Role of endogenous endothelins in the renin system of normal and two-kidney, one clip rats

Renin, endothelial NO synthase and endothelin gene expression in the 2kidney-1clip Goldblatt model of long-term renovascular hypertension

Objective

Numerous reports have shown the influence of renin, nitric oxide (NO) and the endothelin (ET) systems for regulation of blood pressure and renal function. Furthermore, interactions between these peptides have been reported. Aim of our study was to investigate the relative contribution of these compounds in long-term renovascular hypertension/renal ischemia.

Methods

Hypertension/left-sided renal ischemia was induced using the 2K1C-Goldblatt rat model. Renal renin, ET-1, ET-3 and endothelial NO synthase (eNOS) gene expression was measured by means of RNAse protection assay at different timepoints up to 10 weeks after induction of renal artery stenosis.

Results

Plasma renin activity and renal renin gene expression in the left kidney were increased in the clipped animals while eNOS expression was unchanged. Furthermore, an increase in ET-1 expression and a decrease of ET-3 expression was detected in early stenosis.

Conclusions

While renin is obviously involved in regulation of blood pressure and renal function in unilateral renal artery stenosis, ET-1, ET-3 and endothelium derived NO do not appear to play an important role in renal adaptation processes in long-term renal artery stenosis, although ET-1 and ET-3 might be involved in short-term adaptation processes.

Physiology of endothelin and the kidney

Since its discovery in 1988 as an endothelial cell-derived peptide that exerts the most potent vasoconstriction of any known endogenous compound, endothelin (ET) has emerged as an important regulator of renal physiology and pathophysiology. This review focuses on how the ET system impacts renal function in health; it is apparent that ET regulates multiple aspects of kidney function. These include modulation of glomerular filtration rate and renal blood flow, control of renin release, and regulation of transport of sodium, water, protons, and bicarbonate. These effects are exerted through ET interactions with almost every cell type in the kidney, including mesangial cells, podocytes, endothelium, vascular smooth muscle, every section of the nephron, and renal nerves. In addition, while not the subject of the current review, ET can also indirectly affect renal function through modulation of extrarenal systems, including the vasculature, nervous system, adrenal gland, circulating hormones, and the heart. As will become apparent, these pleiotropic effects of ET are of fundamental physiologic importance in the control of renal function in health. In addition, to help put these effects into perspective, we will also discuss, albeit to a relatively limited extent, how alterations in the ET system can contribute to hypertension and kidney disease.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Role of the renin-angiotensin-aldosterone system in collecting duct-derived endothelin-1 regulation of blood pressure

Renal collecting duct (CD)-specific knockout of endothelin-1 (ET-1) causes hypertension and impaired Na excretion. A previous study noted failure to suppress the renin-angiotensin-aldosterone axis in these knockout (KO) mice, hence the current investigation was undertaken to examine the role of this system in CD ET-1 KO. Renal renin content was similar in kidneys from CD ET-1 KO and control mice during normal Na intake; high-Na intake suppressed renal renin content to a similar degree in KO and control. Plasma renin concentrations paralleled changes in renal renin content. Valsartan, an angiotensin receptor blocker (ARB), abolished the hypertension in CD ET-1 KO mice during normal Na intake. High-Na intake + ARB treatment increased blood pressure in CD ET-1 KO, but not in controls. High-Na intake was associated with reduced Na excretion in CD ET-1 KO animals, but no changes in water excretion or creatinine clearance were noted. Spironolactone, an aldosterone antagonist, also normalized blood pressure in CD ET-1 KO mice during normal Na intake, whereas high-Na intake + spironolactone raised blood pressure only in CD ET-1 KO animals. In summary, hypertension in CD ET-1 KO is partly due to angiotensin II and aldosterone. We speculate that CD-derived ET-1 may regulate, via a novel pathway, renal renin production.

Effects of growth hormone on renal renin gene expression in normal rats and rats with myocardial infarction

BACKGROUND

Published data regarding effects of growth hormone (GH) on the renin system are controversial. The aim of this study therefore was to evaluate the effects of GH on the renin system in normal rats and rats with myocardial infarction (MI).

METHODS

Normal rats received 2, 5, or 10 IU GH/kg/day or vehicle subcutaneously for 4 weeks. Furthermore rats with MI were randomized to receive 2 IU GH/kg/day or vehicle for 4 weeks. Subdivision into MI groups (mild, moderate, and large) was by histological determination of infarct size. Renal renin gene expression was assessed by RNAase protection assay and plasma renin activity by radioimmunoassay. In addition, isolated mouse juxtaglomerular cells were exposed to GH for 20 h, and renin secretion rates were assessed.

RESULTS

GH treatment in normal rats for 4 weeks increased body weight, and kidney weight to body weight ratio, but did not affect renin secretion and renal renin gene expression. In rats with large MI, renal renin gene expression increased about fourfold, but was unchanged in rats with small and moderate MI as compared to normal rats. In rats with MI, body weight decreased and this decrease was partially reversed by GH treatment. GH treatment did not change renal renin gene expression, and renin secretion in rats with MI. Renin secretion of isolated juxtaglomerular cells was unaffected by GH.

CONCLUSIONS

Our study demonstrates that GH treatment has no significant effect on renin secretion and on renal renin gene expression in normal rats and in rats with stimulated renin system due to MI in vivo. In isolated juxtaglomerular cells in vitro, renin secretion was also unaffected by GH.

Effects of chronic hypoxia on renal renin gene expression in rats

BACKGROUND

The effects of hypoxia on renin secretion and renin gene expression have been controversial. In recent studies, we have demonstrated that acute hypoxia of 6 h duration caused a marked stimulation of renin secretion and renal renin gene expression. This hypoxia-induced stimulation of the renin-angiotensin system might contribute, for example, to the progression of chronic renal failure and to the development of hypertension in the sleep-apnoea syndrome. For this reason, we were interested in the more chronic effects of hypoxia on renal renin gene expression and its possible regulation.

METHODS

Male rats were exposed to chronic normobaric hypoxia (10% O(2)) for 2 and 4 weeks. Additional groups of rats were treated with an endothelin ET(A) receptor antagonist, LU135252, or a NO donor, molsidomine, respectively. Systolic blood pressure and right ventricular pressures were measured. Renal renin, endothelin-1 and endothelin-3 gene expression were quantitated using RNAase protection assays.

RESULTS

During chronic hypoxia, haematocrit increased to 72+/-2%, and right ventricular pressure increased by a mean of 26 mmHg. Renal renin gene expression was halved during 4 weeks of chronic hypoxia. This decrease was reversed by endothelin receptor blockade (105 or 140% of baseline values after treatment for weeks 3-4 or 1-4). Furthermore, there was a trend of increasing renal endothelin-1 gene expression (to 173% of baseline values) after 4 weeks of hypoxia. Systolic blood pressure increased moderately during 4 weeks of chronic hypoxia from 129+/-2 to 150+/-4 mmHg. This blood pressure increase was higher in rats treated for 4 weeks with an endothelin receptor antagonist (196+/-11 mmHg).

CONCLUSIONS

Chronic hypoxia (in contrast to acute hypoxia) suppresses renal renin gene expression. This inhibition presumably is mediated by endothelins.

Control of the renal renin system by local factors

Local factors, such as prostaglandins (PGs), nitric oxide (NO), and endothelins (ETs), produced in the immediate vicinity of juxtaglomerular (JG) cells can exert significant effects on renin secretion and renin gene expression. PGE2, as the main renotubular PG, and PGI2, as the main endothelial prostanoid, both stimulate renin secretion and renin gene expression by activating cAMP formation in JG cells. Although the direct effect of NO on JG cells is less clear, its overall effect in vivo seems to be to stimulate the renin system. Evidence is emerging that stimulation by NO is related to the cAMP pathway, and cGMP-induced inhibition of cAMP-phosphodiesterase III (PDE-III) may mediate this effect. ETs, on the other hand, appear to inhibit the renin system, in particular in those pathways activated by cAMP, acting via Ca2+- and protein kinase C-related mechanisms. There is increasing evidence that both NO and PGs could be involved in the physiological regulatory mechanisms by which salt intake affects the renin system.

Endothelins in the normal and diseased kidney

Endothelin-1 (ET-1) is a 21-amino acid peptide that potently modulates renal function. ET-1 is produced by, and binds to, most renal cell types. ET-1 exerts a wide range of biologic effects in the kidney, including constriction of most renal vessels, mesangial cell contraction, inhibition of sodium and water reabsorption by the nephron, enhancement of glomerular cell proliferation, and stimulation of extracellular matrix accumulation. ET-1 functions primarily as an autocrine or paracrine factor; its renal effects must be viewed in the context of its local production and actions. This is particularly important when comparing ET-1 biology in the nephron, where it promotes relative hypotension through increased salt and water excretion, with ET-1 effects in the vasculature, where it promotes relative hypertension through vasoconstriction. Numerous studies indicate that ET-1 is involved in the pathogenesis of a broad spectrum of renal diseases. These include those characterized by excessive renal vascular resistance, such as ischemic renal failure, cyclosporine (CyA) nephrotoxicity, radiocontrast nephropathy, endotoxemia, rhabdomyolysis, acute liver rejection, and others. ET-1 appears to play a role in cell proliferation in the setting of inflammatory glomerulonephritides. The peptide also may mediate, at least in part, excessive extracellular matrix accumulation and fibrosis occurring in chronic renal failure, diabetes mellitus, and other disorders. Deranged ET-1 production in the nephron may cause inappropriate sodium and water retention, thereby contributing to the development and/or maintenance of hypertension. Finally, impaired renal clearance of ET-1 may cause hypertension in patients with end-stage renal disease. Many ET-1 antagonists have been developed; however, their clinical usefulness has not yet been determined. Despite this, these agents hold great promise for the treatment of renal diseases; it is hoped that the next decade will witness their introduction into clinical practice.

Endothelins inhibit cyclic-AMP induced renin gene expression in cultured mouse juxtaglomerular cells

We have recently described that endothelins-1 to -3 equipotently inhibit cAMP stimulated renin secretion from cultured mouse juxtaglomerular cells by a process involving phospholipase C activation. This study examined the influence of endothelin-2 on renin gene expression in renal juxtaglomerular cells. To this end we semiquantitated renin mRNA levels by competitive RT-PCR in primary cultures of mouse renal juxtaglomerular cells after 20 hours of incubation. We found that endothelin-2 (0.1 to 100 nmol/liter) did not change basal renin gene expression. The adenylate cyclase activator forskolin (3 mumol/ liter) increased renin mRNA levels to 400% of the controls and this stimulation was dose-dependently attenuated by ET-2 to 250% of the control value. The effect of ET-2 was mimicked by the ETB-receptor agonist sarafotoxin S6c. The kinase inhibitor staurosporine (100 nmol/ liter) increased renin secretion and renin mRNA levels. Combination of staurosporine with forskolin produced the same effects on renin secretion and renin mRNA levels as did staurosporine alone. In the presence of both forskolin and staurosporine ET-2 had no significant effect on renin secretion and renin gene expression. The phorbol ester PMA (30 nmol/ liter), which was used to stimulate protein kinase C activity, attenuated cAMP stimulated renin secretion and renin mRNA levels. Lowering the extracellular concentration of calcium by the addition of 1 mmol/liter EGTA did not inhibit the effect of ET-2 on cAMP induced renin secretion and renin gene expression. These findings suggest that endothelins inhibit cAMP stimulated renin gene expression by an event that is mediated via ETB receptors. This inhibitory effect may in part involve protein kinase C activation.