Regulation of renin release by local and systemic factors.

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.

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.

Neoimmun versus Neoral: a bioequivalence study in healthy volunteers and influence of a fat-rich meal on the bioavailability of Neoimmun.

In two crossover studies with 12 (6 males/6 females) healthy young volunteers each, we compared the bioavailability of Neoimmun capsules with the microemulsion Neoral and the influence of a fat-rich breakfast on the bioavailability of Neoimmun. Each volunteer received a single dose of 200 mg cyclosporine A in each period. Blood samples were taken up to 24 h and analysed for cyclosporine A by high-performance liquid chromatography (HPLC) and photometric detection. The pharmacokinetic parameters were determined by non-compartmental analysis. The treatments were tested for bioequivalence and significant differences. The bioavailability of Neoimmun was significantly lower compared to Neoral, albeit Neoimmun met the bioequivalence criterion (90% confidence interval of AUC 0.80-0.94) or missed the criterion only marginally (90% confidence interval of c (max) 0.75-0.91). The bioavailability of Neoimmun as determined by area under the blood concentration-time curve (AUC) increased by nearly 20% after a fat-rich breakfast. However, mean peak concentrations after food were only higher in male subjects, whereas mean peak concentrations in female subjects were lower compared to fasting administration. In conclusion, our data show that Neoimmun exhibits a lower bioavailability than the microemulsion Neoral and that food has a significant but variable and sex-dependent impact on the bioavailability of Neoimmun capsules.

Reporter gene recombination in juxtaglomerular granular and collecting duct cells by human renin promoter-Cre recombinase transgene.

To assess the feasibility of using the renin promoter for expressing Cre recombinase in juxtaglomerular (JG) cells only, we generated five independent transgenic mouse lines (designated hRen-Cre) expressing Cre recombinase under control of a 12.2-kb human renin promoter. In the kidneys of adult mice Cre mRNA (RT-PCR) was found in the renal cortex, with Cre protein (immunohistochemistry) being localized in afferent arterioles and to a lower degree in interlobular arteries. Cre mRNA levels were regulated in a renin-typical fashion by changes in oral salt intake, water restriction, or isoproterenol infusion, indicating the presence of key regulatory elements within 12.2 kb of the 5'-flanking region of the human renin gene. hRen-Cre mice were interbred with both the ROSA26-EGFP and ROSA26-lacZ reporter strains to assess renin promoter activity from Cre-mediated excision of a floxed stop cassette and subsequent enhanced green fluorescent protein (EGFP) and beta-galactosidase (beta-gal) detection. In adult mice, beta-gal staining and EGFP were observed in afferent arterioles and interlobular arteries, overlapping with Cre protein expression. In addition, intense beta-gal staining was found in cortical and medullary collecting ducts where Cre expression was minimal. In embryonic kidneys, beta-gal staining was detected in the developing collecting duct system beginning at embryonic day 12, showing substantial activity of the human renin promoter in the branching ureteric bud. Our data indicate that besides its well-known activity in JG cells and renal vessels the human renin promoter is transiently active in the collecting duct system during kidney development, complicating the use of this approach for JG cell-specific excision of floxed targets.

Cellular mechanism of renin release.

In this review we aim to give a comprehensive overview over the current knowledge of the cellular control of renin release. We hereby focus on the inhibitory effects of calcium on the exocytosis of renin. After a short introduction into general aspects of the regulation of renin release, including a brief summary on the role of the second messengers cAMP and cGMP, we will discuss parts of the literature on the effects of calcium on the renin system together with recent studies from our laboratory, investigating putative calcium influx and extrusion pathways of juxtaglomerular cells. Finally, as the precise mechanisms by which calcium inhibits the exocytosis of renin are far from being understood, we will present some hypotheses on the intracellular events being involved in the suppression of renin release by calcium.

Low sodium and furosemide-induced stimulation of the renin system in man is mediated by cyclooxygenase 2.

The aim of this study was to examine the effects of highly selective inhibition of cyclooxygenase 2 (COX-2) with rofecoxib on the renin system during long-term stimulation and after short-term stimulation. Six healthy male volunteers received, in a randomized crossover design, a low-sodium diet for days 1 through 9 with or without 25 mg rofecoxib twice daily on days 5 through 9 and, in addition, 20 mg of furosemide intravenously on day 8. Plasma renin activity increased 2 to 3 times over baseline with a low-sodium diet and 5 times over baseline 30 minutes after intravenous furosemide; it was still elevated nearly 5 times on day 9. These effects were completely blocked by rofecoxib. Plasma aldosterone and urinary aldosterone concentrations basically reflected the findings with plasma renin activity. Urinary sodium excretion decreased during a low-sodium diet and increased after intravenous furosemide without being significantly affected by rofecoxib. We have concluded that low-sodium and furosemide-stimulated renin and aldosterone secretion is completely blocked in healthy volunteers during COX-2 inhibition with rofecoxib, suggesting that intact COX-2 is of major importance for stimulation of the renin system under these conditions in man.

Differential roles of the sodium-calcium exchanger in renin secretion and renal vascular resistance.

The intracellular free calcium concentration ([Ca2+]i) is a central regulator of renin secretion and the contractility of vascular smooth muscle cells. As [Ca2+]i results from calcium influx and calcium extrusion, we were interested in the role of the Na+/Ca2+-exchanger as an important calcium-extrusion pathway in the regulation of renin secretion. Therefore, we investigated the effects of inhibiting the Na+/Ca2+-exchanger, either by reducing the extracellular sodium concentration ([Na+]e) or using pharmacological tools, on renin secretion and vascular resistance in the isolated perfused rat kidney model. Stepwise reductions of [Na+]e led to progressive (up to sevenfold) increases in renal vascular resistance ([Na+]e 7 mM) whilst renin secretion rates were not altered significantly. Similarly, pharmacological blockade of the Na+/Ca2+-exchanger by benzamil (100 microM) or KB-R7943 (30 microM) resulted in significant vasoconstrictions without altering basal renin secretion rates. Also renin secretion that was pre-stimulated by isoproterenol (10 nM), blockade of macula densa salt transport by bumetanide (100 microM) or lowering the perfusion pressure to 40 mmHg was not attenuated by Na+/Ca2+-exchanger inhibition, although the vascular resistance increased significantly. In contrast, angiotensin II (100 pM) reduced pre-stimulated renin secretion values by 50%. The subsequent lowering of the [Na+]e however did not augment the inhibition of renin secretion, although the renal vascular resistance increased markedly. We conclude that the Na+/Ca2+-exchanger has no functional role in the regulation of [Ca2+]i in juxtaglomerular cells controlling renin secretion, whereas it markedly affects the preglomerular vascular smooth muscle cells of the renal vasculature.

Functional role of sodium-calcium exchange in the regulation of renal vascular resistance.

Our study aimed to assess a possible functional role of the Na(+)/Ca(2+) exchanger in the regulation of renal vascular resistance (RVR). Therefore, we investigated the effects of an inhibition of the Na(+)/Ca(2+) exchanger either by lowering the extracellular sodium concentration ([Na(+)](e)) or, pharmacologically on RVR, by using isolated perfused rat kidneys. Graded decreases in [Na(+)](e) led to dose-dependent increases in RVR to 4.3-fold (35 mM Na(+)). This vasoconstriction was markedly attenuated by lowering the extracellular calcium concentration, by the L-type calcium channel blocker amlodipine or by the chloride channel blocker niflumic acid. Further lowering of [Na(+)](e) to 7 mM led to an increase in RVR to 7.5-fold. In this setting, amlodipine did not influence the magnitude but did influence the velocity of vasoconstriction. Pharmacological blockade of the Na(+)/Ca(2+) exchanger with KB-R7943, benzamil, or nickel resulted in significant vasoconstriction (RVR 2.5-, 1.8-, and 4.2-fold of control, respectively). Our data suggest a functional role of the Na(+)/Ca(2+) exchanger in the renal vascular bed. In conditions of partial replacement of [Na(+)](e), vasoconstriction is dependent on chloride and L-type calcium channels. A total replacement of [Na(+)](e) leads to a vasoconstriction that is nearly independent of L-type calcium channels. This might be due to an active calcium transport into the cell by the Na(+)/Ca(2+) exchanger.

Measuring Renin secretion from juxtaglomerular cells.

The development of specific receptor inhibitors of angiotensin II and the generation of genetic knockout models of the different components of the renin-angiotensin-aldosterone (RAAS) cascade has confirmed previous and has created new evidence for elementary functions of the RAAS for the body.

Role of endothelins for the regulation of renal renin gene expression.

Endothelin-1, -2 and -3 (ET-1, -2, -3) have suppressive effects on the renin system in different experimental in vitro models, whereas a modulation of renin secretion or renin gene expression by endothelins (ETs) in in vivo studies has not so far been found. In a recent study we observed a significant stimulation of the renin system by acute hypoxia over 6 h in rats. In the study reported here, we investigated the more chronic effects of hypoxia (10% O2 for 4 weeks) on renin gene expression and the influence of the ET system on its regulation. Renin mRNA levels decreased after 2 weeks of hypoxia to 76% of control and after 4 weeks to 49% of control (p < 0.05). Concomitant administration of the ET(A)-receptor antagonist LU135252 led to a significant increase in renin gene expression compared to control or hypoxia alone. ET-1 mRNA increased to 120% after 2 weeks and 173% after 4 weeks of hypoxia (NS), while ET-3 was not affected by hypoxia. We therefore conclude that ETs have a suppressive effect on renal renin gene expression in the setting of chronic hypoxia in rats in vivo.

Effects of chronic hypoxia on renal PDGF-A, PDGF-B, and VEGF gene expression in rats.

BACKGROUND: There is evidence from in vitro studies to suggest that the genes of platelet-derived growth factor (PDGF), and vascular endothelial growth factor (VEGF) are, like the erythropoietin gene, regulated by oxygen tension. Hypoxia-induced stimulation of, for example, PDGF or VEGF might be involved in the pathogenesis of acute or chronic renal failure and in renal 'inflammatory' diseases (glomerulonephritis, vasculitis, allograft rejection). METHODS: Male Wistar rats were exposed to chronic normobaric hypoxia (10% O(2), 90% N(2)) for 4 weeks. Additional groups of rats were treated with the endothelin receptor antagonist LU13525 and the NO donor molsidomine. Renal mRNA levels of PDGF-A, PDGF-B, and VEGF were semiquantitated using RNase protection assays. RESULTS: Renal gene expression of PDGF-A and PDGF-B was neither affected by 2 or 4 weeks of hypoxia nor by concomitant treatment with LU135252 or molsidomine. Chronic hypoxia did also not change VEGF gene expression; however, concomitant treatment with LU135252 increased all VEGF subtypes (188, 164, 120). CONCLUSIONS: The findings of the present study suggest that renal PDGF and VEGF gene expression in vivo during chronic hypoxia for 2 and 4 weeks is not sensitive to tissue hypoxia in contrast to cell culture experiments. During chronic hypoxia with concomitant blockade of endothelin receptors, all VEGF subtypes were increased, suggesting an inhibitory action of endothelins with regard to renal VEGF gene expression.

Store-operated calcium influx inhibits renin secretion.

On the basis of evidence that changes in the extracellular concentration of calcium effectively modulate renin secretion from renal juxtaglomerular cells, our study aimed to determine the effect of calcium influx activated by depletion of intracellular calcium stores on renin secretion. For this purpose we characterized the effects of the endoplasmatic Ca(2+)-ATPase inhibitors thapsigargin (300 nM) and cyclopiazonic acid (20 microM) on renin secretion from isolated perfused rat kidneys. We found that Ca(2+)-ATPase inhibition caused a potent inhibition of basal renin secretion as well as renin secretion activated by isoproterenol, bumetanide, and by a fall in the renal perfusion pressure. The inhibitory effect of Ca(2+)-ATPase inhibition on renin secretion was reversed within seconds by lowering of the extracellular calcium concentration into the submicromolar range but was not affected by lanthanum, gadolinium, flufenamic acid, or amlodipine. These data suggest that calcium influx triggered by release of calcium from internal stores is a powerful mechanism to inhibit renin secretion from juxtaglomerular cells. The store-triggered calcium influx pathway in juxtaglomerular cells is apparently not sensitive to classic blockers of the capacitative calcium entry pathway.

Extracorporal therapy with AN69 membranes in combination with ACE inhibition causing severe anaphylactoid reactions: still a current problem?

The negatively charged membrane AN69 is known to evoke anaphylactoid reactions both without and with concomitant ACE inhibition. Underlying reasons are mainly the induction of bradykinin release due to the negatively charged membrane and the reduced degradation of bradykinin due to ACE inhibition. This complication has been reported repeatedly, but anaphylactoid reactions still occur in clinical practice. We recently had to treat two patients who suffered anaphylactoid reactions during extracorporal therapy with an AN69 membrane and simultaneous ACE inhibition. The first incident occurred in a patient on hemodialysis, the second was in a patient on continuous venovenous hemofiltration. An anaphylactoid reaction induced by an AN69 membrane during continuous, extracorporal treatment in combination with ACE inhibition has not been reported so far. Our report intends to serve as a reminder that the potentially lethal combination of AN69 membranes with ACE inhibitor treatment should be avoided.

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.

Chloride channel blockers attenuate the inhibition of renin secretion by angiotensin II.

The aim of this study was to assess the relevance of chloride channels to the inhibitory effect of angiotensin II (ANGII) on renin secretion. We thus examined the effects of the chloride channels blockers IAA-94 and niflumic acid, the Na-K-Cl cotransport blocker bumetanide and substitution of isethionate for extracellular chloride on the action of ANGII on renin secretion from isolated perfused rat kidneys. Renin secretion prestimulated by isoproterenol (10 nmol/l) was almost completely blocked by ANGII with a concentration yielding a half-maximal response (EC50) of around 150 nmol/l. In the presence of IAA-94 and niflumic acid the EC50 for ANGII was shifted to about 400 nmol/l. In the presence of bumetanide and isethionate renin secretion responded more sensitively to ANGII and the EC50 for ANGII was below 100 nmol/l. On the assumption that the chloride equilibrium potential in renin-secreting cells is more positive than the membrane potential, our findings would suggest that the inhibitory effect of ANGII is enhanced when chloride entry is blocked and attenuated when chloride efflux is impaired. Activation of chloride channels therefore probably contributes to the inhibitory action of ANGII on renin secretion.

Diuretic treatment and diuretic resistance in heart failure.

Diuretic therapy decreases capillary wedge pressure and improves New York Heart Association (NYHA) functional class both in acute and chronic heart failure. In advanced symptomatic heart failure, loop diuretics are generally necessary to improve symptoms of congestion. Diuretic resistance in the edematous patient has been defined as a clinical state in which diuretic response is diminished or lost before the therapeutic goal of relief from edema has been reached. The major causes of diuretic resistance are functional renal failure (prerenal azotemia), hyponatremia, altered diuretic pharmacokinetics, and sodium retention caused by counterregulatory mechanisms intended to reestablish the effective arterial blood volume. Therapeutic approaches to combat diuretic resistance include restriction of fluid and sodium intake, use of angiotensin-converting-enzyme (ACE) inhibitors, changes in route (oral, intravenous) and timing (single dose, multiple doses, continuous infusion) of diuretic therapy, and use of diuretic combinations.