Effect of ouabain on renin secretion in anaesthetized dogs

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Vanadate-induced inhibition of renin secretion is unrelated to inhibition Na,K-ATPase activity

There is evidence that three inhibitors of Na,K-ATPase activity--ouabain, K-free extracellular fluid, and vanadate--inhibit renin secretion by increasing Ca2+ concentration in juxtaglomerular cells, but in the case of vanadate, it is uncertain whether the increase in Ca2+ is due to a decrease in Ca2+ efflux (inhibition of Ca-ATPase activity, or inhibition of Na,K-ATPase activity, followed by an increase in intracellular Na+ and a decrease in Na-Ca exchange) or to an increase in Ca2+ influx through potential operated Ca channels (inhibition of electrogenic Na,K transport, followed by membrane depolarization and activation of Ca channels). In the present experiments, the rat renal cortical slice preparation was used to compare and contrast the effects of ouabain, of K-free fluid, and of vanadate on renin secretion, in the absence and presence of methoxyverapamil, a Ca channel blocker. Basal renin secretory rate averaged 7.7 +/- 0.3 GU/g/60 min, and secretory rate was reduced to nearly zero by 1 mM ouabain, by K-free fluid, by 0.5 mM vanadate, and by K-depolarization (increasing extracellular K+ to 60 mM). Although 0.5 microM methoxyverapamil completely blocked the inhibitory effect of K-depolarization, it failed to antagonize the inhibitory effects of ouabain, of K-free fluid, and of vanadate. A concentration of methoxyverapamil two hundred times higher (100 microM) completely blocked the inhibitory effects of vanadate, but still failed to antagonize the effects of ouabain and of K-free fluid. Collectively, these observations demonstrate that vanadate-induced inhibition of renin secretion cannot be attributed entirely to Na,K-ATPase inhibition, since in the presence of methoxyverapamil, the effect of vanadate differed from the effects of either ouabain (a specific Na,K-ATPase inhibitor) or K-free fluid. Moreover, it cannot be attributed entirely to a depolarization-induced influx of Ca2+ through potential-operated Ca channels, since methoxyverapamil antagonized K-depolarization-induced inhibition of renin secretion much more effectively than it antagonized vanadate-induced inhibition.

Secretion control for active and inactive renin: effect of ouabain on release from rabbit kidney cortex slices

Release of active and inactive renin by rabbit kidney cortex slices was investigated. Inactive renin was estimated as the increase in renin activity after acidification (pH 2.8) of slice supernatant solutions. For kidney slices incubated in complete Krebs bicarbonate buffer, the Na-K-ATPase inhibitor ouabain (100 microM) reduced the secretion of both active (-19.2%) and inactive (-78.9%) forms of renin. In low Na buffers ([Na+] = 23 mM) active renin release was increased and inactive renin was suppressed. Both of these changes were abolished by addition of ouabain (100 microM). The reduction in inactive renin secretion produced by ouabain in complete Krebs buffer did not occur in low [Na+] buffers. In zero Ca2+ buffers containing EGTA (5 mM), secretion of both active and inactive renin was increased but these changes were abolished by addition of ouabain (100 microM). Incubating kidney slices in low Na+, zero Ca2+ media revealed differences between the secretion control mechanisms for the two forms of renin. The separate stimulatory effects of low Na+ and low Ca2+ were not additive for the release of active renin and inclusion of ouabain resulted in similar secretion rates to those under control conditions. For inactive renin secretion, in the absence of Ca2+ release mechanisms still respond to reduction in Na+ with decreased secretion. Conversely, in low Na+ buffers, removal of Ca2+ still promotes inactive renin secretion. These changes were abolished by the addition of ouabain (100 microM). Slices did not change in weight during incubation in media which did not contain ouabain. Addition of this inhibitor to control buffers and low Na buffers did result in an increase in weight. This correlated with the presence of Ca2+ in the buffer and did not appear to be related to [Na+]. These studies again show that the mechanisms regulating the secretion of active and inactive renin are not identical and support the hypothesis that Na+ have differing roles to play in the regulation of these two forms of renin.

Acute effects of digoxin on total systemic vascular resistance in congestive heart failure due to dilated cardiomyopathy: a hemodynamic-hormonal study

The effects of the digitalis glycosides on systemic vascular resistance (SVR) in patients with congestive heart failure (CHF) are controversial. Most investigators report a reduction in total SVR, an action that has been attributed primarily to withdrawal of elevated sympathetic tone. Direct proof of this hypothesis is lacking, however, and the roles played by the renin-angiotensin-aldosterone and vasopressin systems have not been fully explored. Moreover, in several studies of patients with CHF, SVR did not decrease after the administration of digitalis. To clarify these issues, the hemodynamic and hormonal effects of digoxin were correlated in 11 normotensive men in sinus rhythm with CHF due to dilated cardiomyopathy. Patients were evaluated at rest and during submaximal exercise before and 6 hours after the intravenous infusion of 1.0 mg of digoxin (mean serum concentration 1.7 ng/ml). With digoxin therapy, heart rate, pulmonary wedge pressure and right atrial pressure declined and cardiac output increased. Although vasopressin was unchanged, both plasma norepinephrine concentrations and plasma renin activity decreased, the reduction in norepinephrine correlating with the increase in cardiac output. Despite these hemodynamic and hormonal effects, there was no change in total SVR at rest or during exercise. It is concluded that the improvement in cardiac function with digoxin in this patient group was a result of the inotropic properties of the drug, without an associated reduction in impedance. The failure of total SVR to decrease despite decreases in plasma norepinephrine levels and plasma renin activity might be explained by concomitant digitalis-induced vasoconstriction, impaired ability of arterioles to dilate in CHF, or offsetting alterations in other vasoactive hormone systems.

Stimulation of renin secretion by 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8)

The intracellular calcium antagonist 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) prevents the release of Ca2+ from cell storage sites. The effect of this compound on renin secretion from rat renal cortical slices in vitro was investigated. TMB-8 was a potent stimulant of renin secretion within the concentration range 10(-5)M to 5 X 10(-4)M with an optimum concentration of 2 X 10(-4)M. TMB-8 overcame the inhibition of renin secretion by angiotensin II, ouabain, 60 mM KCl and A23187. The results add to the existing evidence that Ca2+ is a common inhibitory messenger for a number of compounds which affect renin release and suggest a role for intracellular calcium stores in the regulation of juxtaglomerular cell Ca2+ levels.

Quinacrine antagonizes the effects of Na,K-ATPase inhibitors on renal prostaglandin E2 release but not their effects on renin secretion

Previous results have demonstrated that two inhibitors of Na-and-K-activated adenosine triphosphatase (ouabain, vanadate) lead to stimulated prostaglandin E2 release and to inhibited renin secretion in the rat renal cortical slice preparation. It was speculated that stimulation of phospholipase A2 activity accounted for the effect on prostaglandin E2 release. We used the same preparation in the present experiments, and showed that another inhibitor of Na-and-K-activated adenosine triphosphatase (K-free incubation medium) stimulates prostaglandin E2 release and inhibits renin secretion. Quinacrine antagonized the stimulatory effects of ouabain, vanadate, and K-free medium on prostaglandin E2 release (consistent with phospholipase A2 involvement), but did not antagonize their inhibitory effects on renin secretion. Collectively, these observations lend further weight to the argument against a mediatory role of prostaglandin synthesis in the renin secretory process.

Ouabain inhibits renin release by a direct renal haemodynamic effect

The effect of intrarenal infusion of ouabain (90 micrograms/kg) on renin release was examined in the anaesthetized dog. Ouabain reduced cortical Na-K-ATPase activity to 23% and outer medullary activity to 18% of the control level. During renal arterial constriction to a perfusion pressure below the autoregulatory range, renin release rose from 1.2 +/- 0.4 to 47.4 +/- 6.9 micrograms/min (P less than 0.001). This response was abolished by ouabain. When superimposed on renal arterial constriction, beta-adrenergic stimulation enhanced renin release from 25.6 +/- 10.7 to 56.9 +/- 9.5 micrograms/min (P = 0.02) at a urinary sodium excretion of 2 +/- 1 mumol/min. After ouabain, the corresponding increment substantially decreased since release rose from 5.6 +/- 2.0 to 19.9 +/- 5.3 micrograms/min only (P = 0.02), at a urinary sodium excretion of 140 +/- 67 mumol/min. When glomerular filtration was reduced to zero by ureteral occlusion in one series, renin release increased to 22.6 +/- 5.1 but was reduced (P less than 0.05) by ouabain to 13.5 +/- 5.5 micrograms/min and superimposed isoproterenol had no effect. According to these observations, ouabain inhibits renin release by a direct effect on the afferent arteriole through constriction of the autoregulating renin-secreting segment.

Calcium ion dependence of myogenic renal plasma flow autoregulation: evidence from the isolated perfused rat kidney

1. The independent roles of Ca2+ and glomerular filtration in the control of renal plasma flow autoregulation were examined in the isolated perfused rat kidney. Control kidneys autoregulated plasma flow between perfusion pressures of 120 and 160 mmHg. 2. Complete ischaemia, induced by clamping of the renal artery cannula for 1 h induced relative vasoconstriction and a loss of autoregulatory capacity. 3. The addition of either nifedipine (10(-7) M) or verapamil (10(-4) M) to the perfusion medium produced vasodilation and a loss of autoregulation. 4. Kidneys that were rendered non-filtering by raising the perfusate albumin concentration from 65 to 100 g/l appeared to shift autoregulatory capacity to a higher pressure range, whereas raising it to 150 g/l reduced autoregulation at all pressures studied. 5. The addition of ouabain (10(-3) M) restored autoregulation to the lower pressure range in non-filtering kidneys perfused with an albumin concentration of 100 g/l. Ouabain-treated non-filtering kidneys displayed a loss of autoregulation when perfused either with nifedipine or with reduced perfusate Ca2+. 6. Plasma flow was reduced in isolated kidneys perfused at pressures of 100 or 150 mmHg when ionized Ca2+ in the medium was raised from 0 to 1.8 mM. However, no decrement in flow was observed in kidneys perfused at 50 mmHg when ionized Ca2+ in the perfusate was raised to the same level. Thus the vasoconstrictive effect of raised ionized Ca2+ was pressure-dependent. 7. We conclude that renal plasma flow autoregulation occurs in the isolated kidney in the absence of glomerular filtration and is at least in part a myogenic phenomenon. Myogenic control of renal plasma flow autoregulation is regulated by smooth muscle permeability to Ca2+. Changes in smooth muscle Ca2+ permeability appear to be pressure-regulated.

Stimulation of renin release by hyperoncotic perfusion of the isolated rat kidney

Renin release was measured in the isolated rat kidney perfused with a recirculating artificial medium containing bovine serum albumin at 6.7 g per 100 ml of 11 g per 100 ml. At the higher concentration of albumin, glomerular filtration ceased and the rate of renin release over 70 minutes of perfusion was increased 6-fold. The addition of ouabain to the perfusate containing 11 g per 100 ml inhibited the release of renin, suggesting that inhibition of Na-K-ATPase or the related changes in cellular volume or composition prevented renin release. Lowering the osmolality of the perfusate by reducing the concentration of sodium chloride also prevented the increase in renin secretion produced by perfusion with 11 g per 100 ml albumin. Increasing the osmolality of the perfusate with mannitol restored the augmented renin release. These results are consistent with the hypothesis that alterations in the volume of certain cells, perhaps in the juxtaglomerular apparatus itself, can control renin release.

Effect of D-600 on inhibition of in vitro renin release in the rat by high extracellular potassium and angiotensin II

1. Renin was secreted by rat renal cortical slices incubated at 37 degrees C in a physiological saline solution. 2. Secretion was nearly abolished by incubation in a medium containing 60 mM-K. Secretion could be restored to the control level by the addition of 5 X 10(-7) M-D-600 (methoxy verapamil) to 60 mM-K medium. 3. Angiotensin II inhibited renin secretion in a concentration-dependent manner. Concentrations of D-600 ranging from 1 to 3 X 10(-6) M (two to sixfold higher than required to block the inhibitory effect of high K) failed to antagonize the inhibitory effect of angiotensin II. 4. Ca is required for the inhibitory effect of angiotensin II, however, as Ca-depletion (incubation of slices in a medium with Na2EGTA and no added CaCl2)( progressively decreased and finally abolished any inhibitory effect. 5. These results confirm and extend previous observations suggesting that Ca plays an inhibitory coupling role in the control of renin secretion from the juxtaglomerular apparatus. Moreover, they suggest that although voltage-sensitive channels exist on juxtaglomerular cells, angiotensin II activates an independent pathway for Ca mobilization.

Separate and combined effects of ouabain and extracellular potassium on renin secretion from rat renal cortical slices

1. Renin secretion of rat renal cortical slices was measured as a function of extracellular K and ouabain concentrations in the incubation medium.2. A sigmoid relationship was found between renin secretion and log K concentration over the range 1.0-4.0 mM. Secretion was maximal at about 2.25 mM-K and half-maximal at about 1.43 mM-K.3. In media containing 4.0 mM-K, ouabain at 10(-8), 10(-7), and 10(-6)M did not affect renin secretion. Higher concentrations of ouabain inhibited secretion. A sigmoid relationship was found between% inhibition of secretion and log ouabain concentration (10(-6)-10(-3)M). Inhibition was half-maximal at 2.3 x 10(-5)M and complete at 10(-3)M-ouabain.4. Lowering extracellular K concentration from 4.0 to 2.25 mM shifted the dose-effect curve of ouabain to the left. At 2.25 mM-K, inhibition of renin secretion was half-maximal at 10(-5)M-ouabain.5. The inhibitory effect of 2 x 10(-5)M-ouabain (twice the dose for 50% inhibition) in media containing 2.25 mM-K was nearly identical to the combined effect of lowering K to 1.43 mM (the concentration required for 50% inhibition) and adding 10(-5)M-ouabain. This observation suggests that ouabain and low extracellular K act at a common site, presumably on Na, K-ATPase activity, to inhibit renin secretion.6. Neither 10(-3)M-ouabain nor K-free medium inhibited renin secretion when the concentration of free Ca in the medium was lowered to < 10(-8)M. Therefore it is proposed that as a result of Na, K-ATPase inhibition, (a) intracellular Na increases, (b) intracellular Ca increases via Na-Ca exchange, provided that extracellular Ca exceeds 10(-8)M, and that (c) Ca accumulation, in some unknown manner, inhibits renin secretion from rat renal cortical slices.

Mechanism by which renin secretion from perfused rat kidneys is stimulated by isoprenaline and inhibited by high perfusion pressure

1. Rat kidneys were completely isolated and perfused to determine the ionic mechanism whereby isoprenaline stimulates and high perfusion pressure inhibits renin secretion. 2. Isoprenaline (2.43 microM) stimulated renin secretion, but K-free medium and ouabain also reduced perfusate flow. 3. Removing Ca from the perfusion medium increased renin secretion threefold when perfusion pressure was 100 mmHg and fourfold when pressure was raised to 150 mmHg. Simultaneously raising the perfusion pressure to 150 mmHg and the Ca concentration to 5 mM inhibited renin secretion. Verapamil (50 microM) prevented this inhibition. Raising Ca also caused vasoconstriction at a pressure of 150 mmHg, but verapamil partially prevented the vasoconstriction. 4. High concentration of K (50 mM) in the perfusion medium also stimulated renin secretion when Ca was removed and inhibited secretion when Ca concentration was raised to 5 mM. Verapamil (50 microM) prevented this inhibition. High K induced vasoconstriction in the presence of high Ca, but verapamil partially prevented the constriction. 5. High concentration of K (50 mM) and high perfusion pressure (150 mmHg) stimulated renin secretion in the absence of Ca and 20 mM-Mg potentiated this effect. This suggests that both sets of stimuli activate renin secretion by different cellular mechanisms, but that both act to lower cytoplasmic Ca in the juxtaglomerular cell. This is discussed. 6. High perfusion pressure inhibited renin secretion and reduced perfusate flow when Na was lowered from 145 to 14.5 or 0 mM whether or not Ca was present. 7. K-free medium and ouabain blocked the elevated renin secretion and reduced the high flow stimulated by high perfusion pressure when Ca was removed from the perfusion medium. 8. These observations suggest that isoprenaline stimulates renin secretion by a mechanism coupled to the Na-K pump. These observations are also consistent with the hypothesis that high renal perfusion pressure inhibits renin secretion by promoting Ca movement into the juxtaglomerular cell and stimulates secretion by a mechanism coupled to the Na-K pump. High perfusion pressure also causes renal vasoconstriction by increasing the Ca permeability of the smooth muscle cells in the afferent arteriole.

Possible mechanism of the inhibitory effect of ouabain on renin secretion from rat renal cortical slices

1. The effects of ouabain on renin secretion by rat renal cortical slices were studied. 2. Renin secretion was inhibited by 10(-3) M-ouabain in the presence of free Ca (10(-4) to 2.6 x 10(-3) M). Inhibition was blocked at Ca less than 10(-8) M. 3. The effect of free Ca on ouabain-inhibition was shown to be independent of the presence of EGTA, completely reversible, and unrelated to passive leakage of renin from non-viable cells, as assessed by simultaneous release of lactate dehydrogenase activity (LDH). 4. It is proposed that, as a result of inhibition of Na, K-ATPase by ouabain, (a) intracellular Na increases in the renin-secreting juxtaglomerular cells, (b) intracellular Ca increases, via an Na-Ca exchange mechanism, and (c) that Ca accumulation, in some unknown manner, inhibits renin secretion.

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.