The control of renin release

An outbreak of histoplasmosis occurred in early May 1970 at a junior high school in Delaware, Ohio; clinical illness occurred in 384 (40 per cent) of the students and faculty, with probably an equal number of subclinical cases. The mode of spread was airborne and was shown epidemiologically to be related to activities on Earth Day, April 22, 1970, when the courtyard in the center of the school, an old bird roost, was raked and swept. Contamination of the entire school building with courtyard air occurred via the school's forced air ventilation system with intakes in the courtyard. Soil samples from the courtyard were positive for Histoplasma capsulatum, but random samples from other areas around the building were negative. In two persons in the building only on April 22, the typical illness developed. Features of the outbreak have important implications for clinicians and public health officials.     

The role of calcium in the control of renin release

The effects of removing external calcium and inhibiting entry of calcium into the cell by treatment with D-600 on renin release from renal cortical slices of male Sprague-Dawley rats were examined. Baseline renin release, angiotensin II (AII)-induced inhibition, and isoproterenol-induced stimulation of renin release were studied. Removal of external calcium by chelation with 5 mM EGTA inhibited basal renin release while treatment with 1 mM EGTA stimulated basal renin release slightly. Incubation of slices with zero calcium medium containing 1 mM EGTA had no effect on isoproterenol-induced stimulation of renin release. In contrast, similar treatment reduced the inhibitory effect of AII from 58.7% of baseline to 85.3% (p less than 0.001). Similarly, blockage of calcium entry into cells with D-600 had no effect on isoproterenol-induced stimulation of renin release but abolished AII-induced inhibition. Replacement of sodium in the bathing medium with choline had no effect on baseline renin release or on AII-induced inhibition of renin release, ruling out the possibility that D-600 altered renin release via an effect on sodium influx. Taken together, the data strongly suggest that AII-induced inhibition of renin release is partially dependent on the presence of external calcium but that isoproterenol-induced stimulation of renin release is not.     

The beta-adrenergic receptors

BACKGROUND: The beta-adrenergic receptors of the myocardium play an important role in the regulation of heart function. The beta-adrenergic receptors belong to the family of G-protein coupled receptors. Three subtypes have been distinguished (beta1-, beta2-, and beta3-adrenoceptors). The receptors consist of seven membrane-spanning domains, three intra- and three extracellular loops, one extracellular N-terminal domain, and one intracellular C-terminal tail. PATHOPHYSIOLOGY: Stimulation of beta-adrenergic receptors by catecholamines is realized via the beta-adrenoceptor-adenylylcyclase-protein kinase A cascade. The second messenger is the cyclic AMP (cAMP). Stimulation of the cascade caused an accumulation of the second messenger cAMP and activated via the cAMP the cAMP dependent protein kinase A (PKA) The PKA phosphorylated, beside other cell proteins, the beta-adrenergic receptors. A phosphorylation of the beta-adrenergic receptors caused - with exception of the beta3-adrenoceptor - an uncoupling and desensitisation of the receptors. Phosphorylation via the G-protein receptor kinase (GRK or betaARK) also caused uncoupling and reduced the beta-adrenergic responsiveness. The uncoupling of the receptor is the prerequisite for receptor internalisation. In the process of internalisation the receptor shifted from the sarcolemma membrane into cytosolic compartments. Chronic beta-adrenergic stimulation caused a down-regulation of the receptors. During this process of desensitisation the expression of the receptor on mRNA and protein level is reduced. CHANGING OF THE RECEPTORS IN THE FAILING HEART: In patients with dilated cardiomyopathy the beta-adrenergic responsiveness of the myocardium is diminished. It was shown that in these patients the expression of the beta1-adrenergic receptor is reduced on the mRNA and protein level. In these patients the expression of the inhibitory G-protein G(i) is increased. Furthermore, the expression of the G-protein receptor kinase is elevated. This kinase induces the uncoupling of the beta-adrenergic receptors. These alterations of the beta-adrenoceptor signal cascade may be induced by an elevated catecholamine release or by agonist-like autoantibodies directed against the beta1-adrenergic receptor found in patients with dilated cardiomyopathy. Both, permanent stimulation with catecholamines and chronic treatment with agonistic anti-beta1-adrenoceptor autoantibodies cause a reduction of the expression of the beta1-adrenoceptor on mRNA and protein level in "in vitro" experiments. Moreover, an over-expression of the beta1-adrenoceptor, the stimulatory G(s) protein, and the protein kinase A induce detrimental alterations of the cardiac function and morphology in transgenic animals. These animals developed heart failure accompanied by an increased mortality rate.     

Direct effect of beta-adrenergic stimulation on renin release by the rat kidney slice in vitro.

Controversy exists regarding the mechanism by which catecholamines stimulate renin secretion in vivo. A sensitive rat kidney slice system was utilized to study the direct effects of adrenergic agonists and antagonists on renin release in vitro. Catecholamines were protected from degradation by the addition of ascorbic acid to the incubation medium. Significant dose-related stimulation of renin release was observed with epinephrine and norepinephrine in concentrations from 1.5 times 10(-9) to 1.5 times 10(-7)M and with isoproterenol in concentrations from 2 times 10(-9) to 2 times 10(-7)M. No significant stimulation was seen with 10(-10)M concentrations of the three agents. Methoxamine (10(-6)M) stimulated renin release significantly (P less than 0.01). The stimulation observed with epinephrine, norepinephrine, or isoproterenol was blocked by d,l- and l-propranolol (2 times 10(-4)M) but not by d-ropranolol (2 times 10(-4)M) or phentolamine (9 times 10(-4)M). Methoxamine-induced stimulation was abolished by d,l-propranolol but not by phentolamine. These data that the in vitro kidney slice system is responsive to physiological concentrations of catecholamines when they are protected from degradation. The results further demonstrate a direct stimulatory role for beta-adrenergic agents on renin release and suggest that alpha-adrenergic effects seen in vivo are mediated indirectly by hemodynamic, vascular, or functional changes in the kidney.     

Evidence that the beta-adrenergic system and prostaglandins stimulate renin release through different mechanisms

In normal man, converting enzyme inhibition (CEI) acutely increases plasma active renin and decreases plasma inactive renin. This reciprocal relationship suggests that conversion of inactive to active renin may be important in the acute response to stimulation of renin secretion. To determine whether the beta-adrenergic system or prostaglandins (PGs) participate in the acute effect of CEI on renin, we administered captopril (50 mg) alone and with either propranolol (P; 80 mg) or a PG cyclooxygenase inhibitor [PI; indomethacin (50 mg) or ibuprofen (800 mg)] to normal subjects ingesting a 25 meq/day Na diet. Supine blood pressure fell by 12 +/- 2 (+/- SE) mm Hg with CEI alone, 10 +/- 1 mm Hg with CEI plus P, and 7 +/- 1 mm Hg with CEI plus PI. Active renin rose 8-fold (P less than 0.01), with a peak at 1-2 h, after CEI and 3-fold (P less than 0.02) in response to CEI plus P or CEI plus PI. P did not block the fall in acid-activated inactive renin compared to CEI alone. The nadir of the inactive renin response to both CEI or CEI plus P occurred at 1-2 h. PI, however, prevented the fall in inactive renin. To extend this observation, we compared the effects of infusion of a vasodilator PG (PGA1; 0.6 micrograms/kg X min) and a pure beta-agonist (isoproterenol; 0.3 micrograms/kg X min). PGA1 increased active renin 2.5-fold and decreased inactive renin by 80% (both P less than 0.02), while isoproterenol increased active renin 4.1-fold, but did not significantly change inactive renin. These data suggest that the beta-adrenergic system and PGs at least acutely stimulate renin production at different steps of its biosynthesis or secretion.     

The role of renal kallikrein and prostaglandin in the control of renin release

Renal prostaglandins and kallikrein are considered to play an important role in the control of renin release. Recently, we have shown that aprotinin, a kallikrein inhibitor, inhibits the stimulation of plasma renin activity (PRA) by either furosemide or a low sodium diet. However, the mechanisms of action of kallikrein are unknown. Since kallikrein may stimulate bradykinin and prostaglandin production, the present study examines the relationship of renal kallikrein and renal prostaglandins in the control of renin release. Furosemide and a low sodium diet stimulated PRA, urinary kallikrein excretion and urinary prostaglandin E2 excretion. Aprotinin and indomethacin inhibited both furosemide and low sodium diet stimulation of PRA. When maximum doses of both aprotinin and indomethacin were given, PRA was more strongly suppressed than by indomethacin alone. The stimulation of urinary kallikrein excretion by furosemide and by a low sodium diet was not inhibited by indomethacin. These results suggest that both renal kallikrein and prostaglandins play an important role in the control of renin release under sodium depletion. Renal kallikrein may also have a direct action on the kidney to release renin.     

The emerging pharmacogenomics of the beta-adrenergic receptors

Beta-adrenergic signaling mechanisms are of central importance to cardiovascular health and disease. Modulation of these pathways represents an important pharmacologic approach to the treatment of heart failure and hypertension. Advances in molecular genetics have identified genetic polymorphisms in the human beta-adrenergic receptor genes; some of this variation predicts changes in protein sequence/structure, and potentially changes in function, of the b-adrenergic receptors. This article reviews the current state of knowledge and understanding of the genetic variation present in the three human beta-adrenergic receptor genes. Already, variation in these genes has been associated with observed differences in several cardiovascular phenotypes. This work has led to the demonstration of functional differences in activity between receptors with certain known polymorphisms and "wild-type" receptors. An understanding of these polymorphisms is key to the development of studies of how differences in drug response/effects may be mediated by these polymorphisms. Such studies are anticipated to provide a foundation for the development of novel pharmacologic approaches where selection of and dosing of cardiovascular therapy is tailored to individuals on the basis of each patient's specific genetic makeup.     

Desensitization of beta-adrenergic receptors by pheochromocytoma

Prolonged stimulation of cells by beta-adrenergic receptor agonists may lead to diminished responsiveness of the cells to subsequent activation by catecholamines. This phenomenon has been termed desensitization; the mechanism(s) for desensitization may involve an apparent loss in the number of beta-adrenergic receptors or an alteration in receptor-effector coupling. We have examined the consequences of prolonged stimulation of beta-adrenergic receptors in an interesting rat model harboring pheochromocytoma. New England Deaconess Hospital rats with transplanted pheochromocytomas developed systolic hypertension and plasma norepinephrine concentrations approximately 40-fold greater than controls. beta-Adrenergic receptors were quantitated in several tissues from controls and rats with transplanted pheochromocytoma using the beta-adrenergic receptor antagonist [125I]iodocyanopindolol. Down-regulation of beta 1-receptors was found in heart tissue (22.8 vs. 13.6 fmol/mg protein; P less than 0.001) and adipocytes (29,400 vs. 2,800 sites/cell; P less than 0.001). Also, maximal isoproterenol-stimulated cAMP accumulation in isolated adipocytes was diminished in pheochromocytomic animals (13.1 vs. 4.9 pmol cAMP/10(5) cells/min; P less than 0.05). Interestingly, there was no change in beta-receptors in lung and mesenteric artery, which predominantly contain beta 2-receptors. Furthermore, the competition curves of isoproterenol in the heart membranes from control and pheochromocytomic rats in the absence and presence of guanylylimidodiphosphate indicated uncoupling of the beta-adrenergic receptors in pheochromocytomic animals. Rats with pheochromocytoma secreting large amounts of norepinephrine provide a valuable model system for studying the in vivo development of desensitization.     

Lack of control of renin release by adrenergic nervous system in the aglomerular toadfish.

Aglomerular toadfish, Opsanus tau, release renin in response to hemorrhage or vasodilator drugs, presumably by stimulating a renal arterial baroreceptor. We aimed to determine whether the adrenergic nervous system and prostaglandins play a role in the control of renin release in unanesthetized toadfish kept in 50% seawater. Isoproterenol (1 microgram/kg) increased plasma renin activity (PRA) fourfold and decreased blood pressure (BP); both effects were abolished by a concomitant infusion of propranolol. Propranolol itself slightly decreased the basal level of heart rate and BP, but not that of PRA. Norepinephrine (1 microgram/kg) increased BP, but did not change PRA. Repeated injection of 6-hydroxydopamine did not alter resting levels of either PRA or BP. Monoamine-specific nerve fluorescence activity could not be demonstrated in association with arterioles of kidneys from intact toadfish or from those treated with monoamine oxidase inhibitor and norepinephrine (5 mg/kg). Furthermore, treatment of toadfish with indomethacin (10 or 20 mg/kg) prevented neither the increase in PRA nor the reduction in BP after a massive hemorrhage. These results indicate that renin release in toadfish primarily occurs in response to a reduction in renal arterial pressure, whereas it appears unlikely that the adrenergic nervous system or prostaglandins have a significant role in the control of renin release.     

Inhibition of hypothalamic somatostatin release by beta-adrenergic antagonists

A number of in vivo studies suggest that hypothalamic somatostatin (SRIF) tone is stimulated by the beta-adrenergic system. Employing dispersed adult male rat hypothalamic cells, we studied the effects of beta-adrenergic antagonists on the release of hypothalamic SRIF. Propranolol, at concentrations of 1-100 microM, had no detectable effect on basal SRIF release, but caused dose-dependent inhibition of SRIF release stimulated by ouabain. Two other beta-adrenergic antagonists, labetolol and metoprolol, also caused inhibition of ouabain-stimulated SRIF release. The alpha 2-agonist clonidine was without effect on SRIF release under basal or stimulated conditions. GH secretion from monolayers of dispersed rat anterior pituitary cells was also examined. Propranolol (1-100 microM) had no significant effect on basal GH secretion or GH secretion stimulated by rat GRF. In conclusion, 1) beta-adrenergic antagonists caused inhibition of stimulated SRIF release; 2) clonidine had no detectable effect on SRIF release; and 3) propranolol did not affect GH secretion in vitro. These findings support the hypothesis that beta-adrenergic antagonists augment GH responsivity by inhibiting hypothalamic SRIF release.     

Proteinase-activated receptors 1 and 2 exert opposite effects on renal renin release

Proteinase-activated receptors (PARs) 1 to 4 are highly expressed in the kidney and are involved in the regulation of renal hemodynamics and tubular function. Since intravascular infusion of the proteinase thrombin, which activates PARs, has been shown to decrease plasma renin activity in rats, we investigated the effects of the respective PAR subtypes on renin release using the isolated perfused mouse kidney model. Thrombin dose-dependently reduced perfusate flow and inhibited renin secretion rates (RSRs) that had been prestimulated by the β-adrenoreceptor agonist isoproterenol. The suppression of RSRs was prevented by the selective PAR1 inhibitor SCH79797, and direct activation of PAR1 by TFLLR mimicked the effects of thrombin on RSRs and vascular tone. Moreover, TFLLR suppressed the stimulations of RSRs in response to the loop diuretic bumetanide, to prostaglandin E(2), or to a decrease in renal perfusion pressure but not in response to a reduction in extracellular calcium. The PAR2-activating peptide SLIGRL concentration dependently increased RSR and perfusate flow. The stimulation of RSRs by SLIGRL was markedly attenuated by N(G)-nitro-L-arginine methyl ester, suggesting an NO-dependent mechanism. Activation of PAR4 by AYPGKF did not modulate RSRs or perfusate flow. PAR1 and PAR2 immunoreactivity were detected in the juxtaglomerular region and were colocalized with renin immunoreactivity. Our data provide evidence that PAR1 activation inhibits renal renin secretion and induces renal vasoconstriction, whereas PAR2 activation stimulates renin release and induces vasodilation mainly via the release of NO.     

Effect of stress on the control of renin release in spontaneously hypertensive rats

Recent reports suggest that centrally induced increases in sympathetic outflow to the kidney have the potential to enhance the sensitivity of pressure-dependent renin release. In the present study, the possibility was investigated that spontaneously hypertensive rats (SHR), which are thought to have increased tonic sympathetic outflow to the kidney, exhibit enhanced renin release in response to reduced renal perfusion pressure. The increase in plasma renin activity in response to a graded suprarenal aortic constriction was determined in conscious young (6-9 weeks of age) and adult (14-16 weeks of age) SHR and age-matched Wistar-Kyoto (WKY) control rats. Under conditions of relatively little stress, the renin response to reduced renal perfusion pressure was not enhanced in young or adult SHR when compared with age-matched WKY rats. That is, this regulatory mechanism was not "reset" in the hypertensive animals. When challenged with an acute stress (air to the face) both age groups of SHR exhibited a significantly enhanced response. Neither age group of WKY rats was affected by the acute air stress. These data suggest that, under unstressed conditions, pressure-dependent renin release probably does not contribute to the elevation of arterial pressure in the SHR. However, under stressful conditions, the contribution of this system may be significant. Intermittent increases in sympathetic outflow to the kidney that can occur in the SHR in response to daily stresses have the potential to render it more sensitive to spontaneous reductions in perfusion pressure. Occasional exaggerated release of renin could then contribute to the hypertensive process.