Preglomerular Vessels Research Articles

Direct evidence that thromboxane mimetic U44069 preferentially constricts the afferent arteriole.

The thromboxane A2 (TXA2) mimetic U44069 has been demonstrated to reduce the GFR and filtration fraction of the normal isolated perfused rat kidney markedly, suggesting a predominant constriction of preglomerular vessels. To assess this possibility directly, effects of U44069 on the renal microvessels of the isolated perfused hydronephrotic kidney were examined. At 10(-6) mol/L, U44069 elicited a 27 +/- 2% decrease in afferent arteriolar (AA) diameter (from 18.8 +/- 0.3 to 13.7 +/- 0.3 micron, P < 0.001). In contrast, efferent arteriolar (EA) diameter decreased by only 9 +/- 1% (from 16.4 +/- 0.5 to 15.0 +/- 0.5 micron, P < 0.001). These effects on both AA and EA were completely reversed by the TXA2 receptor antagonist SQ29548. The calcium antagonist diltiazem reversed U44069-induced AA constriction by 83 +/- 5%. The U44069-induced EA constriction was insensitive to the vasodilator action of diltiazem at concentrations from 10(-8) to 10(-6) mol/L, but at 10(-5) mol/L, diltiazem increased the EA diameter significantly, albeit modestly. Nifedipine also reversed the U44069-induced AA constriction (81 +/- 7%), but failed to inhibit the EA constriction at concentrations from 10(-9) to 10(-6) mol/L. These findings constitute the first direct evidence that a TXA2 agonist preferentially constricts the afferent arteriole. Furthermore, the ability of both the calcium antagonist and SQ29548 to reverse the renal microvascular actions of TXA2 agonists suggests a potential utility of these agents in ameliorating TXA2-induced renal hemodynamic abnormalities.

Angiotensin II-receptor stimulation of cytosolic calcium concentration in cultured renal resistance arterioles

This study provides an initial characterization of basic morphological properties of cultures of vascular smooth muscle cells (VSMC) from rat preglomerular resistance vessels and of the functional coupling of angiotensin II (ANG II) receptors to cytosolic free calcium concentration ([Ca2+]i (fura 2 fluorescence photometry). Renal VSMC were isolated from interlobular arteries and afferent arterioles (< 50 microns) using an iron oxide sieving method and compared with rat aortic VSMC cultured under similar conditions. Quiescent monolayers maintained uniform morphology and [Ca2+]i signaling profile between passages 3 and 10. Arteriolar and aortic VSMC were spindle shaped and expressed smooth muscle-specific alpha-actin and myosin heavy chains SM-1 and SM-2. ANG II caused a rapid increase in [Ca2+]i, followed by a sustained plateau phase at 50-60% of the peak value. The initial maximum [Ca2+]i responses were dose dependent and of similar magnitude in renal arteriolar and aortic VSMC. ANG II (10(-7) M) increased [Ca2+]i from 50 to 240 nM in arteriolar and from 57 to 201 nM in aortic VSMC (P < 0.001 for both). Inhibition of ANG II effects on [Ca2+]i revealed significant signaling through distinct AT-receptor subtypes (losartan and PD-123319 sensitive) in renal arteriolar VSMC. In contrast, only losartan was effective in aortic VSMC. The AT2-receptor ligand CGP-42112 had no effect in either vessel type. Our results demonstrate that cultured arteriolar VSMC have anatomical similarities to aortic VSMC and functional differences in AT-receptor signaling in response to ANG II. This novel preparation should provide a useful approach with which to investigate cellular mechanisms concerning receptor coupling to signaling pathways involved in vascular reactivity of arteriolar VSMC in the microcirculation in general and the kidney in particular.

Localization of endothelin ETA and ETB receptor-mediated constriction in the renal microcirculation of rats.

1. The aim of the study was to visualize endothelin-1 (ET-1)-mediated constriction in renal vessels of cortical and juxtamedullary glomeruli in the split hydronephrotic rat kidney in vivo and to functionally characterize the ET receptor subtypes involved. 2. ET-1 (10(-9) M) constricted preglomerular vessels (by 6-18%) and efferent arterioles (by 11-13%), and decreased glomerular blood flow (GBF, by 55%) of cortical and juxtamedullary glomeruli. 3. The ETA antagonist BQ-123 (10(-6) M), as well as the ETB antagonist BQ-788 (2 x 10(-7) M) and IRL 1038 (10(-6) M), shifted the concentration-response curve of GBF for ET-1 to the right by one order of magnitude. While BQ-123 antagonized ET-1 constriction only in preglomerular vessels, BQ-788 and IRL 1038 were effective both in preglomerular vessels and efferent arterioles. 4. The ETB agonist IRL 1620 (10(-8) M) reduced GBF by 50% and constricted efferent arterioles (by 20-33%) about two times more than preglomerular vessels (by 6-14%). 5. Our results suggest that in renal cortical and juxtamedullary vessels of rats, ET-1-induced preglomerular vasoconstriction is mediated by ETA and ETB receptors, while efferent vasoconstriction is predominantly mediated by ETB receptors, which might have important consequences for the regulation of glomerular filtration pressure by ET.

Cyclosporin nephrotoxicity in heart and lung transplant patients.

Twenty-two patients with heart, lung or heart and lung transplants maintained on cyclosporin for periods ranging from 3 months to 10 years developed renal insufficiency which was investigated by renal biopsy. The histopathological changes were: (i) severe vascular and glomerular damage due to thrombotic microangiopathy (TM); (ii) a form of focal segmental glomerulosclerosis (FSGS); (iii) glomerular ischaemia. Rather than being separate entities, these changes appeared to represent a spectrum of pathology, some biopsies showing all three forms of glomerular injury. In all cases the glomerular changes were accompanied by arteriolar and arterial pathology, and we identified novel ultrastructural changes in the arteriolar endothelial basal lamina. Tubular atrophy was a consistent feature, the severity of which reflected the severity of the glomerular sclerosis, and which appeared to be a consequence of glomerular loss. Our findings are consistent with the nephrotoxic effects of cyclosporin being mediated chiefly via damage to preglomerular vessels and glomerular capillary endothelium. From an analysis of the clinical aspects of these cases, the effects of cyclosporin appear to be to some extent idiosyncratic, and therefore not entirely preventable, but strict monitoring of blood cyclosporin levels is essential to minimize the risk of permanent renal damage. Monitoring urinary protein in addition to plasma creatinine may detect the onset of FSGS, as proteinuria precedes creatinine elevation.

Roles of prostaglandins and nitric oxide in the effect of endothelin-1 on renal hemodynamics.

It is known that endothelin-1 stimulates the release of nitric oxide and prostaglandins in various vascular beds. We designed the present study to analyze the roles of prostaglandins and nitric oxide in the effect of endothelin-1 on the regulation of renal hemodynamics and renin release. We used N omega-nitro-L-arginine methyl ester (L-NAME) and meclofenamic acid to inhibit the production of nitric oxide and prostaglandins, respectively. With a nonfiltering kidney model, renal blood flow was reduced 21% in dogs treated with L-NAME and 18% in dogs treated with meclofenamic acid. Inhibition of nitric oxide and prostaglandins, however, produced opposite effects on estimated glomerular hydraulic pressure: L-NAME increased glomerular hydraulic pressure from 63.1 +/- 0.9 to 64.6 +/- 1.3 mm Hg (P < .01), and meclofenamic acid reduced glomerular hydraulic pressure from 63.3 +/- 1.4 to 59.8 +/- 1.6 mm Hg (P < .01). Endothelin-1 infusion produced a dose-dependent reduction in renal blood flow after blockade of nitric oxide and prostaglandins. The responses of glomerular hydraulic pressure were different in the two groups during endothelin-1 infusion. Endothelin-1 progressively reduced glomerular hydraulic pressure in a dose-dependent fashion in the meclofenamic acid group. However, endothelin-1 slightly increased glomerular hydraulic pressure until the infusion rate reached 5.0 ng/kg per minute. At that rate, endothelin-1 reduced glomerular hydraulic pressure from 63.3 +/- 1.4 to 47.0 +/- 1.4 mm Hg in the meclofenamic acid group (P < .01), a more than 25% reduction, whereas at the same dose, endothelin-1 reduced glomerular hydraulic pressure only less than 2% in the L-NAME group. In addition, blockade of nitric oxide and prostaglandins did not alter the inhibitory effect of endothelin-1 on renin release in the non-filtering kidney. Therefore, the present study demonstrates that the release of nitric oxide and prostaglandins might modulate the effects of endothelin-1 on the renal circulation. The present findings suggest that the differential vasoconstrictive effects of endothelin-1 on preglomerular and postglomerular vessels are associated with its stimulation of nitric oxide and prostaglandin production.

Effect of ramipril, nifedipine, and moxonidine on glomerular morphology and podocyte structure in experimental renal failure

Experimental renal failure causes structural alterations of the kidney. It is still unresolved how these changes are modified by antihypertensive treatment. Purpose of the study. To examine the effects of different antihypertensive agents (ramipril, nifedipine, moxonidine) mainly on glomerular geometry, cell number, cell morphology, and capillarization, in a subtotal nephrectomy model of renal failure. Sham-operated male SD rats and subtotally nephrectomized (SNX) ad libitum-fed rats were examined. Groups of 8-10 SNX rats were left untreated or were treated with ramipril (0.5 mg/kg b.w. per day), nifedipine (20 mg/kg b.w. per day) or moxonidine (10 mg/kg b.w.per day) respectively. After perfusion fixation the kidneys were examined using stereological techniques. Systolic blood pressure (by tail plethysmography) was 110+/-13 mm Hg in sham-op and 119+/-9 in SNX. It was effectively and comparably reduced below normal values by ramipril (89+/-11 mmHg), nifedipine (98+/-23 mmHg) and moxonidine (92+/-11 mmHg). The glomerulosclerosis index (SI) was significantly increased in SNX versus sham-op; it was similarly decreased by ramipril and moxonidine but less so by nifedipine. Vascular damage (preglomerular vessels) was reduced by all treatments whereas tubulointerstitial damage was signficantly reduced only by ramipril and moxonidine. Mean glomerular tuft volume was increased in SNX compared to sham-op. controls and was normalized only by ramipril treatment. Glomerular cells were differentially affected the three antihypertensive agents. After subtotal nephrectomy an increase in podocyte volume and mesangial cell number per glomerulus was noted. Nifedipine, and to a lesser extent ramipril, prevented mesangial cell hyperplasia. In contrast, only the ACE inhibitor ramipril, but not nifedipine or moxonidine prevented podocyte abnormalities, particularly podocyte hypertrophy. (i) Despite comparable reduction in systolic blood pressure, different classes of antihypertensive agents had diverse effects on renal damage in subtotally nephrectomized rat. This observation is consistent with specific, non-hemodynamic actions of anti-hypertensives. (ii) Glomerular and tubulointerstitial damage are prevented by treatment with ACE inhibitors and antisympathotonic agents, but not with the calcium antagonist nifedipine. In contrast, renal vascular changes were also prevented by nifedipine. (iii) Only ACE inhibitors effectively inhibited podocyte hypertrophy and mesangial cell hyperplasia. Whether the superior effect of ACE inhibitors on glomerulosclerosis is related to inhibition of glomerular growth and podocyte hypertrophy as well as preservation of podocyte structure, or whether these findings are merely a passive reflection of greater efficacy, remains unresolved.

Differential regulation of renal glomerular and preglomerular vascular angiotensin II receptors.

The activity of the renin-angiotensin system (RAS) can be influenced by sodium intake and angiotensin II (ANG II) infusion. It has been shown that ANG II can regulate the density of its receptors. Therefore, we investigated the regulation of glomerular and preglomerular vascular ANG II receptors by changes in RAS activity. Sprague-Dawley rats were fed a low- or high-sodium diet or were infused with nonpressor or pressor doses of ANG II for 7 days. ANG II receptor characteristics were determined by radioligand binding assays with use of ANG II antagonists losartan and PD-123319. AT1 was the only receptor type found in membrane preparations from all groups. Glomerular ANG II receptor characteristics in all groups were unchanged compared with their controls, whereas vascular receptor density was significantly downregulated by sodium restriction. We conclude that glomerular and preglomerular vascular ANG II receptors are differentially regulated such that AT1 receptors in preglomerular vessels can be regulated by changes in endogenous RAS. ANG II infusion did not induce any modification of either glomerular or vascular AT1 receptors, suggesting a predominant role of the endogenous local renal RAS.

Preglomerular sudanophilia in L-NAME hypertensive rats: involvement of endothelin.

To characterize alterations of renal vessels occurring during systemic hypertension elicited in rats by 5, 10, and 25 days of treatment by the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME)(20 mg/kg daily), preglomerular vasculatures, consisting of arcuate arteries and their branches, interlobular arteries, and afferent arterioles, were isolated by HCl maceration. Blockade of nitric oxide synthase significantly increased tail-cuff systolic blood pressure by 21 +/- 2% and 42 +/- 3% after 5 and 25 days, respectively. Medias of hypertensive arcuate arterial branches and interlobular arteries but not of afferent arterioles had focal deposits of Sudan black-positive lipid droplets. At 25 days, vessel wall thickness increased by 72 +/- 6% along the sudanophilic areas. Immunostaining of sudanophilic lesions with a panel of antibodies unveiled medial cell proliferation, macrophage invasion, immunoreactive vascular cell adhesion molecule-1, and low-density lipoprotein. The frequency of sudanophilic lesions increased with time to affect 26 +/- 2% and 36 +/- 3% of arcuate arterial branches and interlobular arteries, respectively, at 25 days. Hypertensive L-NAME-treated rats developed glomerular injury probed by albuminuria and glomerular immunostaining for alpha-smooth muscle actin. Administration of the nonselective endothelin antagonist bosentan (30 mg/kg daily) blunted the development of sudanophilic lesions during L-NAME treatment without affecting arterial hypertension or degree of glomerular injury. Therefore, L-NAME hypertension leads to rapid development of focal, inflammatory, proliferative, and sudanophilic lesions along preglomerular vessels, suggesting atherosclerosis-like processes. Furthermore, endothelin is a likely mediator in the development of these lesions.

Characterization of endothelin receptor subtypes in isolated rat renal preglomerular microvessels

Endothelin (ET) increases renal vascular resistance by constriction of post- and preglomerular vessels in the rat. However, ET receptor subtypes in renal microvessels have not been clearly defined. Radioligand binding experiments were performed in isolated arcuate and interlobular arteries as well as branching afferent arterioles to characterize 125I-ET-1 binding sites. Competitive inhibition assays were performed with ET-1, ET-2, ET-3, sarafotoxin 6b (S6b), BQ-123 (a preferential ET A receptor antagonist) and 4-Ala-ET-1 (a preferential ET B receptor agonist). Saturation data revealed a single class of high affinity binding sites with a k d of 0.31 ± 0.03 nM and a B max of 1336 ± 181 fmol/mg protein. Competitive inhibition of 125I-ET-1 binding showed that all assayed compounds displaced 125I-ET-1 in a dose-related manner. ET-1 displaced 100% of 125I-ET-1 binding, displaying monophasic curves with B max and k d values of 1369 ± 170 fmol/mg protein and 0.35 ± 0.04 nM, respectively. ET-2's displacement curves were similar to those of ET-1. ET-3 and S6b inhibited 100% of 125I-ET-1 binding in a biphasic manner, suggesting these peptides bind both high and low affinity sites. BQ-123 displaced about 50% of 125I-ET-1 in a monophasic manner, indicating a single high affinity binding site. 4-Ala-ET-1 displaced 125I-ET-1 in a clearly biphasic manner with an almost equal proportion of high and low affinity binding sites. Our results suggest that both ET A and ET B receptors are expressed in rat renal preglomerular vessels in almost equal proportions. However, the characteristics of competitive inhibition of 125I-ET-1 binding by several agents cannot be fully explained by a two-receptor model.

Expression of vascular endothelial growth factor in renal vascular disease and renal allografts

Vascular endothelial growth factor (VEGF) is a dimeric glycoprotein that exerts a proliferative effect specifically on endothelial cells. VEGF can increase vascular permeability and collagenase activity, is chemotactic for monocytes, and may dilate blood vessels. It can be induced by phorbol ester and cAMP in both mesenchymal and epithelial cells. In vitro cell culture experiments suggest that VEGF is upregulated by oxygen deprivation. In this study we tested whether in vivo acute and/or chronic reduction of renal blood flow by vascular obstruction would result in increased expression of VEGF mRNA and protein. Three normal kidneys, five human kidneys with narrowing of preglomerular vessels by vascular rejection or by vasculitis, and eight kidneys with nephrosclerosis and/or diabetic nephropathy were examined. In situ hybridization with 35S-labelled riboprobes showed a pronounced expression of VEGF mRNA in acutely hypoxic proximal and distal tubules of both the cortex and medulla; VEGF protein was demonstrated in the epithelia of these tubules by immunohistochemistry. In kidneys with chronically reduced blood flow, the majority of atrophic tubules were negative for VEGF mRNA and protein, although interstitial cells expressed VEGF mRNA. In arcuate arteries showing intimal and adventitial fibrosis, some medial smooth muscle cells were positive for VEGF mRNA. In glomeruli with segmental sclerosis, viable podocytes showed a prominent signal for VEGF mRNA. Mesangial cells did not express VEGF in the cases studied. It is possible that hypoxia itself led to the upregulation of VEGF in tubular epithelia and vascular smooth muscle cells. The vasodilatory and permeability-promoting effects of the endothelial growth factor produced by damaged tubular epithelia may constitute a mechanism to alleviate a decrease in blood flow and substrate availability and to re-establish vascular integrity.

Angiotensin II receptor subtypes in the kidney: Distribution and function

Summary: Angirotensin II (AII) is a powerful humoral regulator of body fluid and electrolyte balance and arterial blood pressure. In the kidney, All influences renal haemodynamics and proximal tubular reabsorption of sodium through activation of All that mediate complex signal transduction pathways. Angiotensin II is also implicated in the pathophysiological process of some progressive renal diseases. Pharmacological characterization and molecular cloning of All receptor reveals at least two major subtypes of All receptors, AT1 and AT2, in the kidney and other tissues. the AT1 receptor cDNA encodes a 359 amino acid protein with structure typical of seven transmembrane G‐protein coupled receptors. Two isoforms of AT1 receptor, AT1A and AT1B, are known in rodents, but probably only one occurs in other mammals including humans. the AT2 receptor cDNA, a 363 amino acid protein, shares only 32% identical amino acid residues with AT1 receptor, although it also has a seven transmembrane domain topology. In adult mammalian kidneys, AT1 receptors predominate in the glomerular mesangium, proximal tubular epithelium, renomedullary interstitial cells in the inner stripe of the outer medulla and large preglomerular vessels except those in human and monkey where AT2 receptors predominate. By contrast, in foetal kidneys, AT2 receptors are the major subtype; however, this shows dramatic regulation during development. Physiological studies using AT1 selective antagonists show that the known actions of All on renal haemodynamics, glomerular filtration, and tubular sodium and water transport are mediated by this subtype of All receptors. In addition, AT1 receptors also mediate hypertrophic and mitogenic actions of All on cultured glomerular mesangial cells and proximal tubular epithelial cells, and on extra‐cellular matrix accumulation in animal models of progressive renal diseases. By contrast, blockade of AT2 receptors has no effect on renal haemodynamics, tubular sodium reabsorption or growth properties of All. Overall, All exerts multiple actions in the kidney by interacting with different subtypes of All receptors located on multiple cellular sites.

Effect of furosemide on local and zonal glomerular filtration rate in the rat kidney.

Furosemide has been reported to produce disproportional changes in blood flow in cortical zones and to inhibit tubuloglomerular feedback (TGF), suggesting that furosemide might alter the intracortical distribution of glomerular filtrate. We have tested this hypothesis by a new method for measuring local and total glomerular filtration rate (GFR) based on proximal tubular accumulation of the basic polypeptide aprotinin (mol wt 6513). Local GFR was calculated in tissue samples dissected from outer cortex (OC), inner cortex (IC) and the corticomedullary border zone (CM) from the plasma clearances of two aprotinin tracers injected i.v. before and after a 3 min i.v. infusion of 25 mg kg-1 furosemide. The mean of five samples from each region was used to determine zonal GFR. Isotonic saline was infused at a rate corresponding to urine flow. Furosemide reduced whole kidney GFR from 1.17 to 1.00 mL min-1 and gave a similar reduction of renal artery blood flow. Urine flow increased from 0.6 to 17% of GFR. Haematocrit (approximately 0.48) and plasma protein concentration (approximately 55 mg mL-1) were maintained while the arterial blood pressure tended to decline (118 +/- 5 mmHg to 108 +/- 6 mmHg, P < 0.05). GFR in OC, IC and CM (1.58, 1.18, 0.42 mL min-1 g-1) fell to 87, 88 and 88% of control after furosemide infusion respectively. The furosemide/control ratio for each sample showed a coefficient of variation of about 3%. We conclude that furosemide produced a modest GFR reduction that was uniform throughout the renal cortex. The homogenous GFR response suggests a similar TGF constriction tone in preglomerular vessels of deep and superficial nephrons.

Diversity and variability of smooth muscle phenotypes of renal arterioles as revealed by myosin isoform expression

The contractility and distensibility of renal arterioles are important in the regulation of glomerular filtration. However, little is known regarding the characteristics of contractile proteins in these arterioles. Recently it was demonstrated that vascular smooth muscles contain two types of myosin heavy chain (MHC) isoforms, SM1 and SM2, which are unique molecular markers of smooth muscle cell phenotypes. SM1 is constitutively expressed in all types of smooth muscles, whereas SM2 exists only in mature smooth muscles. We characterized the expression of MHC isoforms as well as the ultrastructural myofilament assembly of renal arteriolar smooth muscles in human, rat and rabbit by immunohistochemical techniques. SM1 and alpha-smooth muscle actin were localized in both the preglomerular vessels (including the afferent arterioles) and efferent arterioles, whereas SM2 was present only in the preglomerular vessels. Renin-producing cells in the afferent arterioles (juxtaglomerular granular cells, JG cells) were positive for alpha-smooth muscle actin but negative for SM2. When renin synthesis was stimulated, the more proximal afferent arteriolar smooth muscles turned renin-positive and SM2 disappeared. Glomerular mesangial cells did not show immunoreactivities for SM1, SM2 or alpha-smooth muscle actin. The difference in MHC isoform expression in these arterioles was also reflected by ultrastructures; the afferent arteriolar smooth muscles contained abundant myofilaments including thick filaments, whereas the efferent arteriolar smooth muscles had a few myofilaments composed only of thin microfilaments. The JG cells displayed a myofilament assembly similar to that in the efferent arteriolar smooth muscles. We conclude from these observations that smooth muscles in pre-and postglomerular arterioles, the glomerular mesangial cells and JG cells differ in phenotypes, suggesting that they may have different contractile properties which may be critically involved in the regulation of glomerular filtration.

Chronic L-NAME hypertension in rats and autoregulation of juxtamedullary preglomerular vessels

The impact of chronic NG-nitro-L-arginine methyl ester (L-NAME)-induced hypertension (20 mg.kg-1.day-1 po, for 25 days) on pressure responsiveness was assessed in vessels ranging from arcuate arteries (ArcA) to juxtaglomerular afferent arterioles (JAA), using videomicroscopy and blood-perfused juxtamedullary nephron (JMN) preparations. Respective tail-cuff pressures of control and L-NAME rats were 127 +/- 2 (n = 8) and 173 +/- 4 mmHg (n = 5). Corresponding vessels of both groups had similar calibers at 60 mmHg. Increasing blood perfusion pressure to 200 mmHg constricted control ArcA and JAA by 26 +/- 4% (n = 20) and 43 +/- 5% (n = 15), respectively. Instead, a respective 3 +/- 4% (n = 15) and 21 +/- 9% (n = 6) pressure-induced dilation occurred in L-NAME vessels, and 86 +/- 2% of glomeruli expressed alpha-smooth muscle actin. Responses to acetylcholine (1 microM) but not to nitroprusside (1 mM) were impaired by L-NAME. Maximal relaxation induced by Mn2+ (10 mM) revealed equal basal tone and similar passive viscoelastic properties in control and L-NAME vessels. No vascular hypertrophy was found in L-NAME vessels. Chronic L-NAME hypertension is therefore associated with a selective loss of vascular autoregulation in JMNs, which may contribute to glomerular injury.

Effects of urodilatin in the rat kidney: Comparison with ANF and interaction with vasoactive substances

We compared the effects of urodilatin (URO) and atrial natriuretic factor (ANF) in normal and hydronephrotic kidneys (HNK) of rats. Furthermore, the impact of blocking different vasoactive hormones on the action of natriuretic peptides on vessels of cortical (C) and juxtamedullary (JM) glomeruli was studied in HNK by using URO. In normal kidneys, effects of URO and ANF (1.2, 2.4, 4.8, 12, and 19.10(-11) mol.kg-1.min-1 i.v.) were not significantly different. At 12.10(-11) mol.kg-1.min-1, URO and ANF increased urine flow 5.4 +/- 1.7 and 3.0 +/- 0.8-fold, increased urinary sodium excretion 20.7 +/- 8.8 and 10.3 +/- 4.0-fold, and decreased blood pressure by 13 +/- 2% and 12 +/- 1%, respectively (mean +/- SEM). In HNK, URO and ANF (0.4, 0.9, and 2.0.10(-11) mol.kg-1.min-1 i.v. and local application of 0.5, 1.0, and 2.0.10(-9) M) dose-dependent dilated preglomerular vessels (max approximately 20%), constricted efferent arterioles (max approximately 15%), and increased glomerular blood flow of C glomeruli in an identical fashion. Comparing URO effects on C and JM arterioles (0.4 and 0.9.10(-11) mol.kg-1.min-1 i.v.), JM responses were about one third of C responses. Angiotensin converting enzyme inhibition (ACEI, 2.10(-6) mol.kg-1 quinapril i.v.), combined ACEI and cyclooxygenase inhibition (CYOI, 2.8.10(-5) M indomethacin), and endothelin (ET) receptor blockade (10(-6) M BQ 123 and IRL 1038) diminished preglomerular vasodilation (C and JM) caused by URO infusion. Efferent vasoconstriction (C and JM) caused by URO was exaggerated by blockade of nitric oxide synthesis (10(-5) M L-NAME) and abolished by combined ACEI and CYOI, by bradykinin receptor blockade (4.10(-8) M Hoe 140), and by ET blockade. CYOI attenuated only JM efferent constriction. Our results show that URO and ANF possess equipotent vascular and similar natriuretic effects in the rat kidney. The magnitude of preglomerular vasodilation, which is directly mediated by these peptides, depends on the basal level of endogenous vasoconstrictors, while efferent vasoconstriction may be mediated by the secondary release of ET.

Vascular effects of parathyroid hormone and parathyroid hormone-related protein in the split hydronephrotic rat kidney.

1. The effects of locally applied parathyroid hormone-related protein (PTHRP), a putative autocrine/paracrine hormone, on vascular diameters and glomerular blood flow (GBF) in the split hydronephrotic rat kidney were studied. As PTHRP interacts with parathyroid hormone (PTH) receptors in all tissues tested so far, the effects of PTHRP were compared with those of PTH. 2. Preglomerular vessels dilated in a concentration- and time-dependent manner that was almost identical for PTH and PTHRP. A significant preglomerular vasodilation (5-17%) occurred at a threshold concentration of 10(-10) mol l-1 PTH or PTHRP, which raised GBF by 20 +/- 2 and 31 +/- 4%, respectively (means +/- S.E.M., n = 6). PTH or PTHRP (10(-7) mol l-1) increased preglomerular diameters (11-36%) and GBF (60 +/- 10 and 70 +/- 8%, respectively) to near maximum. The most prominent dilatation was located at the interlobular artery and at the proximal afferent arteriole. 3. Efferent arterioles were not affected by either PTH or PTHRP. 4. Estimated concentrations of half-maximal response (EC50) for preglomerular vasodilatation and GBF increase were in the nanomolar to subnanomolar range. 5. After inhibition of angiotensin I-converting enzyme by 2 x 10(-6) mol kg-1 quinapril I.V. (n = 6), 10(-8) mol l-1 PTHRP dilated preglomerular vessels and efferent arterioles (9 +/- 1% proximal and 6 +/- 1% distal). 6. We conclude that the renal vasculature of the hydronephrotic kidney is highly sensitive to vasodilatation by PTH and PTHRP, which, in addition, may constrict efferent arterioles by stimulating renin release.(ABSTRACT TRUNCATED AT 250 WORDS)

Nephrotoxicity of FK-506 in the rat. Studies on glomerular and tubular function, and on the relationship between efficacy and toxicity

Recent studies in liver and kidney transplant recipients revealed a nephrotoxic adverse effect of the new macrolide immunosuppressant FK-506. Therefore the effect of FK-506 0.1 to 0.8 mg per kg per day was investigated in rats using clearance methods including lithium clearance. In rats given FK-506 or placebo during 1 week the nephrotoxicity of FK-506 was characterized by a slight reduction of inulin clearance. The end proximal delivery as measured by the lithium clearance was decreased by FK-506. In rats treated for 4 weeks with FK-506 0.8 mg/kg/day the glomerular filtration rate (GFR) had decreased to 23% of the GFR found in controls (P < 0.001), while end proximal delivery was only 8% of normal. Renal histopathological investigation showed a slight but statistically significant increase of tubular basophilia and atrophy in FK-506-treated rats. Skin transplantation studies in the same rat strain showed a dose-dependent immunosuppressive effect of FK-506. FK-506 0.8 mg/kg was significantly more immunosuppressive than 0.2 or 0.4 mg/kg, so it was concluded that the lower doses of FK-506 did not fully exploit the drug's immunosuppressive potential. Thus in a dosage inside the therapeutic range defined from skin transplantations, FK-506 generated a number of toxic effects including a considerable nephrotoxic effect. The FK-506 induced changes in glomerular and tubular function was a close match to the changes found in cyclosporin A nephrotoxicity. The present study suggests that FK-506 nephrotoxicity is caused by constriction of preglomerular vessels.

Role of angiotensin II in salt-induced increase in blood pressure and renal injury during chronic nitric oxide inhibition

Visualization of renal autoregulation in the split hydronephrotic kidney of rats. The autoregulatory ability of the renal vascular system in the split hydronephrotic kidney was quantitated with intravital microscopy. The luminal diameters of the arcuate and interlobular arteries as well as the afferent and efferent arterioles were measured. Glomerular blood flow was determined by the dual slit technique. In the first series of experiments, the renal perfusion pressure was reduced by graded clamping of the abdominal aorta. Pressure reduction from 118 mm Hg to 95 mm Hg induced dilation of all preglomerular vessels except for the distal afferent arteriole; there was no change in the efferent arteriole and the blood flow was maintained. Further pressure reductions to 71 and 43 mm Hg caused additional dilations of the preglomerular vessels, a marginal reduction in diameter of proximal efferent arterioles and flow reductions by 15% and 41%, respectively. In the second series, systemic blood pressure was increased by continuous i.v. infusions of norepinephrine (NE). NE constricted pre- and postglomerular vessels except for the distal afferent arteriole; glomerular flow was decreased. Reduction of renal perfusion pressure during NE infusion to the preinfusion value did not diminish glomerular blood flow, but reduced the constrictor response to NE in preglomerular vessels. In a third series of experiments we examined the effect of atrial natriuretic factor (ANF) on renal autoregulation. Addition of ANF (10−9 to 10−7M) to the renal bath induced a dose-dependent dilation of all preglomerular vessels and a constriction of the efferent arteriole. Pressure reduction from 120 to 95 mm Hg resulted in a further preglomerular vasodilation. These experiments demonstrate that autoregulation is mediated primarily by diameter changes in all preglomerular vessels excluding the distal segment of the afferent arteriole. Further, these data suggest that ANF induced dilation of preglomerular vessels does not impair the myogenic response of these vessels.

The roles of endogenous vasolidators in the control of glomerular hemodynamics

Responses of in vivo renal microvessels to dopamine. The split hydro-nephrotic kidney preparation was used to directly observe the effects of locally applied dopamine on the in vivo diameters of renal vessels. Dopamine (1 × 10-6 to 3 × 10-5 M) produced a concentration–dependent dilation of the arcuate and interlobular arteries and afferent arterioles. Efferent arterioles near the glomeruli also dilated to dopamine but the dilation was less than that of the preglomerular vessels. Higher dopamine concentrations (3 × 10-4 and 1 × 10-3 M) produced more variable effects, with a tendency for the arcuate and interlobular arteries and the afferent and efferent arterioles away from the glomeruli to decrease in diameter. After pretreatment with haloperidol, dopamine (1 × 10-6 to 1 × 10-4 M) did not dilate any pre- or postglomerular vascular segment, but the tendency for pre- and postglomerular constrictions with higher dopamine concentrations were not abolished. Pretreatment with phentolamine and propranolol enhanced the dilator response of the pre- and postglomerular vessels (except the afferent arterioles near glomeruli and efferent arterioles near welling points) to dopamine (3 × 10 -5 and 1 × 10-4 M), and abolished the reductions in diameter produced by the high dopamine levels. These data indicate that the dilator effect of dopamine is mediated by interactions with specific dopaminergic receptors, while alpha and beta adrenergic receptors appear to mediate a constrictor influence observed with high dopamine concentrations. The overall effect of dopamine on the renal vessel diameters thus appears to depend on the balance of dilator and constrictor stimuli mediated by multiple receptors.

Atrial natriuretic factor, angiotensin II, and the slow component of renal autoregulation

Autoregulation of renal blood flow is highly efficient and is mediated partly by tubuloglomerular feedback (TGF), which couples regulation of blood flow to that of sodium excretion. Atrial natriuretic factor (ANF) dilates preglomerular resistance vessels, in which autoregulation occurs, and has been reported to inhibit TGF. This study addressed potential actions of ANF on the slow, TGF-mediated, component of autoregulation. Renal blood flow was measured by an electromagnetic flow probe in Sprague-Dawley rats anesthetized by halothane or isoflurane while renal perfusion pressure was manipulated by a servo-controlled clamp placed on the aorta between the renal arteries. Progressive reduction of perfusion pressure to 60 mmHg (1 mmHg = 133.3 Pa) induced resetting of autoregulation to operate at the reduced pressure and to defend lower renal blood flow. Infusion of ANF at a dose shown to reliably increase sodium excretion did not affect autoregulation or its resetting. Because resetting is angiotensin II dependent, the converting enzyme inhibitor Enalaprilat was used to provide angiotensin II blockade. As expected, autoregulation did not reset to operate at reduced perfusion pressure. Again ANF was without effect. In a third experiment, relaxation of resistance was assessed in response to repeated steps of perfusion pressure between 65 and 75 mmHg. Time constants of constriction and dilatation were recovered by fitting to a single exponential before and during ANF infusion. Time constants ranged form 0.045 to 0.055 Hz, were consistent with operation of TGF, were not different for constriction or dilatation, and were unaltered by ANF; nor did ANF affect the magnitude of constriction or dilatation.(ABSTRACT TRUNCATED AT 250 WORDS)