Importance of calcium in renal renin release

Effects of KRN2391-induced hypotension on the endocrine system and carbohydrate metabolism in halothane-anesthetized dogs

PURPOSE

The hemodynamic profiles of KRN2391-induced hypotension have been reported to be a hyperdynamic state. However, the endocrine effects of KRN2391-induced hypotension remain to be elucidated. We investigated the endocrine and metabolic effects of KRN2391-induced hypotension on the plasma concentrations of catecholamines, aldosterone, cortisol, glucose, and lactic acid and on plasma renin activity.

METHODS

Eight dogs were anesthetized with 087% halothane in oxygen. After a baseline period, mean arterial pressure (MAP) was lowered to 60 mmHg for 60min by the infusion of KRN2391.

RESULTS

KRN2391-induced hypotension resulted in a 50% decrease (P<0.01) in MAP due to a 80% reduction (P<0.01) in systemic vascular resistance associated with a 224% increase (P<0.01) in cardiac index. Plasma norepinephrine concentrations increased (P<0.01) after 60 min of hypotension. Plasma epinephrine concentrations and plasma renin activity both increased (P<0.05) during the hypotensive period. Plasma aldosterone concentrations remained unchanged during the hypotensive period, but then increased (P<0.05) after termination of KRN2391. Plasma cortisol concentrations remained unchanged throughout the observation period. Plasma glucose concentrations increased (P<0.01) during the hypotensive period. Plasma lactic acid concentrations increased (P<0.01) throughout the observation period.

CONCLUSION

KRN2391-induced hypotension activates the sympathetic nervous system and consequently may modulate the renin-angiotensin-aldosterone axis and carbohydrate metabolism.

Conserved mechanism of negative gene regulation by extracellular calcium. Parathyroid hormone gene versus atrial natriuretic polypeptide gene

We found that a negative calcium-responsive element (nCaRE) originally reported in the human parathyroid hormone gene is conserved among several genes. The results of the present study show that expression of one of the genes, the rat atrial natriuretic polypeptide gene, was negatively regulated in the heart by extracellular calcium by using an in vivo infusion system. Moreover, transfection of the cultured cells revealed that this DNA element conferred negative regulation by extracellular calcium on the reporter gene. It is suggested that there is a gene family whose expression is negatively regulated by extracellular calcium through this conserved DNA motif, nCaRE.

Stretch-induced atriopeptin secretion in the isolated rat myocyte and its negative modulation by calcium

Cellular mechanism(s) regulating atriopeptin secretion and processing by the atrial myocyte are currently unknown. Osmotic stretch of isolated atrial myocytes as well as potassium chloride depolarization were potent stimuli of atriopeptin secretion. Release was potentiated by buffering either extracellular calcium with EGTA or intracellular calcium with the intracellular chelator, BAPTA AM. Atrial release of atriopeptin was inhibited after administration of ionomycin which elevates intracellular calcium. Fetal or early neonatal ventricular myocytes actively synthesize atriopeptin. Atriopeptin secretion by ventricular myocytes was also markedly potentiated by osmotic stretch as well as KCl depolarization. Only the 126 amino acid prohormone was secreted by the stretch-stimulated atrial and ventricular myocyte. These data suggest that stretch of the myocyte plasma membrane is a major stimulus for atriopeptin secretion and that atriopeptin secretion is not stimulated by raising intracellular calcium and appears to be negatively modulated by this cation. Like the atrial myocyte, the ventricular myocyte possesses the cellular mechanism(s) necessary to secrete atriopeptin by a regulated mechanism.

Effect of atriopeptin III on renin release in vitro

The ability of atriopeptin III (AP) to directly inhibit renal renin release has not been resolved. This issue was examined in a series of experiments performed in a system of rat renal cortical slices (dry weight 1.91 mg) in which the goal was to explore the effects of AP on renin release induced by cyclic AMP (cAMP)-coupled stimuli or by agents which are believed to decrease intracellular calcium (Cai). Concentration response relationships were initially established for all test agents. The cAMP stimuli utilized were isoproterenol (10(-5) M), forskolin (10(-5) M), and dibutyryl cAMP (3 X 10(-4) M); each of these agents produced a significant increase in renin release in the system (with isoproterenol a 59% increase, with forskolin 37%, and with dibutyryl cAMP 52%). The addition of AP (2.09 X 10(-8) M, a minimum inhibitory concentration derived from preliminary studies) significantly blunted these increases; in the case of the dibutyryl cAMP-stimulated renin release, the inhibition was partial as a significant 25% increase in renin occurred in the presence of AP. The addition of the calcium channel blocking agent diltiazem (10(-4) M) resulted in a significant increase in renin release (364 to 567 ng X mg-1, p less than .05) which was not blocked by the addition of AP. Similarly, TMB-8 (0.6 X 10(-4) M), another agent thought to lower Cai, also resulted in increased renin release (455 to 810 ng X mg-1), p less than .01) which was also unaffected by the addition of the AP. In summary, these results show that AP is capable of partially inhibiting renin release in vitro, particularly renin release coupled to cAMP action. In contrast, renin release induced by a decline in Cai appears to be unaffected by the addition of AP.