Intrarenal factors in the control of renin secretion

Intrarenal renin-angiotensin system

Phylogenetically the renin-angiotensin system (RAS) is an ancient regulatory system which has attracted the attention of researchers for about a century. As a result of their efforts, different types of RAS inhibitors are now widely used as therapeutic medicines. The scientific enthusiasm toward RAS remains undiminished and new findings and discoveries are to be expected. Early investigators described the role of RAS in the local control of renal hemodynamics. This correlated well with the morphology of juxtaglomerular apparatus (JGA). Recently developed imaging techniques has allowed for in vivo visualization of cellular functions and the use of molecular biological tools have shed new light on the morphology and physiology of renal RAS, especially in connection with the tubular system. RAS has gained recognition to be more than just an endocrine regulatory system for regulating hemodynamics and water/salt metabolism. RAS is a local tissue and/or cellular regulator with a wide range of effects exerted via various receptors. Local RAS is crucially involved in basic physiological processes like ontogenesis and cell proliferation as well as pathophysiological conditions such as inflammation and tissue fibrosis. These findings may open new frontiers for novel therapeutic approaches. This review focuses only on some specific - less discussed and recently described or hypothesized - morphological and functional aspects of intrarenal RAS, including in vivo imaging of RAS, its effects on juxtaglomerular apparatus and possible cooperative mechanisms among various local renal RAS systems.

The effects of combined angiotensin converting enzyme inhibition and beta-adrenoceptor blockade on plasma renin activity in anaesthetized dogs

1. The effects of beta-adrenoceptor blockade on the changes in plasma renin activity (PRA) following angiotensin enzyme (ACE) inhibition were investigated in pentobarbitone-chloralose anaesthetized dogs. 2. ACE-inhibition, with enalapril (2 mg kg-1), caused a significant reduction in systemic arterial blood pressure (BP) with little or no effect on cardiac function, and a significant elevation of plasma renin activity (PRA). By contrast beta-adrenoceptor blockade with atenolol (1 mg kg-1), caused a similar reduction in BP but in addition, significantly reduced cardiac function and PRA. 3. A combination of enalapril with atenolol, caused a significant reduction in BP, cardiac function and PRA, hence there was no elevation of PRA, as was seen following ACE-inhibition with enalapril alone. 4. The observations with beta-adrenoceptor blockade alone, show that there is an important homeostatic role for the renal sympathetic innervation, mediated by beta-adrenoceptors, in controlling basal renin levels. Furthermore, the renal sympathetic innervation appears to be an important contributor to the renin release caused by an ACE-inhibitor as the additional presence of a beta-adrenoceptor blocking agent will prevent this release. 5. BW B385C (2 mg kg-1), which combines both ACE-inhibition and beta-adrenoceptor blocking properties, also produced reductions in BP and cardiac function similar to those seen with the enalapril/atenolol combination. In addition, for an equivalent degree of ACE-inhibition by BW 385C, to that seen with enalapril alone, the elevation of PRA was attenuated. 6. A combination of ACE-inhibition and beta-adrenoceptor blocking activity in a single entity, such as BW B385C, therefore also produces a reduced renin release when compared with an ACE-inhibitor, such as enalapril. This provides further confirmation of the importance of the renal sympathetic innervation in the renin response to ACE-inhibition, and supports the concept of combining ACEinhibition with beta-adrenoceptor blockade.

Dissociation of the responses of the renin-angiotensin system and sympathetic nervous system to a vasodilator stimulus in congestive heart failure

The ability of neurohumoral reflex control mechanisms to respond to a vasodilator mediated alteration in hemodynamic status was studied. A sodium nitroprusside infusion was administered to 5 normal subjects and 47 patients with severe congestive heart failure resulting in significant decreases in mean arterial pressure and in systemic vascular resistance. As expected in normals the vasodilator stimulus caused a reflex activation in both the renin-angiotensin system and sympathetic nervous system as measured by increased plasma renin activity and plasma norepinephrine, respectively. In the patients with heart failure, plasma renin activity rose similarly in response to nitroprusside (+63% in heart failure, 100% in normals, P = NS) while plasma norepinephrine remained essentially unchanged (+11% in heart failure, 98% in normals, P less than 0.01). These data demonstrate that the neurohumoral dysfunction seen in patients with heart failure is not uniform. In patients with severe congestive heart failure the renin-angiotensin system apparently is activated by mechanisms other than sympathetic nervous stimulation. This intact reflex humoral response may still function in opposition to the beneficial hemodynamic effects produced by direct vasodilators such as nitroprusside.

Renal function and renin secretion after administration of ouabain and ouabain plus furosemide in conscious sheep

The effects of ouabain or ouabain and furosemide on renal function and renin secretion were studied in conscious isovolemic sheep. The sheep received a continuous renal arterial infusion of papaverine, 7 mg/min, throughout the experiment. Ouabain alone (7 X 10(-7) M in the renal plasma) produced significant decreases in glomerular filtration rate (GFR) and renal plasma flow (RPF) but not in renal perfusion pressure. Plasma [K+] rose after ouabain administration. Fractional (FENa) and absolute (UNaV) Na+ excretion were 2.9 +/- 1.0% (mean +/- SE) and 78 +/- 54 muEq/min, respectively, during the papaverine infusion and rose to 19 +/- 5.1% (P less than 0.05) and 528 +/- 116 muEq/min (P less than 0.01) after ouabain administration. Despite the large changes in Na+ reabsorption, renin secretion was not stimulated. During the control period, renin secretion was 281 +/- 131 ng/min and the average renin secretion after ouabain administration was 310 +/- 78 ng/min (not significant). A smaller dose of ouabain (2 X 10(-7) M) infused into the renal artery with 40 mg of furosemide, iv, did not decrease GFR but RPF was suppressed. FENa and UNaV averaged 4.4 +/- 1.6% and 121 +/- 44 muEq/min, respectively, while papaverine was infused into the renal artery and increased to 18 +/- 4.8% (P less than 0.05) and 636 +/- 209 muEq/min (P less than 0.05) after ouabain and furosemide were infused. Renin secretion was 118 +/- 62 ng/min during the control period and averaged 240 +/- 67 ng/min after ouabain plus furosemide. The difference was not statistically significant. Thus ouabain alone does not stimulate renin secretion in the conscious, isovolemic sheep despite a presumed increase in [NaCl] at the macula densa and inhibition of NaCl transport by the loop of Henle. Ouabain also blocks the normal stimulatory effects of furosemide on renin secretion.

Control of renin release: a review of experimental evidence and clinical implications

Present knowledge of the mechanisms regulating release of renin is reviewed with particular emphasis on neural factors. Evidence is given for a direct effect of renal innervation on beta adrenergic receptors in juxtaglomerular cells, and for the involvement of reflex release of renin in conditions such as tilting and acute salt depletion. Participation of neural and nonneural mechanisms of control is also shown to occur in other conditions, such as aortic constriction and hemorrhage. The view is held that neural sympathetic factors might explain some of the renin disturbances found in essential hypertension. First, in patients with high renin hypertension part of the hypertension is renin-dependent, and these pressor levels of renin seem to be neurally induced since they can commonly be suppressed by beta adrenoreceptor blocking agents. Second, the hypothesis is presented that patients with low renin hypertension, at least those who have no volume disturbance, have a blunted sympathetic control of renin release. Therefore a sufficiently precise test of sympathetic activity, and possibly of body fluid volumes, should be associated with renin profiles for a better understanding of the pathophysiology of arterial hypertension and as a better guide to therapeutic management. Indeed, most of the available antihypertensive drugs act on sympathetic activity, body fluid volume or renin, and this multifaceted profile would provide more rational guidelines for treatment.

The Signal Perceived by the Macula Densa during Changes in Renin Release

Renin secretion was studied in acute experiments in dogs during occlusion of the ureter alone (experiment 1), ureteral occlusion with a superimposed intrarenal infusion of papaverine (experiment 2), or ureteral occlusion with a superimposed intravenous ethacrynic acid injection (experiment 3) and following release of the ureteral occlusion (all three experiments). In all three experiments, ureteral occlusion increased renin secretion four- to fivefold. In experiment 2, papaverine, an inhibitor of smooth muscle contractility, was infused intrarenally during the last 20 minutes of ureteral occlusion, but renin secretion was unchanged by the infusion. Renin secretion decreased rapidly and was at the control level 5, 12½, and 27½ minutes after release of the occlusion, but urinary sodium concentration and excretion rate increased markedly during this period. In experiment 3, a superimposed injection of ethacrynic acid failed to alter renin secretion during ureteral occlusion. After release of the ureter, renin secretion remained unchanged for the first 5 minutes. Although renin secretion had decreased 12½ and 27½ minutes after release of the occlusion, it was still elevated twofold above the control level. Again, sodium excretion increased markedly and urinary sodium concentration was high after ureteral release. These findings support the hypothesis that the rate of renin release is inversely related to the rate of sodium transport by the macula densa cells.