GFR), the urinary flow (V) and osmolar clearance (Cosm) were studied in two groups of isolated perfused rabbit kidneys; each group had four isolated rabbit kidneys. RVV was added to the perfusion system to obtain the final concentration of 10 ⃞g/ml. Results: Immediate decreases in PP and RVR caused by the venom were significantly apparent (p < 0.05) in the first 15 min after RVV administration. A gradual rise in both PP and RVR occurred 15 min after the initial reduction of the first phase, but its remained below pretreatment values. The GFR, V, and Cosm decreased significantly throughout experiments after venom perfusion (p < 0.05). The total fractional sodium excretion increased significantly after venom perfusion throughout experiments, while significant reductions (p < 0.05) of renal tubular handling of sodium were apparent for proximal absolute reabsorption of sodium and proximal fractional reabsorption of sodium including marked reductions of distal absolute reabsorption of sodium and distal fractional reabsorption of sodium of the venom treated kidney. Optical microscopy of treated kidney tissue showed acute tubular necrosis at the end of experiment. Conclusion: The present study suggests that an administration of RVV in the isolated rabbit kidney causes direct acute nephrotoxicity and acute alterations of main functional parameters that are probably mediated by either the direct action of venom components or an indirect effect from vasoactive mediators released from renal cells of the RVV-treated kidney.

Cooperative mechanisms involved in chronic antidiuretic response to bendroflumethiazide in rats with lithium-induced nephrogenic diabetes insipidus

Previous studies of central diabetes insipidus suggested that thiazides acutely exerted a paradoxical antidiuresis by either indirectly activating volume-homeostatic reflexes to decrease distal fluid-delivery, or directly stimulating distal water-reabsorption. This study investigated whether the direct and indirect actions of bendroflumethiazide (BFTZ) simultaneously cooperated and also whether the renal nerves were involved in inducing long-term antidiuresis in nephrogenic diabetes insipidus (NDI). BFTZ or vehicle was gavaged into bilateral renal denervated and innervated rats with lithium-induced NDI for 10 days, constituting four groups. At one day before (D0) and one, five and ten days after starting administration of BFTZ or vehicle, rats were placed in metabolic cages to collect urine for 6 hours. BFTZ-treatment in both renal innervated and denervated rats caused equivalent reductions in urine-flow, creatinine clearance, lithium clearance and free-water clearance, but rises in urine-osmolality, fractional proximal reabsorption and fractional distal reabsorption at all days compared to D0, as well as to those of their relevant vehicle-received group. Therefore, the chronic antidiuretic response to BFTZ in conscious NDI rats was exerted through a concomitant cooperation of its direct distal effect of stimulating water-reabsorption and its indirect effect of reducing distal fluid-delivery by activating volume-homeostatic mechanisms, which appeared independent of the renal nerves.

PROXIMAL SODIUM REABSORPTION IS AN INDEPENDENT PREDICTOR OF HYPERTENSION: 2D.05

Objective: Given the association between salt-sensitivity of blood pressure (BP) and risk of future hypertension (HPT), we investigated the predictive role of alterations in segmental renal tubular sodium handling in the development of HPT during the 8 year follow-up of the Olivetti Heart Study participants. Methods: A selected sample (n = 314) of OHS population was examined at baseline and after 8 years. The participants were included if they were normotensive (SBP/DBP <140/90 mm Hg without anti-hypertensive treatment) and had normal renal function (creatinine clearance - CrCl> 60 ml/min) at baseline. Proximal and distal fractional tubular sodium reabsorption were calculated using the clearance of exogenous lithium. Results: The baseline sample characteristics were: age 49.3 ± 6.8 yrs, BMI 26.4 ± 2.8 kg/m2, SBP/DBP 119.8 ± 9.6/78.9 ± 6.4 mm Hg, CrCl 90.9 ± 18.2 mL/min (M ± SD). The HPT incidence in 8 years was 52%. The participants who developed HPT (group A) compared with those who did not (group B) had higher baseline SBP (122.4 ± 8.4 mm Hg vs 117.0 ± 10.0, p < 0.0001), DBP (80.6 ± 5.6 VS 77.1 ± 6.7 mm Hg, p < 0.0001), BMI (27.0 ± 2.9 vs 25.9 ± 2.6 kg/m2, p < 0.0001) and fractional proximal reabsorption of sodium (75.8 ± 6.3 vs 73.8 ± 7.3 %, p = 0.01). At logistic regression analysis using standardised variables, a 1SD-higher proximal sodium reabsorption at baseline predicted a 44% greater risk of HPT in 8 years (95% C.I. 13–84, p = 0.003), independently of baseline SBP (OR: 1.88, 95% C.I. 1.43–2.47, p < 0.001), BMI (OR: 1.51, 95% C.I. 1.16–1.95, p = 0.002), age and creatinine clearance used as an index of glomerular filtration rate. Conclusions: In this sample of healthy adult male population, proximal sodium reabsorption indexed to glomerular filtration rate was an independent predictor of future HPT.

Diabetic Natriuresis is Due to the Loss of Insulin Dependent Proximal Tubular Sodium Reabsorption

This study tested the role of hypo‐insulinemia in mediating the natriuresis in Type I diabetes. A 6‐day diabetic period was induced using either decreased insulin administration to induce hyperglycemia (D group), or clamped insulin administration (DG group) at the control‐period infusion rate and hyperglycemia was induced by infusing glucose continuously i.v. During diabetes, blood glucose increased from normal to 449±40 mg/dl in the D group, and 470±56 mg/dl in the DG group. Control urinary sodium excretion averaged 71±5 mEq/day and 69±7 in the D and DG groups respectively. Urinary sodium excretion increased during diabetes in the D group averaging 125±24 by day 5 while decreased to 54±9 by day 5 in the DG group. Fractional sodium reabsorption (FNaR) decreased by day 5 in the D group (99.8±0.02 vs. 98.2±0.08%, p&lt;0.05) but tended to increase by day 5 in the DG group. Estimated proximal fractional sodium reabsorption (PFNaR) decreased from control during diabetes in the D group while PFNaR increased during the diabetic period in the DG group. This was corroborated by fractional lithium reabsorption, an indicator of PFNaR, which increased during the diabetic period in the DG group from 73±6% to 89±4 day 2 and 92±1 day 5, p&lt;0.05) These data suggest that the decrease in plasma insulin in Type I diabetes plays a significant role in mediating the diabetic natriuresis by decreasing proximal tubular sodium reabsorption. (HL56259)

Segmental Regulation of Sodium and Water Excretion by TRPV1 Activation in the Kidney

Our previous studies show that activation of the transient receptor potential vanilloid type 1 (TRPV1) channels by a selective agonist, capsaicin (CAP), given unilaterally into the renal pelvis leads to increases in urine flow rate (Uflow) and urinary sodium excretion (UNa) bilaterally, although the mechanisms underlying enhanced renal excretory function are unknown. The present study was designed to determine the contribution of each of the renal segments to enhanced renal excretory function when TRPV1 expressed in sensory nerve fibers innervating the renal pelvis is activated. To accomplish the goal, LiCl was given intravenously to male Wistar rats while the left renal pelvis (LRP) was perfused with vehicle or CAP with or without a selective TRPV1 antagonist, capsazepine (CAPZ). Uflow and clearance of creatinine, lithium, sodium, and water, either filtered or fractionally, were determined in both kidneys. LRP perfusion of CAP at 2.4 nmol increased Uflow (microL.ming; ipsilaterally from 6.6 +/- 0.6 to 14.6 +/- 2.2 and contralaterally from 7.4 +/- 0.7 to 13.9 +/- 1.8, P < 0.05) and UNa (micromol.ming; ipsilaterally from 0.6 +/- 0.2 to 1.8 +/- 0.3 and contralaterally from 0.7 +/- 0.2 to 1.9 +/- 0.4, P < 0.05). Ipsilateral blockade of the TRPV1 with CAPZ at 24 nmol prevented CAP-induced increases in Uflow and UNa bilaterally. Creatinine, lithium, sodium, and free water clearance (ml.min) were increased in CAP (1.47 +/- 0.27, 0.44 +/- 0.05, 0.026 +/- 0.004, 0.41 +/- 0.05, respectively) compared to vehicle (0.72 +/- 0.12, 0.25 +/- 0.05, 0.010 +/- 0.001, 0.24 +/- 0.05), CAPZ+CAP (0.83 +/- 0.13, 0.24 +/- 0.03, 0.014 +/- 0.002, 0.23 +/- 0.03), and CAPZ (0.88 +/- 0.05, 0.21 +/- 0.01, 0.010 +/- 0.001, 0.20 +/- 0.01) groups (P <or= 0.01). Filtered sodium load, distal delivery of sodium, and distal sodium reabsorption (muEq.min) were also increased in CAP (202.2 +/- 33.3, 61.3 +/- 7.4, 57.6 +/- 7.4, respectively) compared to vehicle (97.7 +/- 16.6, 33.6 +/- 5.8, 32.2 +/- 5.9), CAPZ+CAP (110.5 +/- 16.3, 32.5 +/- 4.5, 30.7 +/- 4.3), and CAPZ (118.0 +/- 4.5, 27.9 +/- 1.2, 26.8 +/- 1.2) groups (P <or= 0.01). In contrast, fractional lithium and sodium excretion, absolute proximal reabsorption, fractional proximal reabsorption, fractional distal sodium, and water reabsorption were not different among groups. Therefore, activation of the TRPV1 expressed in primary afferent nerves innervating the renal pelvis leads to diuresis and natriuresis in both kidneys. The TRPV1-induced sodium and water excretion appears to be mediated by increases in glomerular filtration rate and distal tubular delivery of sodium but not by suppression of renal proximal and distal tubular reabsorption, suggesting a key role of segmental regulation of renal function by TRPV1-positive primary sensory nerves in the maintenance of sodium and water homeostasis.

Renal response to an acute protein challenge in pregnant women with borderline hypertension

The renal reserve (RR), assessed after an oral protein challenge or the intravenous administration of amino acids, is still present in healthy pregnant women (NP), although resting glomerular filtration rate (GFR) and renal plasma flow (RPF) increase progressively throughout normal gestation. No studies have addressed this issue in hypertensive gravidas; the aim of this trial was to evaluate renal response to an acute protein load (PL) in NP and pregnant women with borderline hypertension (HP). Five NP, eight HP and eight healthy non-pregnant women (CG) were evaluated. After fasting overnight, all subjects received an oral water load (20 mL/kg of body weight), the urinary output was then replaced orally with equal volumes of water. After two 30 min periods, an 80 g PL was provided. Creatinine clearance (CCr) was measured every 30 min from 1 h before and for 4 h following PL. Participants remained recumbent during the study, bladder emptiness was assessed by ultrasound immediately after each micturition. Baseline CCr was taken as the average of two 30 min periods before PL and peak Ccr as the maximal CCr recorded thereafter. The groups were similar with regard to age, weight or gestation age. Baseline CCr (NP: 118.5+/-6.0, HP: 127.4+/-6.7 and CG: 99.8+/-2.9 mL/min, P=0.004 (CG vs NP and HP), increased after PL to NP: 223.5+/-9.8 to HP: 178.5+/-13 and to CG: 149.1+/-4.0 mL/min, P<0.0004 (CG vs HP, CG vs NP and NP vs HP)). Peak minus baseline CCr was 97.3+/-10.1; 46.3+/-12.7 and 48.3+/-4.8 for NP, HP and CG, respectively (P<0.006 HP vs CG and NP). The peak CCr was obtained significantly earlier in both pregnant groups (Period 3) compared with the healthy non-pregnant women (Period 5) (P=0.02). The fractional proximal reabsorption of sodium (FPRNa+) at peak CCr was similar in the groups (NP: 0.74+/-0.01 HP: 0.78+/-0.02 and CG: 0.74+/-0.03, P=not significant (NS)) as was the distal delivery of sodium (DDNa+) (NP: 5.8+/-0.5; HP: 4.1+/-0.5 and CG: 4.3+/-0.4 meq/min, P=NS). Fractional excretion of urea (%) increased from 91.4+/-5.5 to 105.5+/-9.8%; 80.7+/-8.0 to 97.3+/-9.8; and 44.4+/-7.8 to 86.0+/-7.1 in NP, HP and CG, respectively (P=NS). There was a trend towards a poorer maternal and fetal outcome in the HP group. Mid-term borderline HP failed to increase CCr as much as NP did after a protein challenge, suggesting altered functional response of the nephron or lessened sensitivity of renal vasculature to additional vasodilator stimuli. These results support the interest of additional prospective studies with a larger number of patients to confirm these findings and evaluate the value of RR tests as predictors of outcome of pregnancies at risk.

Abstract 584: Simultaneous Antagonism of the V2 Receptor with Tolvaptan and Activation of the Natriuretic Peptide A-Receptor with B-Type Natriuretic Peptide: A Novel Strategy to Physiologically Enhance Water and Sodium Excretion Without Adversely Affecting Renal Function in a Canine Model of Congestive Heart Failure

Background: Water and sodium (Na) retention are hallmarks of congestive heart failure (CHF). While widely employed, conventional diuretics may adversely affect renal function. Therefore, physiological strategies that enhance water and Na excretion while maintaining renal function are a high priority. Arginine vasopressin (AVP) increases water reabsorption after binding to the V2 receptor in the collecting duct (CD). In contrast, B-type natriuretic peptide (BNP) decreases Na reabsorption via activation of the NPR-A receptor also in the CD and at higher doses also in the proximal tubule. We hypothesized that coadministration of the V2-receptor antagonist tolvaptan (TLV) and BNP would mediate a potent diuresis and natriuresis without adversely affecting renal hemodynamics in experimental CHF. We also assessed plasma and urinary excretion of AVP. Methods: Severe CHF was induced in 2 groups of dogs by tachypacing. On day 11 cardiorenal function was assessed. One group received TLV alone (0.1 mg/kg IV bolus; n=6), whereas the other group received infusion of BNP (50 ng/kg/min) in addition to TLV (n=5). Results: Renal hemodynamics were preserved in both groups. TLV alone increased urine flow from 0.07±0.01 to 0.48±0.09 mL/min, while urinary Na excretion (UNaV) remained unchanged. TLV+BNP-50 increased urine flow from 0.17±0.07 to 2.58±0.47 mL/min and UNaV from 1±1 to 77±23 uEq/min while decreasing proximal fractional Na reabsorption (all p&lt;0.05 vs. TLV alone). TLV alone increased plasma aldosterone and distal fractional Na reabsorption, both of which were prevented with BNP-50. In addition, while plasma AVP did not change with TLV or TLV+BNP, urinary AVP excretion markedly increased with TLV+BNP (from 16±5 to 694±196 pg/min) as compared to TLV alone (from 1±1 to 69±23 pg/min). Conclusions: TLV and BNP coadministration enhances aquaresis and natriuresis through proximal and distal tubular actions without changing renal hemodynamics. Further, TLV with BNP uniquely enhances urinary elimination of AVP warranting further study. We conclude from these findings in experimental CHF that simultaneously antagonizing the V2 receptor by TLV and activating the NPR-A receptor with BNP may be a novel and physiologic diuretic and natriuretic strategy in overt CHF.

Co-Targeting of the V2 Receptor with Tolvaptan and the Natriuretic Peptide A-Receptor with B-Type Natriuretic Peptide Enhances Water and Sodium Excretion without Adversely Affecting Renal Function: A Novel Physiologic Approach to Sodium and Water Retention in Experimental Heart Failure

Background: Water and sodium (Na) retention are hallmarks of congestive heart failure (CHF). While widely employed, conventional diuretics may adversely affect renal function. Therefore, physiological strategies that enhance water and Na excretion while maintaining renal function are a high priority. Arginine vasopressin increases water reabsorption after binding to the V2 receptor in the collecting duct in the distal nephron. In contrast, B-type natriuretic peptide (BNP) decreases Na reabsorption via activation of the NPR-A receptor also in the distal nephron and at higher doses also in the proximal tubule. We hypothesized that coadministration of the V2-receptor antagonist tolvaptan (TLV) and BNP would mediate a potent diuresis and natriuresis without adversely affecting renal function in experimental CHF. Methods: Severe HF was induced in 3 groups of dogs (n = 6 each) by tachypacing. On day 11 cardiorenal function was assessed in an acute study. One group received TLV alone (0.1 mg/kg IV bolus), whereas the two other groups received a low or high dose infusion of BNP (10 and 50 ng/kg/min, respectively) in addition to TLV. ∗p < 0.05. Results: Glomerular filtration rate and renal blood flow were preserved in all groups. TLV alone increased urine flow from 0.07 ± 0.01 to 0.48 ± 0.09∗ mL/min, while urinary Na excretion (UNaV) remained unchanged. TLV + BNP-10 slightly increased urine flow∗ and UNaV∗ compared to TLV alone. TLV + BNP-50 increased urine flow from 0.17 ± 0.07 to 2.58 ± 0.47∗ mL/min and UNaV from 1 ± 1 to 77 ± 23∗ μEq/min while decreasing proximal fractional Na reabsorption (all ∗compared to both other groups). TLV alone increased plasma aldosterone and distal fractional Na reabsorption, both of which were prevented with BNP-50. Interestingly, TLV + BNP-50 increased free water clearance as compared to TLV alone. Conclusions: TLV alone acts as a physiologic aquaretic without changing UNaV or renal hemodynamics. BNP coadministered with TLV enhances diuresis and natriuresis through proximal and distal tubular actions without changing renal hemodynamics or aldosterone. We conclude that TLV via V2 receptor antagonism and BNP via NPR-A activation may be a novel and physiologic diuretic and natriuretic strategy in overt CHF warranting further investigation.

Impaired Glomerular and Tubular Function as a Short-Term Effect of Sirolimus Treatment in the Rat

Aims: To investigate acute and short-term effects of sirolimus (SRL) on glomerulo-tubular function, blood pressure (BP), and renal morphology in the rat. Methods: Male Sprague-Dawley rats, weighing initially 140–180 g were treated with SRL in three series: SRL 0.2, 0.4, or 0.8 mg/kg/day intraperitoneally for up to 28 days after skin allo-transplantation from Lewis donors (to establish a dosage with significant immunosuppressive effect). SRL 0.4 mg/kg intravenously (acute effects). SRL 0.4 mg/kg/day intraperitoneally for 7 days (short-term effects). Inulin, lithium (C_Li) and sodium clearance, and intra-arterial BP were measured in conscious catheterized rats. Morphological kidney studies were completed after post-mortem fixation. Results: Maximum immunosuppressive effect was achieved with SRL 0.4 mg/kg/day. SRL acutely increased GFR and C_Li, whereas fractional proximal reabsorption (PFR) declined. In the short-term study SRL had opposite effects on GFR and C_Li, unaffected proximal tubular reabsorption and PFR, raised BP, diminished food consumption, and slower increase in body weight. Morphological changes were non-specific. Conclusion: In a dosage giving maximum immunosuppressive effect, SRL revealed acute effects on glomerular and proximal tubular function thus indicating increased outflow from the proximal tubules whereas one week of SRL treatment produced a change resembling the known nephrotoxic effects of the calcineurine inhibitors.

Glomerular hyperfiltration in type 1 diabetes mellitus results from primary changes in proximal tubular sodium handling without changes in volume expansion

Glomerular hyperfiltration plays a role in the pathophysiology of diabetic nephropathy. An increase in the glomerular filtration rate (GFR) could result from primary actions at the glomerular/vascular level or could be the consequence of a primary increase in proximal tubular sodium reabsorption resulting in systemic volume expansion. Recently it was hypothesized that an increase in sodium reabsorption may lead to glomerular hyperfiltration through the tubulo-glomerular feedback mechanism (tubular-hypothesis) without volume expansion. We have studied 54 normoalbuminuric patients with type 1 diabetes. The GFR was measured by inulin clearance. Proximal and distal sodium reabsorption were calculated according to standard formulas using the free water clearance technique. Plasma volume, measured by the (125)I-albumin method, atrial natriuretic peptide (ANP) and the second messenger cyclic guanosine-3,5-monophosphate (c-GMP) were used as markers of extracellular volume expansion. Glomerular hyperfiltration (GFR >or= 130 mL min(-1) 1.73 m(-2)) was present in 14 out of 55 patients with diabetes (25%). There were no differences in plasma volume between normo-(NF) and hyper-filtrating (HF) patients (2933 +/- 423 in NF vs. 3026 +/- 562 mL in HF, NS). Also plasma ANP and c-GMP levels were not significantly different between the groups. The fractional proximal reabsorption of sodium was significantly increased in HF [fPRNa(+) (%) 90.1 +/- 2.0 vs. 91.5 +/- 1.6, P = 0.02]. There were no differences in distal sodium reabsorption or distal sodium load (approximately macula densa concentration of NaCl) in both groups. Our data suggest that the primary event in diabetic glomerular hyperfiltration is an increase in proximal tubular sodium reabsorption. They do not support the hypothesis that systemic volume expansion or ANP mediate glomerular hyperfiltration in patients with normoalbuminuric type 1 diabetes. As such, changes in tubular sodium handling most probably influence tubulo-glomerular feedback.

Micropuncture determination of nephron function in mice without tissue angiotensin-converting enzyme

To determine the role of the local renin-angiotensin system in renal function, micropuncture was performed on two lines of mice in which genetic changes to the angiotensin-converting enzyme (ACE) gene markedly reduced or eliminated the expression of renal tissue ACE. Whereas blood pressure is low in one line (ACE 2/2), it is normal in the other (ACE 1/3) due to ectopic hepatic ACE expression. When normalized for renal size, levels of glomerular filtration rate [GFR; microl x min(-1) x g kidney wt(-1) (KW)] and single-nephron GFR (SNGFR; nl x min(-1) x g KW(-1)) were similar between wild-type (WT) and ACE 1/3 mice, while both measures were significantly reduced in ACE 2/2 mice (WT: 500 +/- 63 and 41.7 +/- 3.5; ACE 1/3: 515.8 +/- 71 and 44.3 +/- 3.3; ACE 2/2: 131.4 +/- 23 and 30.3 +/- 3.5). Proximal fractional reabsorption was not significantly different between WT and ACE 1/3 mice (51 +/- 3.5 and 49 +/- 2.3%), and it was increased significantly in ACE 2/2 mice (74 +/- 3.5%). Infusion of ANG II (50 ng x kg(-1) x min(-1)) increased mean arterial pressure by approximately 7 mmHg in all groups of mice and reduced SNGFR in WT and ACE 1/3 mice (to 30.9 +/- 2.8 and 31.9 +/- 2.5 nl x min(-1) x g KW(-1)) while increasing it in ACE 2/2 mice (to 55.3 +/- 5.3 nl x min(-1) x g KW(-1)) despite an increase in total renal vascular resistance. The tubuloglomerular feedback (TGF) response was markedly reduced in ACE 1/3 mice (stop-flow pressure change -2.5 +/- 0.9 mmHg) compared with WT despite similar blood pressures (-8.3 +/- 0.6 mmHg). In ACE 2/2 mice, TGF was absent (-0.7 +/- 0.2 mmHg). We conclude that the chronic lack of ACE, and presumably ANG II generation, in the proximal tubule was not associated with sustained proximal fluid transport defects. However, renal tissue ACE is an important contributor to TGF.

Cardiorenal actions of a unique chimeric peptide to CNP and BNP in experimental chronic heart failure

Background: CNP, a member of the natriuretic peptide system (NPS) of endothelial origin, is a potent venodilator with minimal natriuretic actions. CNP exerts venodilating actions via generation of cGMP after binding to the NPR-B receptor. BNP, a member of the NPS of myocardial origin, is the most potent natriuretic peptides and mediates its natriuretic properties via the NPR-A receptor. Both CNP and BNP possess a 17-amino acid ringed structure while BNP has a 6-amino acid COOH-terminus that CNP lacks. We recently designed and synthesized a novel 28-amino acid chimeric peptide termed “CBNP” which possesses the natriuretic actions of BNP and the venodilating actions of CNP. This chimeric protein possesses the 22-amino acid structure of CNP and the 6-amino acid COOH-terminus of BNP. To date its biological actions in experimental chronic heart failure (HF) are undefined. Methods: In 5 anesthetized dogs with experimental chronic HF produced by rapid ventricular pacing at 180 beats/min for 10 days, cardiorenal function was assessed in response to CBNP (50 ng/kg/min IV). Results: CBNP increased urinary sodium excretion (41±23 to 108±25 μEq/min∗), fractional sodium excretion (0.3±0.1 to 1.3±0.7 %∗) and glomerular filtration rate (29±5 to 39±4 ml/min∗), with a decrease in proximal fractional reabsorption of sodium (85±5 to 79±6 %∗). These renal responses were paralleled by increases in plasma cGMP (27±3 to 32±3 pmol/ml∗) and urinary cGMP excretion (1532±278 to 2344±589 pmol/min∗). In addition, CBNP also decreased right atrial pressure (5.8±0.4 to 3.4±0.8 mmHg∗), pulmonary capillary wedge pressure (18.9±0.9 to 15.6±0.5 mmHg∗) and cardiac output (2.7±0.3 to 2.0±0.2 l/min∗) consistent with a decrease in cardiac preload. Conclusion: These cardiorenal actions were unassociated with increases in plasma renin activity. The current study suggests that CBNP possess the venodilating actions of CNP and the natriuretic actions of BNP in experimental chronic HF. Thus, CBNP may have therapeutic potential as a venodilating and natriuretic agent in heart failure. (∗p<0.05 vs baseline)

Renal, cardiovascular and endocrine responses of fetal sheep at 0.8 of gestation to an infusion of amino acids.

Amino acid infusions increase renal blood flow (RBF) and glomerular filtration rate (GFR) and stimulate tubular reabsorption in adults. To characterize the effects of amino acids on fetal renal haemodynamics, tubular sodium reabsorption, acid-base homeostasis and plasma renin levels, 11 chronically catheterized fetal sheep aged 121 +/- 1 days (term ~150 days) were infused I.V. for 4 h with alanine, glycine, proline and serine (0.1, 0.1, 0.06 and 0.06 mmol min(-1), respectively) in 0.15 M saline at 0.165 ml min(-1). Eight control fetuses were given saline. During amino acid infusion, plasma amino acid levels increased up to 20-fold (P < 0.005). GFR increased by 50 +/- 8 % (P < 0.001); there was only a small transient increase in RBF. Proximal fractional sodium reabsorption fell from 74.6 +/- 2.9 to 55.5 +/- 5.4 % (P < 0.005). Distal sodium delivery increased, but a smaller percentage of this distal sodium load was reabsorbed (P < 0.005). Thus fractional sodium reabsorption fell from 95.5 +/- 0.9 to 81.4 +/- 2.0 % (P < 0.005). There was a large diuresis, natriuresis, kaliuresis and increase in osmolar excretion (P < 0.005). Plasma sodium and chloride concentrations fell (P < 0.005). Plasma osmolality did not change. Plasma renin levels fell (P < 0.05), cortisol levels increased (P < 0.05), and there was a compensated metabolic acidosis. Thus the fetal sheep kidney demonstrated a remarkable functional capacity to respond to amino acid infusion. The increase in filtration fraction and the lack of an increase in RBF suggest that efferent arteriolar vasoconstriction occurred, a very different response from the renal vasodilatation seen in adult animals.

Site of action of moxonidine in the rat nephron.

Moxonidine is a centrally acting antihypertensive agent which has been found to exert its blood pressure lowering effect by interaction with (alpha2-adrenoceptors and imidazoline receptors of the I(1)-type. These receptors have also been demonstrated to be present in the rat kidney. In the present study, clearance and micropuncture techniques were applied to anaesthetized rats to localize the site of action of moxonidine within the nephron. The clearance data show that moxonidine (0.25 mg/kg i.v., followed by a continuous i.v. infusion of 0.25 mg/h) induced a marked increase in urine flow and urinary excretion of sodium, chloride and potassium. The changes in urine flow and urinary solute excretion were accompanied by an enhanced glomerular filtration rate. The micropuncture experiments revealed that moxonidine significantly increased glomerular filtration rate of superficial nephrons, and significantly inhibited fractional reabsorption of fluid, sodium, potassium and chloride by similar amounts (by 9.0%-9.8%) in superficial proximal tubules. Regarding fluid and sodium reabsorption, the proximal effect of moxonidine was continuously weakened by a compensatory increase of reabsorption in the loop of Henle and the subsequent distal nephron segments. The inhibitory effect of moxonidine on fractional proximal potassium reabsorption was completely compensated in the loop of Henle, but the drug induced a net secretion of potassium into the segments lying beyond the early distal tubule, probably as a consequence of the increased tubule fluid and sodium load delivered to them. The experiments have identified the proximal tubule as the principal nephron site where the diuretic action of moxonidine arises. The proximal effect may be related to the increased glomerular filtration rate and to a direct inhibitory interaction of moxonidine with the proximal Na+/H+ exchanger.

Blood pressure lowering, renin suppressing and natriuretic properties of a new designer peptide BD-NP

Brain natriuretic peptide (BNP) is a 32-amino-acid peptide of cardiac origin with natriuretic and vasodilatory properties which is currently being developed for the treatment of congestive heart failure (CHF). Dendroaspis natriuretic peptide (DNP) is a recently discovered 38-amino-acid peptide structurally similar to ANP, BNP and CNP with potent natriuretic and vasoactive actions whose immunoreactivity is present in human plasma and elevated in human CHF. Data suggest that DNP is the natriuretic peptide most resistant to degradation by neutral endopeptidase due to its long C-terminus. Based upon the potent biological properties of both peptides we designed and synthesized a chimeric peptide BD-NP which consists of the 26 amino acid ring structure of BNP and the 15 amino acid C-terminus of DNP. We then assessed the cardiorenal actions of synthetic BD-NP in normal dogs in vivo. The therapeutic potential of parenterally administered BD-NP was assessed upon cardiorenal and endocrine function in 6 anesthetized dogs and compared with the placebo control group (n=7) which received saline. Intravenous synthetic BD-NP was infused after baseline measurements at 10 and 50 ng/kg/min. * p<0.05 vs. Baseline. Administration of low and high dose BD-NP decreased MAP (from baseline 133±6 to 122±4 and 104±3* mmHg), RAP (from 2.8±0.4 to 1.7±0.3* and 1.2±0.3* mmHg), PAP (from 16.7±0.8 to 15.3±0.5 and 12.4±0.4* mmHg) and PCWP (from 5.2±0.5 to 3.5±0.5* and 1.8±0.4* mmHg). BD-NP increased GFR (from 31±3 to 46±5* and 46±4* ml/min) without changes in RBF. BD-NP enhanced natriuresis (UNaV from 14.8±10 to 121.8±52* and 158.8±57* μEq/min) and diuresis (UV from 0.27±0.1 to 1.25±0.3* and 2.22±0.5* ml/min) with a decrease in proximal fractional reabsorption of sodium (from 83.8±5.0 to 68.6±3.8* and 61.7±3.8* %). Both doses of BD-NP decreased plasma renin activity (from 8.7±1.0 to 4.1±0.6* and 5.8±1.0* ng/ml/hr). In contrast, all parameters were unchanged with placebo. We report that this new designer peptide BD-NP potently lowers blood pressure, reduces cardiac filling pressures, increases glomerular filtration rate, augments natriuresis and diuresis and possesses renin-suppressing properties. These findings support a possible therapeutic role for this new designer peptide in the treatment of cardiovascular diseases such as hypertensive crisis and acute decompensated CHF.

Normal Responses of Atrial Natriuretic Factor and Renal Tubular Function to Sodium Loading in Hypertension-Prone Humans

Background: In order to explore the hypothesis of an atrial natriuretic factor (ANF) deficiency in prehypertension, we compared the response to sodium loading on ANF and renal function in subjects with positive and negative histories of hypertension. Methods: Twenty-two offspring of hypertensive parents (OH) and 20 offspring of normotensive parents (ON) were studied after 4 days of low (50 mmol/day) or high (300 mmol/day) dietary sodium intake. The diets were allocated randomly. Blood pressure (BP), renal function, plasma concentration of ANF, cyclic guanosine monophosphate (cGMP), renin, angiotensin I and II, aldosterone, endothelin and catecholamines were determined during a clearance period of 90 min on both diets. Neurohormones were measured by radioimmunoassays. Renal function was determined by simultaneous measurements of 51Cr-ethylenediaminetetraacetate (a marker of glomerular filtration rate), lithium and sodium clearances. Results: Supine systolic and diastolic BPs were significantly elevated in OH, with both low and high dietary sodium intake. There was no difference in ANF and cGMP concentrations on the low sodium diet. Increasing sodium intake caused a similar increase in ANF in OH and ON but cGMP did not change significantly. As expected the activity of the renin-angiotensin-aldosterone system was decreased by enhancing sodium intake but with both low and high sodium intake plasma renin concentration was significantly higher in OH than in ON. Activation of the sympathetic nervous system with low sodium intake was indicated by a moderate increase in plasma concentrations of epinephrine and norepinephrine in both groups. The renal effects were characterized by significant increases in GFR, lithium and sodium clearances with increasing sodium intake. There were no differences between OH and ON. Estimated values of fractional proximal and distal tubular sodium reabsorption decreased significantly and in a similar way in both OH and ON. Conclusion: These results indicate that the renal and neuroendocrine responses to dietary sodium loading are similar in both OH and ON. The only difference was a higher BP and an elevated plasma renin concentration on both dietary regimens in OH compared with ON. In particular, in OH and ON an identical increase in plasma ANF concentration in response to sodium loading was found. Thus, this study cannot support the hypothesis of a dysregulation of ANF in hypertension-prone humans.

Diltiazem modulates cyclosporin A induced renal hemodynamic effects but not its effect on plasma endothelin‐1

Cyclosporin A (CSA) has been reported to induce major acute renal hypoperfusion that may be antagonised by calcium channel blockers. The vasoconstrictive peptide endothelin‐1 (ET‐1) has been proposed as a mediator of CSA induced hypoperfusion. We investigated the acute effects of the new CSA formulation (Sandimmun Neoral) in 8 renal transplant patients on triple immunosuppressive therapy before and following slow‐release diltiazem treatment in a dose of 90‐120 mg b.i.d for 4 weeks. CSA significantly increased mean arterial blood pressure by 6±2 mmHg (p&lt;0.05) during the first 3 h after administration. This effect was abolished by diltiazem treatment, also reducing blood pressure by 12±3 mmHg (p&lt;0.05) 3‐9 h after administration. CSA administration induced a maximum reduction in renal blood flow of 20±8% (p &lt;0.05) 5 h after ingestion and a concomitant reduction in glomerular filtration rate of 18±7% (p &lt;0.05). The filtration fraction increased by a maximum of 13±7% (p &lt;0.05) after 4 h as did the calculated fractional proximal reabsorption by 14±4% (p &lt; 0.05). All these acute renal effects were abolished by diltiazem administration. Concurrent with the maximum renal hemodynamic effects. plasma ET‐1 was elevated with a peak increase of about 40% 4‐5 h after CSA ingestion. Diltiazem treatment had no effect on the increase in plasma ET‐1 following CSA administration. These findings suggest that CSA induced acute vasoconstriction and renal hypoperfusion are mediated by ET‐1 and that diltiazem treatment abolishes these pharmacodynamic effects of CSA despite persistent increase of plasma ET‐1 levels.

Sensitivity of the segmental renal arterioles to angiotensin II in the aging rat

With advancing age the old rat kidney becomes tonically vasoconstricted by endogenous angiotensin II (ANGII) (C. Baylis. Am. J. Kid. Dis., 22 (1993) 842). The present study was designed to investigate the sensitivity of the cortical glomerular microvasculature of the old rat kidney (19–22 months of age) to exogenous ANGII, using the in vivo micropuncture technique. In the baseline state, glomerular blood pressure ( P GC) in old male rats was higher compared to young rats (4–5 months of age). During exogenous ANGII infusion (40 ng/kg/min), a significant rise in arterial blood pressure and renal vasoconstriction occurred in both young and old rats. In young rats, the ANGII induced fall in renal plasma flow (RPF) and glomerular plasma flow ( Q A) was accompanied by a rise in P GC and thus the glomerular hydrostatic pressure gradient, with little change in K f. Therefore, the glomerular filtration rate (GFR) and single nephron GFR (SNGFR) were unchanged by ANGII infusion in young rats. In old rats, RPF and Q A fell, a rise occurred in P GC and also a fall was seen in the glomerular capillary ultrafiltration coefficient ( K f), thus GFR and SNGFR fell significantly. The magnitude of the pressor and renal vasoconstriction response to ANGII were not affected by age; of interest, ANGII increased preglomerular and efferent arteriolar resistance ( R A, R E) and P GC by similar amounts in young and old rats. SNGFR was reduced in old rats, due to the marked ANGII-induced decline in K f. Neither absolute nor fractional proximal reabsorption were affected by ANGII infusion in either young or old rats. By 19–22 months of age, old rats had much more injured glomeruli than young rats. These data demonstrate that the afferent and efferent arterioles had similar sensitivity to exogenous pressor dose of ANGII in both young and old rats, but K f was more sensitive to ANGII in old rats leading to a significant fall in SNGFR.

Proximal tubular reabsorption of growth hormone and sodium/fluid in normo- and microalbuminuric insulin-dependent diabetes mellitus.

Proximal tubular dysfunction may be implicated in the pathogenesis of diabetic nephropathy. An investigation of proximal tubular function was carried out by assessing proximal tubular sodium-reabsorption and low molecular weight protein excretion in a group of patients with type 1 diabetes mellitus. Normoalbuminuric [group A, n = 6, albumin excretion rate (AER) mean (range) 4 (0-10) micrograms/min], and microalbuminuric [group B, n = 6, AER 88 (35-198) micrograms/min] patients with type 1 diabetes were compared with matched controls. Simultaneous lithium and growth hormone (GH) clearance and urinary beta 2-microglobulin excretion were assessed. Fasting plasma glucose at the start of the study was [median (range)] 13 (10.2-15.1), 9.3 (5.9-15) and 4.1 (4.0-5.0) mmol/l in groups A, B and controls, respectively, with a mean coefficient of variation during the study of 3.9% (group A) and 5.2% (group B). There was no significant difference in plasma glucose levels between patients in groups A and B. Urinary GH excretion was raised in the patients with microalbuminuria (group B; P < 0.05), although there was no difference in serum GH clearance rate between the patient groups and controls. Urinary GH correlated with B 2-microglobulin in the diabetic subjects (r = 0.665, P < 0.05) and with the degree of microalbuminuria in group B patients (r = 1, P < 0.01). Urinary GH was also greater than 10 microU, the median value observed in the controls, in 5 of 6 (83%) patients in group A. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) measured by constant infusion of 51Cr-ethylene diamine tetra-acetic acid (EDTA) and I125-para-amino hippuric acid (PAH), respectively, showed relative hyperfiltration in the normoalbumiruric group compared with controls (P < 0.05) and group B (P < 0.05). Absolute proximal reabsorption of sodium and of water (APRNa and APRH2O) was significantly higher in group A patients (P < 0.05). Although GFR was significantly higher in group A patients, no differences were found in fractional proximal reabsorption of sodium and water (FPRNa+H2O) or end proximal delivery between the patient groups and controls. Therefore, the measurement of protein reabsorptive capacity provides a more sensitive marker of renal tubular impairment in type 1 diabetes than sodium/fluid reabsorptive capacity. In patients with microalbuminuria, both glomerular and tubular damage may coexist. Our results stress the usefulness of markers of renal tubular function in monitoring the course of diabetic nephropathy. This study also shows that assessment of GH clearance has promise as a marker of renal tubular protein reabsorptive capacity.

Acute Ureteral Obstruction and Glomerulotubular Function in Rats

Urinary tract obstruction is a common cause of acute renal failure (ARF). During unilateral ureteral obstruction (UUO) arteriolar vasoconstriction, increase in tubular pressure, and ultrafiltrate retrodiffusion occur. We studied renal function of rats with surgical UUO for 24 hr. After this period of UUO, the contralateral kidney was removed and the right ureter was deobstructed. The control uninephrectomized group consisted of normal rats submitted to left uninephrectomy (UNx). Functional studies were performed 12 and 24 hr, and 7 days after deobstruction and UNx. We measured creatinine clearance, and fractional excretion of sodium and lithium. Using conventional formulas we calculated fractional proximal and distal sodium reabsorption. Initially we observed a reduction in glomerular filtration rate (GFR) after deobstruction (12 and 24 hr). However, after 7 days, the GFR was significantly higher in deobstructed rats than in controls (340.3 ± 18.3 vs. 286.4 ± 9.3 μL/min/100 g, p < 0.01). The dry kidney weight was also increased in these rats. The fractional sodium excretion was increased in deobstructed rats, mainly in early studies (12 and 24 hr). Whereas fractional proximal reabsorption was reduced in both groups, the fractional distal reabsorption was significantly decreased in the deobstructed group compared to UNX controls (93.9 ± 0.9 vs. 98.9 ± 0.1% after 24 hr, p < 0.01). Our data showed that UUO influenced both glomerular and tubular functions. A salient finding was the overcorrection of GFR 7 days after deobstruction. The renal release of hormones and growth factors could mediate these alterations in renal function through their vascular, tubular, and proliferative actions.