Decreased Renin Release Research Articles

The role of Ca 2+ in the control of renin release from dog renal cortical slices

Using a continuous superfusion system of dog renal cortical slices, we studied the role of Ca 2+ in the intracellular control mechanism for renin release. The calcium ionophore A23187 (10 μM) produced a significant decrease in renin release. This effect was abolished in the absence of extracellular Ca 2+. Moreover, pretreatment with the calmodulin inhibitor W-7 ( N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide, 20 μM) completely prevented the inhibitory effect of A23187 (10 μM). The β-adrenoceptor agonist isoproterenol (1, 10 and 100 μM) produced a concentration-dependent increase in renin release. Pretreatment with W-7 (20 μM) potentiated the stimulation of renin release induced by isoproterenol (1 μM). These results suggest that A23187-induced inhibition of renin release is mediated by the activation of calmodulin via an increase in intracellular Ca 2+ and β-adrenoceptor-stimulated renin release is modulated by intracellular Ca 2+ mobilization.

Influence of the M235T polymorphism of human angiotensinogen (AGT) on plasma AGT and renin concentrations after ethinylestradiol administration.

The T235 allele of the angiotensinogen (AGT) gene is associated with plasma AGT concentration and pregnancy-induced hypertension. The aim of this study was to compare changes in the circulating renin-angiotensin system after short-term (2 days) and repeated (7 days) administration of 50 microg ethinylestradiol (EE) in homozygous normotensive men (TT and MM). After repeated EE administration, renin stimulation was induced by a single oral dose of 40 mg furosemide, followed by 50 mg captopril, 12 h later. The short-term administration of EE did not induce a significant differential genotype-dependent increase in AGT concentration. In the 7-day study, TT subjects had higher peak plasma AGT concentrations than MM subjects. The more pronounced AGT increase in TT subjects resulted in similar plasma renin activity at a lower plasma active renin concentration, with a higher plasma renin activity/active renin ratio. The difference between genotypes in renin secretion resulted in readjustment of angiotensins production. In conclusion, the T235 allele of the AGT gene is associated with greater stimulation of AGT secretion in plasma after EE administration. In the short-term, complete readjustment of the circulating renin-angiotensin system occurs, through a decrease in renin release, which blunts the effects of the increase in AGT concentration.

Intravertebral ANG II effects on plasma renin and Na excretion in dogs at constant renal artery pressure.

Studies were performed to determine whether intravertebral angiotensin II infusion (iva ANG II) decreases renin release by increasing renal perfusion pressure (RPP) and to investigate possible effects of iva ANG II on renal function. RPP was electronically servocontrolled in 12 conscious dogs equipped with chronic vascular catheters and a suprarenal aortic balloon constrictor while iva ANG II was infused bilaterally for 60 min at 0.33 ng.kg-1.min-1. Without servocontrol, iva ANG II increased mean arterial pressure (MAP) from 101 +/- 4 to 106 +/- 5 mmHg, urine flow (V) from 0.36 +/- 0.03 to 0.45 +/- 0.04 ml/min, and sodium excretion (UNaV) from 36.2 +/- 7.0 to 62.7 +/- 6.6 mumol/min. Plasma renin activity (PRA) decreased from 6.9 +/- 0.7 to 5.0 +/- 0.6 ng ANG I.ml-1.3 h-1. With servocontrol, iva ANG II increased MAP from 102 +/- 4 to 109 +/- 5 mmHg while RPP remained constant with a variation of less than +/- 1 mmHg. PRA did not change significantly (5.9 +/- 0.3 to 7.0 +/- 0.7 ng ANG I.ml-1.3 h-1). V decreased from 0.33 +/- 0.02 to 0.26 +/- 0.01 ml/min, and UNaV decreased from 49.0 +/- 5.7 to 29.7 +/- 4.4 mumol/min. The data provide evidence that iva ANG II decreases renin release by increasing RPP and stimulating the renal baroreceptor and/or the macula densa mechanisms. In addition, at constant RPP, ANG II exerts a central action to decrease UNaV.

Tumor necrosis factor-alpha and interleukin-1 beta inhibit the synthesis and release of renin from human decidual cells.

Cytokines modulate hormone expression in many cell types, including the expression of renin in juxtaglomerular cells. However, the effect of cytokines on the expression of renin from extrarenal cells is unknown. In this paper, we have examined whether tumor necrosis factor-alpha (TNF alpha) and interleukin-1 beta (IL-1 beta) modulate the release of renin from human decidual cells. Continuous exposure of primary decidual cell cultures from term pregnancies to TNF alpha and IL-1 beta caused dose-dependent inhibition of renin release. The maximal inhibitions by TNF alpha and IL-1 beta were 75.5% and 55.2%, respectively, and the half-maximal effective doses of TNF alpha and IL-1 beta were 30 and 1.1 pmol/L, respectively. The decrease in renin release by the cytokines was statistically significant on days 2-5 (P > 0.001 at each time) and was accompanied by inhibition of renin synthesis and renin messenger ribonucleic acid levels. The renin messenger ribonucleic acid levels in cells exposed for 4 days to TNF alpha (50 ng/mL) or IL-1 beta (50 pg/mL) were 58.0% and 37.7% less than those in control cells, respectively. As decidual macrophages express TNF alpha and IL-1 beta, the results of this study strongly suggest a paracrine role for cytokines in the regulation of decidual renin expression. The effect of these cytokines on renin expression in decidual cells is opposite that in juxtaglomerular cells.

Decreased renin release and constant kallikrein secretion after injection of L-NAME in isolated perfused rat kidney

A possible role of the endothelial L-arginine/NO pathway in the control of renal hemodynamics, renin release and kallikrein secretion was studied in an isolated rat kidney model perfused in a closed-circuit. N G-nitro-L-arginine methyl ester (L-NAME, 1–50 μM), an inhibitor of nitric oxide biosynthesis, caused a dose-dependent increase in perfusion pressure (PP) and a dose-dependent decrease in renal perfusate flow. Renin release was inhibited independently of a rise in PP. L-NAME did not change the urinary kallikrein secretion. These results confirm the intervention of the L-arginine/NO pathway in the vasodilation of this isolated perfused kidney model and demonstrate the inhibitory effect of L-NAME on renin release. They suggest that nitric oxide synthesis plays a role in stimulating renin release and is not involved in the regulation of urinary kallikrein secretion.

Role of intrarenal angiotensin II in mediating renal response to volume expansion

Natriuresis induced by extracellular volume expansion (ECVE) is accompanied by a decrease in renin release and by an increase of renal interstitial hydrostatic pressure (RIHP). This study was undertaken to examine, in anesthetized dogs, the relative role of intrarenal angiotensin II (ANG II) changes in mediating natriuresis, diuresis, and increases in RIHP induced by two different levels of volume expansion (1.5 and 5% body wt in 45 min) with isotonic saline. Intrarenal ANG II levels were maintained in the right kidney throughout the experiment by simultaneously infusing captopril (0.8 micrograms.kg-1.min-1) and ANG II (1 ng.kg-1.min-1) into the right renal artery. In response to 5% ECVE, increases in RIHP, natriuresis, and diuresis were inhibited in the right kidney by 55, 40, and 47% respectively, when compared with the left kidney. Significant increases occurred in plasma atrial natriuretic peptide (ANP) levels during 5% ECVE. Maintenance of constant intrarenal ANG II levels during 1.5% ECVE completely abolished the increment of RIHP and diuresis and inhibited the natriuretic response by 80% in the right kidney when compared with the left kidney. Plasma ANP levels did not change during the 1.5% ECVE. No differences between kidneys were found when the intrarenal effects of ANG II were blocked with saralasin before saline loading. These results suggest that increases in RIHP, natriuresis, and diuresis during ECVE are partially mediated by decreases in intrarenal ANG II levels. Furthermore, these results indicate that the role of intrarenal ANG II levels in mediating the renal response to ECVE is more important when plasma ANP levels do not change than when they are increased.

Antihypertensive Action of Beta-Adrenoceptor Blocking Drugs: The Role of Intrarenal Mechanisms

The different modes of antihypertensive action of beta-adrenoceptor blocking drugs are reviewed with a special emphasis on renal mechanisms of action. Beta-adrenoceptors occur in the kidney at the level of the juxtaglomerular and tubular cells. Direct intrarenal effects of beta-blockers include the decrease in renin release and the enhanced tubular excretion of fluid and electrolytes. Beta-blockers also influence renal function in a more indirect way by altering the activity of different extrarenally formed hormones with an effect on fluid and electrolyte excretion, such as angiotensin II, atrial natriuretic factors and prostaglandins. Several beta-blockers have an additional effect on the renal vasculature. One of these, tertatolol, is a renal vasodilator beta-blocker in different species. The possible mechanisms of renal vasodilatation of tertatolol are reviewed. It is concluded that the shift of the renal pressure-diuresis curve is an essential feature in the mode of long-term antihypertensive action of beta-blockers.

Calmodulin-independent stimulation of renin release by exposure of rat kidney cortical slices to calcium.

1. Effects of calcium-interacting agents on the calcium-induced stimulation of renin release from kidney cortical slices pretreated with calcium-free medium were examined. 2. The exposure of calcium to the slices pretreated with calcium-free medium enhanced the release of renin, followed by a decreased response in the release. The amount of lactate dehydrogenase released from the slices did not correlate with that of renin. 3. High potassium depolarization significantly potentiated the decreased response of renin release, with no influence on the stimulation of the release by exposure to calcium. 4. The decrease in renin release was attenuated by calcium-interacting agents, such as nifedipine, TMB-8 and W-7, but these agents were without effect on the stimulation of the release by exposure to calcium. 5. Thus, the calcium-calmodulin system apparently is not involved in the increased response of renin release following exposure to calcium.

Effects of chlorpropamide on loop of Henle function and plasma renin

We have suggested that inhibition of renin release by sodium chloride is related to increased absorptive solute transport in the loop of Henle. In the rat, we have shown that reduced chloride transport in the loop is associated with increased renin release. Based on indirect evidence, it has been suggested that chlorpropamide (CPMD) increases loop solute transport. This study directly evaluates the effect of CPMD on loop chloride transport and plasma renin activity (PRA) in the male Sprague-Dawley rat. Loop of Henle chloride reabsorption (measured by recollection micropuncture) and PRA were determined before and after acute infusion of CPMD (N = 8) or vehicle (N = 8). Although delivery to the loop was not significantly changed, CPMD increased (P less than 0.05) absolute loop chloride reabsorption from 1798 pEq/min +/- 200 SE to 2453 pEq/min +/- 206 SE. PRA was decreased (P less than 0.01) from 9.2 ng/ml/hr +/- 1.0 SE to 5.6 ng/ml/hr +/- 0.8 SE following CPMD infusion. Comparable vehicle infusion did not alter loop chloride reabsorption or PRA. Arterial pressure, and whole kidney and single nephron glomerular filtration rates were unchanged following infusion of CPMD or vehicle. These results demonstrate that an increase in loop chloride reabsorption is associated with a decrease in renin release. This observation is consistent with the hypothesis that renin release is inversely related to the magnitude of chloride transport in the thick ascending limb of the loop of Henle.

EFFECTS OF AROTINOLOL, AN α‐ AND β‐ADRENOCEPTOR ANTAGONIST, ON RENIN RELEASE FROM RAT KIDNEY CORTICAL SLICES

The effects of arotinolol on changes in renin release in rat kidney cortical slices in response to isoproterenol (IP) or norepinephrine (NE), were studied in comparison with those of AC-623, a main metabolite of arotinolol, and other typical adrenoceptor antagonists. Arotinolol, at concentrations of 10(-8) to 10(-4) mol/l, inhibited the increasing effect of 10(-6) mol/l IP on renin release, in a concentration-dependent manner. Similar results were observed with AC-623, propranolol or labetalol, although the inhibitory potencies of these agents were considerably lower than that of arotinolol. The blocking effect of arotinolol on the 10(-5) mol/l NE-induced decrease in renin release was much less potent than seen with other alpha-adrenoceptor blocking agents such as prazosin, phenoxybenzamine and labetalol. These data suggest that the potent blocking effects of arotinolol and its metabolite on the increased renin release in response to beta-adrenoceptor stimulation may contribute to the antihypertensive effect of this agent.

Possible role of adenosine in the macula densa mechanism of renin release in rabbits.

This study was designed to examine: (a) the effects of adenosine and its analogues on renin release in the absence of tubules, glomeruli, and macula densa, and (b) whether adenosine may be involved in a macula densa-mediated renin release mechanism. Rabbit afferent arterioles (Af) alone and afferent arterioles with macula densa attached (Af + MD) were microdissected and incubated for two consecutive 30-min periods. Hourly renin release rate from a single arteriole (or an arteriole with macula densa) was calculated and expressed as ng AI X h-1 X Af-1 (or Af + MD-1)/h (where AI is angiotensin I). Basal renin release rate from Af was 0.69 +/- 0.09 ng AI X h-1 X Af-1/h (means +/- SEM, n = 16) and remained stable for 60 min. Basal renin release rate from Af + MD was 0.20 +/- 0.04 ng AI X h-1 X Af + MD-1/h (n = 6), which was significantly lower (P less than 0.0025) than that from Af. When adenosine (0.1 microM) was added to Af, renin release decreased from 0.72 +/- 0.16 to 0.24 +/- 0.04 ng AI X h-1 X Af-1/h (P less than 0.025; n = 9). However, when adenosine was added to Af + MD, no significant change in renin release was observed. N6-cyclohexyl adenosine (an A1 adenosine receptor agonist) at 0.1 microM decreased renin release from Af from 0.69 +/- 0.14 to 0.39 +/- 0.12 ng AI X h-1 X Af-1/h (n = 5, P less than 0.05). However, 5'-N-ethylcarboxamide adenosine (an A2 adenosine receptor agonist) either at 0.1 microM or at 10 microM had no effect. Theophylline, at a concentration (10 microM) that does not block phosphodiesterase but does block adenosine receptors, increased renin release from Af + MD from 0.21 +/- 0.03 to 0.46 +/- 0.08 ng AI X h-1 X Af + MD-1/h (P less than 0.05; n = 8). The results are consistent with the hypotheses that adenosine decreases renin release via the activation of A1 adenosine receptors, and that adenosine may be an inhibitory signal from the macula densa to juxtaglomerular cells.

Radioisotopic renal function studies in essential hypertension.

Renal and intrarenal distribution of flow together with an index of cardiac output is measurable using a single injection of 123I-o-iodohippurate. The argument is presented that in the treatment of essential hypertension one should avoid using drugs that reduce renal blood flow, a property of chronic therapy with most beta-blockers or diuretics. Reduction of renal blood flow is a stimulus to salt retention and renin release which tend to maintain hypertension. Drugs which increase renal blood flow, thereby augmenting the renal loss of salt and decreasing renin release, tend to reduce hypertension in the long term.

Adenosine-induced decrease in renin release: dissociation from hemodynamic effects

Adenosine has been reported to produce a biphasic renal blood flow (RBF) response (vasoconstriction followed by a return of flow to control level) and a decrease in glomerular filtration rate (GFR) when infused into the kidney. Intrarenal adenosine infusion also leads to a decrease in renin release. By altering the hemodynamic response to adenosine, we sought to determine whether the decrease in renin release depends on vascular or filtration-induced events. In nine dogs with nonfiltering kidneys, adenosine infusion (3 X 10(-7) mol/min) resulted in a biphasic RBF response and an inhibition of renin release (309 +/- 53 vs. 71 +/- 26 ng ANG I/min). In 11 dogs treated with verapamil (10 micrograms X kg-1 X min-1) no vasoconstriction or decrease in GFR occurred; however, renin release was inhibited by adenosine (1,300 +/- 159 vs. 534 +/- 225 ng ANG I/min). In a third group of nine dogs whose ureteral pressure was raised to 80 cmH2O, adenosine infusion produced a sustained vasoconstriction and an inhibition of renin release (3,086 +/- 1,144 vs. 328 +/- 130 ng ANG I/min). These experiments, in which the renin release effects of adenosine are dissociated from the hemodynamic effects, lead us to conclude that the inhibition of renin release produced by adenosine does not depend either on the vascular or filtration-induced effects of adenosine.

Modulation of noradrenaline release in the pithed rabbit: a role for angiotensin II.

This study in the pithed rabbit with electrically stimulated sympathetic outflow (spinal region, T-8; 3 Hz) was conducted to determine the contribution of the renin-angiotensin system to noradrenaline release in vivo. The rate of noradrenaline release (spillover) into the plasma was determined from the endogenous plasma noradrenaline level and the simultaneously determined noradrenaline plasma clearance. In the pithed rabbit, infusion of angiotensin II (0.1 microgram/kg/min i.v.) failed to increase the noradrenaline release rate and only slightly increased blood pressure. On the other hand, the angiotensin-converting enzyme inhibitor captopril (1 mg/kg i.v.) decreased both blood pressure and the noradrenaline release rate. Bilateral nephrectomy was performed to reduce endogenous angiotensin II formation; and in this case, infusion of angiotensin II markedly increased the noradrenaline release rate and blood pressure, whereas captopril had no effect on either parameter. These results suggest that angiotensin II modulates noradrenaline release in vivo through activation of facilitatory prejunctional angiotensin II receptors, and that in the pithed rabbit these receptors are probably maximally activated by endogenously synthesized angiotensin II. The actions of angiotensin II on noradrenaline release open the possibility that increases in blood pressure in the pithed rabbit--by decreasing renin release via intrarenal baroreceptors and hence decreasing angiotensin II formation--may lead to decreased noradrenaline release. This was investigated using phenylephrine (6 micrograms/kg/min i.v.), a selective alpha 1-adrenoceptor agonist, and adrenaline (1 microgram/kg/min i.v.), and alpha 1/alpha 2-agonist. Both drugs increased blood pressure and decreased the noradrenaline release rate. After bilateral nephrectomy, the inhibitory effect of phenylephrine on noradrenaline release was abolished, whereas that of adrenaline was maintained.(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonistic effect of theophylline on the adenosine-induced decreased in renin release.

The action of theophylline on the adenosine-induced decrease in renin release was studied in anesthetized dogs. Adenosine inhibited renin release, decreased GFR and fractional sodium excretion, and decreased the concentration of angiotensin II in the renal lymph. Theophylline (5 mumol/min intrarenally) had no significant effect on GFR or RBF yet produced a significant increase in the release of renin and the fractional excretion of sodium. The intrarenal infusion of adenosine (3 X 10(-7) mol/min) during theophylline infusion produced no effect on GFR or RBF, but fractional sodium excretion and renin release were significantly decreased. Adenosine was infused at a lower dose (3 X 10(-8) mol/min) during theophylline (5 X 10(-6) mol/min) infusion in a second group of dogs. With the exception of fractional sodium excretion, all effects of adenosine were effectively antagonized by theophylline. Theophylline at 5 X 10(-6) mol/min, which stimulates renin release and effectively antagonizes the renal effects of adenosine, had no detectable effect on cAMP measured in renal cortex. Furthermore, no change in cortical cAMP was observed until theophylline was increased 50-fold over the dose effective in antagonizing adenosine. These findings demonstrate that theophylline, at concentrations having no effect on cortical cAMP, antagonizes the effect of adenosine on renin release. The results are also consistent with the view that theophylline stimulates renin release by a mechanism other than its action on cAMP.

Mechanism of suppressed renin-angiotensin system in spontaneously hypertensive rat (SHR).

Plasma renin concentration (PRC) of SHR decreased with the elevation of blood pressure. In addition in vitro study of basal renin release from kidney slices obtained from SHR produced similar results. However kidney renin activity and total renin content in kidney of SHR were not less than those of Wistar rats. These results indicate that the decreased renin release from kidney might be responsible for the suppression of renin-angiotensin system of SHR after the development of hypertension. Administration of fusaric acid, a dopamine-beta-hydroxylase inhibitor markedly decreased urinary excretion of catecholamine and prostaglandin E in both SHR and Wistar rat and abolished the development of hypertension in SHR. PRC of SHR which became normotensive by fusaric acid administration (normotensive SHR), increased up to the level of control Wistar rats. Therefore suppressed renin-angiotensin system was not observed in normotensive SHR. Nevertheless basal renin release from kidney slices obtained from normotensive SHR was still less than that from control Wistar rats. As urinary catecholamine and prostaglandin E excretion were decreased by fusaric acid administration, these well-known renin stimulators could not be responsible for the reversal of suppressed renin-angiotensin system in normotensive SHR. Rather decreased arterial blood pressure in normotensive SHR lowered renal perfusion pressure, and led to the stimulation of renin release up to the levels of control Wistar rats by the activation of renal arteriolar baroreceptors.

Interaction between perfusion pressure and sympathetic nerves in renin release by carotid baroreflex in conscious dogs.

1. The effect of bilateral carotid occlusion on carotid sinus pressure, systemic blood pressure, heart rate, renal blood flow, renal-venous and arterial plasma renin activity was studied in twelve trained conscious foxhounds on a normal sodium diet (4.7 mmol/kg per day). 2. When renal perfusion pressure was allowed to rise with systemic pressure by 51.0 +/- 6.3 mmHg during carotid occlusion, renin release decreased by 72.3 +/- 22.2 ng/min (78% of control; P less than 0.05) while renal blood flow remained at its resting level of 232.7 +/- 20.1 ml/min (n = 8 dogs). 3. When renal perfusion pressure was maintained constant at 93.0 +/- 3.6 mmHg during carotid occlusion (suprarenal aortic cuff), renin release increased by 154.6 +/- 60.4 ng/min (73% of control; P less than 0.05), again there was no significant change of renal blood flow (n = 7 dogs). 4. After beta-adrenergic blockade carotid occlusion increased systemic blood pressure by 47.7 +/- 7.8 mmHg, decreased renin release by 34.6 +/- 9.9 ng/min (67% of control; P less than 0.05) and had no effect on renal blood flow (n = 4 dogs). 5. When renal perfusion pressure was controlled at its resting level, no significant change of renin release and renal blood flow was observed during carotid occlusion in the surgically denervated kidney (n = 3 dogs) or in the intact kidney after beta-adrenergic blockade (n = 4 dogs). 6. It is concluded that a reduction of carotid sinus pressure in the conscious dog increases renin release by a direct beta-adrenergic stimulation without exerting vasomotor effects provided renal perfusion pressure is maintained at control level. The vascular receptor mechanism can effectively counteract the stimulating influence of the renal sympathetic nerves when perfusion pressure is allowed to rise.

Renin-Angiotensin System in Phlorhizin Compared with Alloxan Diabetes in the Rat

In alloxan-treated diabetic rats, plasma renin activity (PRA) is decreased. One possible mechanism that may explain the decreased PRA is an increased delivery of sodium to the macula densa produced by the glucose osmotic diuresis, resulting in decreased renin release. To evaluate this possible mechanism, rats with phlorhizin diabetes, which produces a glucose osmotic diuresis without hyperglycemia, were studied and compared with rats with alloxan-induced diabetes. Whereas phlorhizin-treated rats had low blood glucose and alloxan-treated rats had elevated glucose, the glucose osmotic diuresis was similar in the two groups. PRA and plasma renin concentration (PRC) were significantly increased in the phlorhizin group. In the alloxan group, PRA was decreased and angiotensin II sensitivity increased, both significantly. Plasma renin substrate (PRS) remained adequate in each group. These results suggest that the decreased PRA in alloxan-induced diabetes is due neither to factors associated with the glucose osmotic diuresis including changes in renal tubular sodium not to decreased PRS.

Effect of somatostatin on plasma renin activity.

Intravenous infusion of somatostatin in mongrel dogs caused a significant decrease in the peripheral plasma renin activity (PRA) enhanced by pentobarbital sodium anesthesia or furosemide treatment. However, the inhibitory activity vanished within 10 min after termination of somatostatin infusion. Intrarenal arterial infusion of somatostatin decreased furosemide-enhanced PRA in renal vein by 24.0%, 16.6% and 8.6% in dose of 0.1, 0.5 and 1.0 microgram, respectively. On the other hand, high doses of the peptide (50-200 microgram) failed to decrease. The changes in PRA occurred in the absence of any alteration in blood pressure during the intravenous infusion under furosemide treatment. In an in vitro study, the addition of somatostatin in doses of 0.01 and 0.05 microgram suppressed the renin release in dog renal cortical cell suspension by 74.3% and 53.6%, respectively. Therefore, in both intrarenal arterial infusion and the cell suspension system, somatostatin was increasingly effective in decreasing renin release towards the lower end of the dose range tested. These results suggest that the effect of somatostatin on hyperreninemia may involve an inhibition of renin release at the cell level in the kidney.

The Effects of Calcium and Calcium-Ionophores (X 537 a and a 23187) on Renin Release in the Isolated Perfused Rat Kidney

1. Calcium depletion from the medium of the isolated perfused kidney reduced renin release and renal perfusate flow. 2. Reintroduction of calcium increased renin release and perfusate flow. 3. The ionophore X 537 A (0.1-4 micromol/1) increased renin release both in the presence and absence of calcium in the medium. 4. The ionophore A 23187 (1-10 nmol/1) increased as well as decreased renin release. There was a positive correlation between direction of this effect and renal perfusate flow. 5. The results are compatible with the view that the effects of calcium and ionophores on renin release are the sum of direct and indirect effects of these agents, the predominant indirect effect being the modification of vascular tone.