Sodium Reabsorption In Distal Nephron Research Articles

Pattern Recognition versus Pathogenesis: Electrolytes in a Patient with Adrenal Insufficiency.

Pattern Recognition versus Pathogenesis: Electrolytes in a Patient with Adrenal Insufficiency.

The Effect of Epidermal Growth Factor Inhibition on Distal Nephron Sodium Reabsorption in Mice

The Effect of Epidermal Growth Factor Inhibition on Distal Nephron Sodium Reabsorption in Mice

Myristoylated alanine-rich C kinase substrate-like protein-1 regulates epithelial sodium channel activity in renal distal convoluted tubule cells.

The epithelial sodium channel (ENaC) regulates blood pressure by fine-tuning distal nephron sodium reabsorption. Our previous work has shown that ENaC gating is regulated by anionic phospholipid phosphates, including phosphatidylinositol 4,5-bisphosphate (PIP2). The PIP2-dependent regulation of ENaC is mediated by the myristoylated alanine-rich protein kinase C substrate-like protein-1 (MLP-1). MLP-1 binds to and is a reversible source of PIP2 at the plasma membrane. We examined MLP-1 regulation of ENaC in distal convoluted tubule clonal cell line DCT-15 cells. Wild-type MLP-1 runs at an apparent molecular mass of 52 kDa despite having a predicted molecular mass of 21 kDa. Native MLP-1 consists of several distinct structural elements: an effector domain that is highly positively charged, sequesters PIP2, contains serines that are the target of PKC, and controls MLP-1 association with the membrane; a myristoylation domain that promotes association with the membrane; and a multiple homology 2 domain of previously unknown function. To further examine MLP-1 in DCT-15 cells, we constructed several MLP-1 mutants: WT, a full-length wild-type protein; S3A, three substitutions in the effector domain to prevent phosphorylation; S3D mimicked constitutive phosphorylation by replacing three serines with aspartates; and GA replaced the myristoylation site glycine with alanine, so GA could not be myristoylated. Each mutant was tagged with either NH2-terminal 3XFLAG or COOH-terminal mCherry or V5. Transfection with MLP mutants modified ENaC activity in DCT-15 cells: activity was highest in S3A and lowest in S3D, and the activity after transfection with either construct was significantly different from WT. In Western blots, when transfected with 3XFLAG-tagged MLP-1 mutants, the expression of the full length of MLP-1 at 52 kDa increased in mutant S3A-MLP-1-transfected DCT-15 cells and decreased in S3D-MLP-1-transfected DCT-15 cells. Several lower molecular mass bands were also detected that correspond to potential presumptive calpain cleavage products. Confocal imaging shows that the different mutants localize in different subcellular compartments consistent with their preferred location in the membrane or in the cytosol. Activation of protein kinase C increases phosphorylation of endogenous MLP-1 and reduces ENaC activity. Our results suggest a complicated role for proteolytic processing in MLP-1 regulation of ENaC.

Discovery, characterization, and preclinical development of a Kir4.1 (KCNJ10) inhibitor for the treatment of hypertension

Kir4.1 (encoded by KCNJ10) inward rectifier potassium channels are emerging as key regulators of sodium reabsorption in the distal nephron of the kidney. Kir4.1 is expressed in the basolateral membrane of the thick ascending limb of Henle, distal convoluted tubule, and collecting duct of nephron. Kir4.1 can form homotetrameric channels or heterotetrameric channels with Kir5.1, although the most common channel subtype in the kidney is the heteromeric Kir4.1/5.1 form. Genetic ablation of Kir4.1 in mice or humans causes severe salt and water wasting resembling clinical phenotypes associated with diuretic use. These human validation data and recent animal studies suggest that Kir4.1 might represent a novel diuretic target for the management of hypertension through inhibition of distal nephron sodium reabsorption. The aim of this study was to develop a selective small‐molecule inhibitor for evaluating the therapeutic potential of renal Kir4.1 channels. From a high‐throughput screen of 76,575 compounds from the Vanderbilt Institute of Chemical Biology library, we identified an inhibitor, termed VU992, which inhibits homotetrameric Kir4.1 with an IC50 of ~0.9 μM and heterotetrameric Kir4.1/5.1 with an IC50 of ~ 9 μM. Lead optimization failed to identify analogs with improved potency toward Kir4.1. The selectivity screening against other Kir channels revealed that VU992 does not block Kir1.1, Kir2.x, Kir6.1/SUR1 and Kir7.1 channels. The voltage dependent unblock suggested that VU992 probably binds in the pore of the Kir4.1 channel. Computational docking analysis using a Kir4.1 homology model revealed several possible interacting residues in the pore. Experimental binding site analysis using mutagenesis and patch clamp electrophysiology suggested that the pore lining glutamic acid residue at 158 (E158) is crucial for the binding of VU992 to the Kir4.1. Mutating E158 to corresponding asparagine residue in Kir1.1 (E158N) abolished the VU992 block completely. Pharmacokinetic studies in rats showed that VU992 does not cross the blood brain barrier and therefore should not inhibit Kir4.1 channels expressed in the CNS. Finally, oral administration of VU992 dose‐dependently increased urine output and Na+ and K+ loss in volume‐loaded, freely behaving rats. This study shows proof‐of‐concept that homotetrameric Kir4.1 might represent a molecular target for a novel class of diuretic for the treatment of hypertension. Its basolateral localization might offer advantages over other diuretic targets that are located on the luminal membrane of the nephron.Support or Funding InformationNIH 5R21 NS073097‐01S1This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Loss of Protein Kinase C Alpha Disrupts Mean Arterial Pressure and Diurnal Rhythm

The sodium‐chloride cotransporter (NCC) is an essential regulator of sodium transport in the distal convoluted tubule (DCT). As such, functional aberrations will lead to dysregulation of sodium homeostasis and blood pressure. Although this role is well accepted, the mechanisms accounting for increased distal nephron sodium reabsorption is not completely understood. Previously published data from the Pharmacogenomic Evaluation of Antihypertensive Responses (PEAR) Study revealed that a single nucleotide polymorphism (SNP) in PKC‐α increases sensitivity to thiazide diuretics. To date, it is unknown whether this SNP is a gain or loss of function of PKC‐α. To investigate whether PKC‐α may be altering blood pressure via NCC, PKC‐α KO (PKC KO) and PKC Control (CTL) mice were used to measure blood pressure via radiotelemetry after normal and high salt (HS, 4%) dietary manipulations and Western blot analysis performed using kidney lysates. Using radiotelemetry (DSI) measurements obtained over an active and sleeping time periods, we report a significant decrease in mean arterial pressure (MAP) in the PKC KO mice as compared to control at baseline (111±1.3mmHg vs 120±2.4mmHg, n=12–16, p<0.001). When comparing active vs. sleeping MAP, PKC KO mice had depressed MAP during both time periods, yet blood pressure differences (day vs. night) double during active periods (5.4±1.3mmHg PKC KO vs. 10.9±3.9mmHg PKC CTL, n=6–7). These differences were abolished with HS chow feeding (17.3mmHg PKC KO vs. 16.1mmHg PKC CTRL, n=4). To determine a possible mechanism for this phenomenon, whole kidney lysate was used to assay for total NCC protein. Total NCC protein was significantly increased in PKC KO mice at baseline, and continued during HS chow feeding, as compared to PKC CTL. In addition, PKC KO mice exhibited a diuresis with both normal and HS feeding. Our data suggest that lack of PKC‐α leads to decreased MAPs and diuresis, which may lead to increased total NCC expression to compensate. In addition, PKC KO mice exhibited an aberrant diurnal rhythm, which was recovered with HS diet suggesting that PKC‐α may be necessary to maintain sodium homeostasis.Support or Funding InformationNIDDK085097‐ RSH 2T32DK7656‐21 Emory Renal Division‐BMW

Abstract 318: The Role of PKC-alpha in NCC-Mediated Hypertension

The sodium-chloride cotransporter (NCC) is an essential regulator of sodium transport in the distal convoluted tubule (DCT); functional aberrations will lead to dysregulation of blood pressure. Although this role is well-accepted, the mechanisms accounting for increased distal nephron sodium reabsorption is not completely understood. Previously published data from the Pharmacogenomic Evaluation of Antihypertensive Responses (PEAR) Study has demonstrated that a polymorphism in PKC-alpha may lead to an increased sensitivity to thiazide diuretics. To investigate whether PKC-alpha may be altering blood pressure via NCC, PKC-alpha KO (PKC KO) and PKC Control (Ctl) mice were used to measure blood pressure via radiotelemetry after normal and high salt (HS, 4%) dietary manipulations and Western blot analysis performed using kidney lysates. Using radiotelemetry (DSI) measurements obtained over an 18 hour ‘active’ time period, we report a significant increase in mean arterial pressure (MAP) in the PKC KO mice as compared to control (123.2±1.1 mmHg vs. 102.0±0.77 mmHg, p<0.001). With HS feeding, MAP increased in both groups; in addition MAP remained significantly higher in the PKC KO mice as compared to the Ctl after 21 days (139.5±0.3 mmHg vs. 124.7±1.1 mmHg, p<0.01). To determine a possible mechanism for this phenomenon, whole kidney lysate was used to assay for total NCC protein. Total NCC protein was almost 3 fold higher in the PKC KO mice as compared to Ctl (in arbitrary units, 208±16 vs. 61±30, p<0.01). Herein we report that lack of PKC-alpha leads to an increase in MAP, which may be accounted for by increased total NCC expression and a subsequent increase in distal nephron reabsorption. We believe that PKC-alpha may be negative regulator of NCC protein expression, and that aberrations in this pathway will lead to hypertension.

Renal potassium handling in the pregnant rat (892.13)

The fetus requires a large amount of potassium for normal development. To accommodate this need the normal pregnant rat accumulates ~6.3 mEq of potassium over the course of gestation, over 70% of which is retained during late pregnancy. However, the mechanism of potassium retention during pregnancy is largely unknown. The purpose of this study was to examine the mechanism permitting increased potassium reabsorption in the setting of high circulating aldosterone and enhanced distal nephron sodium reabsorption in the pregnant rat. Recent work on the WNK kinases has provided interesting targets for divergent modulation of sodium and potassium handling in the distal nephron. We measured the mRNA expression of WNK1, WNK3, WNK4, and ROMK in the renal cortex and outer medulla of virgin (V, n=5), mid pregnant (MP, n=5), and late pregnant (LP, n=7) rats using quantitative real‐time PCR. We found increased WNK3 expression in the outer medulla of LP rats compared to V (156 ±15 vs 100±10 % Control). The expression level of the other genes tested did not appear to differ with pregnancy state in either the cortex or the outer medulla. ROMK is the major potassium secretory channel and WNK3 is an inhibitor of ROMK activity. Therefore, the potassium retention of late pregnancy could be due in part to increased outer medullary WNK 3 expression as well as a failure of the expected aldosterone mediated transcriptional increase in ROMK.

Collecting Duct Renin Synthesis and Secretion are Stimulated by Angiotensin (Ang) II via Protein Kinase C (PKC) Activation and cAMP Accumulation

In rats and mice, Ang II type 1 receptor (AT1R) activation stimulates renin synthesis via PKC activation in the rat inner medulla CD (IMCD) cells. Because cAMP is an important second messenger for the Ang II signal transduction, we hypothesized that Ang II stimulates CD renin synthesis and release via PKC activation and cAMP accumulation. In mouse cortical CD cell line (M‐1 cells) treated with Ang II (8 hrs) we found increased renin mRNA in a dose‐dependent manner. PKC inhibition with Calphostin prevented this response. Likewise, Ang II increased renin and prorenin protein levels at 15 min with a sustained response for 2 hrs. Renin content measured in the cell culture media augmented in Ang II‐treated cells by 6 hrs (Ang II: 31±4 vs. 19±4 ng Ang I/hr/ml; p<0.05). Importantly, Ang II (10−7M) increased cAMP levels by 1 min (148±32 pmol/mg protein) reaching a peak at 30 min (290±27 vs. 44±0 pmol/mg protein; p<0.05). This response was prevented by PKA inhibition with H89 or PKI. Forskolin, an adenlyate cyclase activator, increased renin transcript in a dose‐dependent manner. Taken together, Ang II stimulates renin synthesis and secretion via PKC activation and cAMP stimulation in M‐1 cells; suggesting that during Ang II‐dependent hypertension, increased apical secretion of renin by the CD cells into the luminal fluid may augment intratubular Ang II formation, thus contributing to enhance distal nephron sodium reabsorption.

Counterpoint: Activation of the Intrarenal Renin-Angiotensin System is the Dominant Contributor to Systemic Hypertension

Our discussion regarding the dominant mechanism responsible for hypertension calls to mind the famous poem, “The Blind Men and the Elephant,” by John Godfrey Saxe based on an ancient fable from India. In essence, the blind men described the elephant based on their specific encounter, thus

Reduced Distal Nephron Sodium Reabsorption in Cyp1a1‐Ren2 Transgenic Rats with Inducible ANG II‐Dependent Malignant Hypertension

The present study was performed to determine the ANG II‐dependence of distal nephron sodium reabsorption in Cyp1a1‐Ren2 transgenic rats [strain name: TGR(Cyp1a1‐Ren2)] with inducible ANG II‐dependent malignant hypertension. To assess distal nephron sodium reabsorptive function, we compared sodium excretion before and after blockade of the two main distal nephron sodium transporters by intravenous administration of amiloride (AMIL; 25 μg) plus bendroflumethiazide (BFTZ; 25 μg) in pentobarbital sodium‐anesthetized male Cyp1a1‐Ren2 transgenic rats (n=5). Cyp1a1‐Ren2 transgenic rats induced with 0.3% I3C for 8–10 days exhibited a markedly elevated mean arterial pressure (MAP; 185±5 vs. 131±3 mmHg, P<0.001) compared with non‐induced rats (n=5). Administration of AMIL+BFTZ did not alter MAP in either group, but elicited a greater increase in urinary sodium excretion in the non‐induced control rats than in the hypertensive rats (6.36±1.08 vs. 3.13±0.42 μEq/min, P<0.05). Distal nephron sodium delivery was not different between the two groups (7.43±1.41 vs. 5.24±0.86 μEq/min). However, absolute distal nephron sodium reabsorption and fractional reabsorption of distal nephron sodium delivery were markedly lower in the hypertensive rats than in the normotensive rats (3.13±0.42 vs. 6.36±1.08 μEq/min and 63.37±7.86 vs. 87.97±4.58 %, respectively, P<0.05 in both cases). These findings demonstrate that distal nephron sodium reabsorptive function is markedly decreased in Cyp1a1‐Ren2 rats with ANG II‐dependent malignant hypertension. Such impaired distal nephron sodium reabsorptive function suggests that ANG II‐dependent stimulation of distal nephron sodium reabsorption does not contribute substantively to the hypertension in these rats.

ANP-mediated inhibition of distal nephron fractional sodium reabsorption in wild-type and mice overexpressing natriuretic peptide receptor

Atrial natriuretic peptide (ANP) elicits natriuresis; however, the relative contributions of proximal and distal nephron segments to the overall ANP-induced natriuresis have remained uncertain. This study was performed to characterize the effects of ANP on distal nephron sodium reabsorption determined after blockade of the two major distal nephron sodium transporters with amiloride (5 microg/g body wt) plus bendroflumethiazide (12 microg/g body wt) in male anesthetized C57/BL6 and natriuretic peptide receptor-A gene (Npr1) targeted four-copy mice. The lower dose of ANP (0.1 ng x g body wt(-1) x min(-1), n = 6) increased distal sodium delivery (DSD, 2.4 +/- 0.4 vs. 1.6 +/- 0.2 mueq/min, P < 0.05) but did not change fractional reabsorption of DSD compared with control (86.3 +/- 2.0 vs. 83.9 +/- 3.6%, P > 0.05), thus limiting the magnitude of the natriuresis. In contrast, the higher dose (0.2 ng x g body wt(-1) x min(-1), n = 6) increased DSD (2.8 +/- 0.3 mueq/min, P < 0.01) and also decreased fractional reabsorption of DSD (67.4 +/- 4.5%, P < 0.01), which markedly augmented the natriuresis. In Npr1 gene-duplicated four-copy mice (n = 6), the lower dose of ANP increased urinary sodium excretion (0.6 +/- 0.1 vs. 0.3 +/- 0.1 mueq/min, P < 0.05) and decreased fractional reabsorption of DSD compared with control (72.2 +/- 3.4%, P < 0.05) at similar mean arterial pressures (91 +/- 6 vs. 92 +/- 3 mmHg, P > 0.05). These results provide in vivo evidence that ANP-mediated increases in DSD alone exert modest effects on sodium excretion and that inhibition of fractional reabsorption of distal sodium delivery is requisite for the augmented natriuresis in response to the higher dose of ANP or in Npr1 gene-duplicated mice.

Familial Hyperkalemia and Hypertension: Pathogenetic Insights Based on Lithium Clearance

Familial hyperkalemia and hypertension (FHHt) is caused by mutations in WNK kinases. Its pathogenesis is not completely understood. Our objective was to investigate the mechanism of hypercalciuria in FHHt. We conducted a study of a large family with FHHt and WNK4 Q565E mutation and of control subjects at a referral medical center. Forty-six members of a family with FHHt and WNK4 Q565E mutation, 23 of them affected, and 12 control subjects participated. Urinary calcium and sodium concentrations, endogenous lithium clearance, age of hypertension appearance were assessed. In 40 urine samples of 20 affected subjects, urinary calcium was correlated to urinary sodium (r = 0.567; P = 0.0001). In 28 urinary samples of 22 unaffected members, no correlation was found (r = 0.285; P = 0.14). Mean ratio of urinary calcium to urinary sodium was 2.7-fold higher in affected compared with unaffected members (58.7 +/- 25.9 vs. 22.1 +/- 14.0 micromol/mmol, P < 0.0001). Endogenous lithium clearance in eight affected members was about 50% lower than in 12 controls (16.2 +/- 7.7 vs. 28.8 +/- 9.8 ml/min, P = 0.0073). Hypertension was detected in males 12 yr earlier than in females (26.0 +/- 7.5 vs. 37.9 +/- 11.3 yr; P = 0.031). Hypercalciuria in FHHt seems to be dependent on urinary sodium. According to molecular studies, FHHt patients are presumed to have increased distal nephron sodium reabsorption and therefore decreased proximal reabsorption of sodium, lithium, and calcium. The observed decreased lithium clearance reflects probable abnormal renal handling of lithium, i.e. distal nephron lithium reabsorption. Therefore, hypercalciuria may result from proximal nephron aberration. Finally, earlier appearance of hypertension in males may be the result of sex-hormone activity.

Enhanced Distal Nephron Sodium Reabsorption in Chronic Angiotensin II–Infused Mice

Chronic angiotensin II (Ang II) infusions enhance urinary excretion of angiotensinogen, suggesting augmentation of distal nephron sodium reabsorption. To assess whether chronic Ang II infusions (15 ng/min for 2 weeks) enhance distal nephron sodium reabsorption, we compared sodium excretion before and after blockade of the 2 main distal nephron sodium transporters by IV amiloride (5 mg/kg of body weight) plus bendroflumethiazide (12 mg/kg of body weight) in male C57/BL6 anesthetized control mice (n=10) and in chronic Ang II-infused mice (n=8). Chronic Ang II infusions increased systolic blood pressure to 141+/-6 mm Hg compared with 106+/-4 mm Hg in control mice. After anesthesia, mean arterial pressure averaged 97+/-4 mm Hg in chronic Ang II-infused mice compared with 94+/-3 mm Hg in control mice, allowing comparison of renal function at similar arterial pressures. Ang II-infused mice had lower urinary sodium excretion (0.16+/-0.04 versus 0.30+/-0.05 microEq/min; P<0.05), higher distal sodium reabsorption (1.74+/-0.18 versus 1.12+/-0.18 microEq/min; P<0.05), and higher fractional reabsorption of distal sodium delivery (91.1+/-1.8% versus 77.9+/-4.3%; P<0.05) than control mice. Urinary Ang II concentrations, measured during distal blockade, were greater in Ang II-infused mice (1235.0+/-277.2 versus 468.9+/-146.9 fmol/mL; P<0.05). In chronic Ang II-infused mice treated with spironolactone (n=5), fractional reabsorption of distal sodium delivery was similarly augmented as in chronic Ang II-infused mice (94.6+/-1.7%; P<0.01). These data provide in vivo evidence that there is enhanced distal sodium reabsorption dependent on sodium channel and Na(+)-Cl(-) cotransporter activity and increased urinary Ang II concentrations in mice infused chronically with Ang II.

AT1receptor-mediated enhancement of collecting duct renin in angiotensin II-dependent hypertensive rats

Angiotensin II (ANG II)-infused rats exhibit increases in distal nephron renin expressed in principal cells of connecting tubules and collecting ducts. This study was performed to determine whether the augmentation of distal nephron renin involves ANG II type 1 (AT1) receptor activation. Male Sprague-Dawley rats (200-220 g) were divided into three groups: 1) sham operated (n = 8); 2) ANG II infused (80 ng/min, 13 days, n = 8); and 3) ANG II infused plus AT1 receptor blocker (ARB), olmesartan (5 mg/days, n = 8). ANG II infusion increased systolic blood pressure (BP; 178 +/- 4 vs. 122 +/- 1 mmHg; P < 0.001) and suppressed plasma renin activity (PRA; 0.08 +/- 0.1 vs. 5.3 +/- 0.8 ng ANG I x ml(-1) x h(-1)). ARB treatment prevented the increase in BP (113 +/- 6 mmHg) and led to increases in PRA (15.8 +/- 1.5 ng ANG I x ml(-1) x h(-1)). Renin protein levels measured in the kidney medulla, to avoid contribution from juxtaglomerular apparatus cells, were higher in ANG II-infused rats [1.64 +/- 0.3 vs. 1.00 +/- 0.1 densitometric units (DU) compared with sham-operated rats; P < 0.05], and ARB treatment prevented this increase (1.01 +/- 0.1). Similarly, renin immunoreactivity increased in medullary collecting ducts of ANG II-infused compared with sham-operated rats (2.5 +/- 0.3 vs. 1.0 +/- 0.2 DU; P < 0.001), which was also prevented by ARB (1.01 +/- 0.06). Renin qRTPCR in ANG II-infused rats showed higher mRNA levels in the kidney medulla compared with sham-operated rats (5.5 +/- 2.3 vs. 0.04 +/- 0.02 ratio to GAPDH mRNA levels; P < 0.001); however, renin transcript levels were normalized in the ARB-treated rats. These data demonstrate that the augmentation of distal nephron renin in ANG II-infused hypertensive rats is AT1 receptor mediated. The augmented distal tubular renin may contribute to increased intratubular ANG II levels and distal nephron sodium reabsorption in ANG II-dependent hypertension.

GI262570, a peroxisome proliferator-activated receptor {gamma} agonist, changes electrolytes and water reabsorption from the distal nephron in rats.

Peroxisome proliferator-activated receptor-gamma (PPARgamma) agonists have been shown to have significant therapeutic benefits such as desirable glycemic control in type 2 diabetic patients; however, these agents may cause fluid retention in susceptible individuals. Since PPARgamma is expressed selectively in distal nephron epithelium, we studied the mechanism of PPARgamma agonist-induced fluid retention using male Sprague-Dawley rats treated with either vehicle or GI262570 (farglitazar), a potent PPARgamma agonist. GI262570 (20 mg/kg/day) induced a plasma volume expansion. The plasma volume expansion was accompanied by a small but significant decrease in plasma potassium concentration. Small but significant increases in plasma sodium and chloride concentrations were also observed. These changes in serum electrolytes suggested an activation of the renal mineralocorticoid response system; however, GI262570-treated rats had lower plasma levels of aldosterone compared with vehicle-treated controls. mRNA levels for a group of genes involved in distal nephron sodium and water absorption are changed in the kidney medulla with GI262570 treatment. In addition, due to a possible rebound effect on epithelial sodium channel (ENaC) activity, a low dose of amiloride did not prevent GI262570-induced fluid retention. On the contrary, the rebound effect after amiloride treatment potentiated GI262570-induced plasma volume expansion. This is at least partially due to a synergistic effect of GI262570 and the rebound from amiloride treatment on ENaCalpha expression. In summary, our current data suggest that GI262570 can increase water and sodium reabsorption in distal nephron by stimulating the ENaC and Na,K-ATPase system. This may be an important mechanism for PPARgamma agonist-induced fluid retention.

Renal sodium transport in healthy elderly and elderly with systolic hypertension

It is accepted that lithium clearance expresses the delivery of sodium and water. This study was designed to investigate renal sodium transport in healthy and hypertensive elderly using lithium clearance (Cli) and fractional lithium clearance (FEli). The clearance studies were performed and analyzed in young healthy volunteers (n = 29, 26.2 ± 3.7 years), elderly with normal blood pressure (n = 49, 67.8 ± 6.1 years) and elderly with systolic hypertension (n = 37, 68.5 ± 7.2 years, blood pressure ≥ 160/95 mmHg). FENa of healthy elderly group was greater than that of young group (1.36 ± 0.49% vs. 1.06 ± 0.54%, P < 0.05), but there was no difference between the healthy elderly group and hypertensive elderly group. In the healthy elderly with normal GFR (subgroup 1, n = 27), FELi was still significantly higher than that of young group (26.1 ± 10.0 vs. 18.6 ± 4.0%, P < 0.001). The absolute proximal reabsorption of sodium (APRNa) and the fractional proximal reabsorption of sodium (FDRNa) in subgroup 1 were obviously lower than that of the young group (11.9 ± 1.8 vs. 13.3 ± 3.1 mmol/min, P < 0.05 and 76.5 ±6.4% vs. 81.5 ± 4.0%, P < 0.005). On the other hand, no significant difference was found in absolute or fractional distal reabsorption of sodium (ADRNa or FDRNa) between subgroup 1 and the young group. ADRNa/filtered sodium load (FLNa) in subgroup 1 was significantly higher than that of the young group (22.6 ± 6.8% vs. 17.6 ± 3.9%, P < 0.05), whereas there were no differences in CLi, FELi, APRNa, ADRNa, FDRNa or ADRNa/FLNa between the healthy elderly group and the aged systolic hypertensive group. It is concluded that in ageing, sodium reabsorption in the proximal nephron is reduced alone with a relative enhancement of sodium reabsorption in distal nephron, which might be a compensation in sodium balance of old persons. Abnormalities of sodium reabsorption were unlikely to be responsible for systolic hypertension in the elderly.

Inhibition of cyclic GMP phosphodiesterases augments renal responses to atrial natriuretic factor in congestive heart failure

Atrial natriuretic factor (ANF), a cardiac peptide hormone with potent natriuretic and vasodilator actions, mediates its biologic responses via increases in intracellular cyclic guanosine monophosphate (cGMP). Recognizing that phosphodiesterases degrade cGMP and that congestive heart failure (CHF) is characterized by reduced renal responses to ANF, the authors hypothesized that cGMP phosphodiesterases limit the renal actions of exogenous and endogenous ANF in the presence of experimental CHF. In anesthetized dogs with severe CHF and avid sodium retention produced by rapid ventricular pacing, the authors explored the renal actions of M&B 22,948 (Rhône-Poulenc, Essex, UK), an inhibitor of cGMP-specific phosphodiesterases. High-dose intrarenal cGMP phosphodiesterase inhibition (PDI), with minimal effects upon systemic hemodynamics and hormones, significantly enhanced sodium excretion. This occurred primarily by decreasing distal nephron sodium reabsorption while enhancing renal cGMP generation. In separate groups of dogs, low-dose intrarenal cGMP PDI potentiated the actions of exogenous ANF on glomerular filtration and distal nephron sodium reabsorption, leading to enhanced natriuresis in the presence or absence of severe CHF. These studies support a link between ANF and the renal actions of cGMP PDI, and indicate that cGMP phosphodiesterases may contribute to sodium retention in advanced CHF by limiting the renal actions of increased endogenous ANF.

Effects of 1 M NaCl cerebroventricular injection on renal and baroreceptor reflex functions.

Our purpose was to study the influence of the stimulation of the cerebroventricular system on some mechanisms related to hydrosaline equilibrium and blood pressure regulation. Renal function and blood pressure (group 1) as well as the baroreceptor reflex (group 2) were studied. In group 1, we measured diuresis, natriuresis, creatinine clearance, lithium clearance, and blood pressure in control rats and after stimulation of the cerebroventricular system with 1 M NaCl solution. In group 2, we evaluated the baroreceptor reflex, producing an increase of blood pressure with an injection of phenylephrine to obtain baroreceptor reflex curves--characterized by threshold, point of inflection, heart period range, gain, and systolic pressure corresponding to half the heart period range (SBP50)--in control and experimental rats injected with saline and 1 M NaCl solution, respectively. In group 1 experimental rats, we observed a significant increase in diuresis, natriuresis, blood pressure, and glomerular filtration rate. A substantial increase was also registered in sodium filtered load and reabsorbed sodium in the proximal convoluted tubule and distal nephron. No differences were observed either in fractional proximal tubule or in distal nephron sodium reabsorption. In group 2 experimental rats, mean arterial blood pressure, threshold, point of inflection, and SBP50 were significantly higher than in control rats. By contrast, a decrease in gain and heart period range was observed. No difference was obtained in heart rate. Our results demonstrate that the increase of the natriuresis is due, at least in part, to an increase in sodium filtered load.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular and renal actions of C-type natriuretic peptide.

Studies were performed in two groups of anesthetized dogs (n = 5 per group) to determine the cardiovascular and renal actions of synthetic C-type natriuretic peptide (CNP). Systemic infusion of CNP (group 1; 10 and 50 ng.kg-1.min-1 iv) resulted in marked cardiovascular hemodynamic effects characterized by a decrease in mean arterial pressure, cardiac output, and atrial pressures in association with a decrease in sodium excretion. Bolus administration of CNP (group 2; 5 micrograms/kg iv) to minimize cardiovascular hemodynamic changes resulted in only a transient decrease in arterial pressure. Sodium excretion decreased despite a return of arterial pressure to baseline. These biological responses were associated with increases in plasma guanosine 3',5'-cyclic monophosphate (cGMP) in both groups but with no change in urinary cGMP. With both systemic infusion or bolus administration of CNP, significant increases in plasma aldosterone were observed in association with increases in distal nephron sodium reabsorption. This study demonstrates that CNP exhibits profound systemic hemodynamic actions and is indirectly, or perhaps directly, antinatriuretic.

Renal sodium and water handling in hypothyroid patients

The mechanism responsible for renal tubular abnormalities in sodium and water excretion in hypothyroid patients is poorly understood. To evaluate the possible contribution of the reduced glomerular filtration rate of hypothyroidism to these abnormalities, tubular function in hypothyroid patients was compared with that in patients with chronic renal failure and in normal subjects. The lithium clearance method and oral water loading were used to evaluate parameters of tubular sodium and water handling, respectively. The hypothyroid and the chronic renal failure patients were selected to have similar reductions in glomerular filtration rate. As compared to the normal subjects, the hypothyroid and chronic renal failure patients had a decrease in proximal sodium reabsorption and an increase in distal sodium reabsorption. The changes in tubular handling of sodium were not different in the hypothyroid and the chronic renal failure patients. Maximal urinary flow rate and free water clearances were similarly reduced in the hypothyroid patients and the chronic renal failure patients. For all subjects studied, proximal sodium reabsorption and maximal urinary volume were directly correlated with the glomerular filtration rate, and distal nephron sodium reabsorption was proportionate to delivery of sodium from the proximal tubule. The results suggest that the abnormalities in tubular sodium and water handling in hypothyroid patients are comparable to those present in other patients with a similar degree of renal insufficiency. Thus, the tubular abnormalities in hypothyroidism may be a consequence of the associated decrease in glomerular filtration rate.