Increased basal activity of the HPA axis and renin-angiotensin system in congenital learned helpless rats exposed to stress early in development.

Learned helpless behavior has been successfully bred in rats and designated as a genetic animal model of human depression and/or anxiety. Since congenital learned helpless animals have an impaired stress response in adulthood, we examined the effects of early stressors (at postnatal day 7, 14 or 21) on the hypothalamic-pituitary-adrenal axis and the renin-angiotensin system. The functioning of the hypothalamic-pituitary-adrenal axis was monitored through changes in corticosterone plasma levels in the adult animals after acute exposure to cold stress and maternal deprivation early in development. Renin-angiotensin system functioning was assessed by plasma renin activity. Unstressed congenital learned helpless rats had corticosterone levels that were similar to control animals (congenital non-learned helpless rats not stressed during development), but unstressed plasma renin activity levels of congenital learned helpless rats were lower than congenital non-learned helpless rats. There was a step-wise increase in corticosterone plasma levels in the congenital learned helpless rats with age of acute presentation of either cold stress or maternal deprivation stress (day 7, 49%; day 14, 84%; and day 21, 543% for cold stress). However, these baseline corticosterone levels were significantly lower in congenital learned helpless rats compared to congenital non-learned helpless controls. Similarly, in response to early exposure to cold stress and maternal deprivation, there was an increase in plasma renin activity levels of congenital learned helpless rats with age of presentation to either stressors. However, this increase in plasma renin activity levels was not evident in congenital non-learned helpless controls. Taken together, these results suggest that exposure to stress early in development has long-term effects on both the hypothalamic pituitary-adrenal axis and the renin-angiotensin system, two neuroendocrine indicators of stress responsivity.

Hydrogen and potassium regulation of (pro)renin processing and secretion.

H and K ions play central roles in prorenin processing and secretion, and prorenin is abnormally expressed in H and K disorders. At the surface membrane of juxtaglomerular (JG) cells, K is sensed and regulated by K channels (coupled to Cl channels and activated by excess Ca), Na-K-adenosinetriphosphatase, and a KCl/H exchange transporter (regulated by Ca). In JG cell granular membrane, K flux is regulated by K channels and a KCl/H exchange transporter (activated by Ca). H channels and a H pump reside in the granular membrane, which maintain H concentration in the granular matrix at least two orders of magnitude greater than in cytosol. The H pump may also be responsible for maintaining the acidic matrix required for maximal prorenin processing to renin by prohormone convertase for human renin (PCren), the prorenin convertase. These molecules form the core of a chemiosmotic system, which appears to regulate both prorenin processing and renin secretion. Renin secretion and prorenin processing appear to be of more than causal significance in clinical disorders characterized by chemiosmotic imbalance. A critical review of the literature supports the following general conclusions. First, hyperrenin state defines the initial phase in the pathogenesis of heart disease, diabetes mellitus, and hypertension. Second, low-renin syndrome defines the transition-to-establish phase in the pathogenesis of heart disease, diabetes mellitus, and hypertension in which the key feature is renin secretory hyporesponsivity. Third, renin disorders are usually associated with other endocrine disorders (polyendocrinopathies types I, II, and III), suggesting that renin may be an important molecule in the processing of chemiosmotic forces. The key chemiosmotic molecules (K and H) are also important in the processing and export of most (if not all) hormones. Thus, by regulating K and H homeostasis, renin may regulate the endocrine system.

Regulation of renin processing and secretion: chemiosmotic control and novel secretory pathway.

The renin-angiotensin-aldosterone system (RAAS) plays an important role in cardiovascular and electrolyte regulation in health and disease. Juxtaglomerular cells in the kidney regulate endocrine RAAS by physiologically controlling conversion of prorenin and secretion of renin. The classical baroceptor, neurogenic, and macula densa mechanisms regulate renin expression at the cellular level by Ca2+, adenosine 3',5'-cyclic monophosphate (cAMP), and chemiosmotic forces (K+, Cl-, and water flux coupled to H+ movement). The baroceptor mechanism (through Ca2+) activates K+ and Cl- channels in the surface membrane and deactivates a KCl-H+ exchange chemiosmotic transporter in the secretory granular membrane. The neurogenic mechanism (through cAMP) promotes prorenin processing to renin. The macula densa mechanism (through K+ and Cl-) involves the processing of prorenin to renin. Ca2+, by inhibiting the KCl-H+ exchange transporter, prevents secretory granules from engaging in chemiosmotically mediated exocytosis. cAMP, on the other hand, by stimulating H+ influx, provides the acidic granular environment for prorenin processing to renin. It is concluded that, in the presence of a favorable chemiosmotic environment, prorenin is processed to renin, which may then be secreted by regulative degranulation or divergence translocation, a novel secretory pathway used by several secretory proteins, including renin.

Chemiosmotic control of renin release from isolated renin granules of rat kidneys.

1. Renin-containing granules were isolated, characterized, and used to gain insight into a possible chemiosmotic mechanism of renin secretion. 2. Renin granules were obtained by a modification of the sucrose gradient method, which yielded a 67-fold purification of renin granules as assessed by marker enzymes, or a modification of the Percoll gradient, which yielded a 230-fold enrichment of renin granules. 3. Granular renin content was increased by chronic sodium deprivation and hypophysectomy. 4. Renin release from granules was inversely related to osmotic strength (150-900 mosmol l-1). pH had a biphasic effect on renin release, with greater stimulation at both acidic (pH 5) and alkaline (pH 8 and 9) pH. The pH effect was dependent on Cl-; raising Cl- stimulated release. This effect was abolished by-oligomycin and N,N'-dicyclohexylcarbodiimide (DCCD) at pH 5, but not at pH 8; the effect was enhanced by NH4+. 5. Either valinomycin or carbonyl cyanide m-chlorophenylhydrazone (CCCP) alone was without effect; but in combination they caused a potent stimulation at all pHs. Nigericin stimulated renin release at all pHs, but its effect required K+. 6. Raising K+ stimulated renin release from granules, whereas raising Na+ was without effect. Lowering Ca2+ below 10(-6) M significantly stimulated renin release. 7. Taken together, the evidence is consistent with the chemiosmotic hypothesis for the control of renin release from granules and may have some implications for the regulation of renin secretion from juxtaglomerular cells.

Stimulation of renin secretion by ethacrynic acid is independent of Na(+)-K(+)-2Cl- cotransport.

This study investigated the cellular mechanism of stimulation of renin secretion by the loop diuretic ethacrynic acid (EA) in rabbit renal cortical slices. The diuretic rapidly stimulated renin secretion reversibly and in a concentration-dependent manner. The stimulation was independent of the presence of Na+, Cl-, Ca2+, or other loop diuretics (furosemide and bumetanide) in the incubation media, suggesting that the stimulation in vitro was not dependent on the inhibitory effect of the diuretic on Na(+)-K(+)-2Cl-cotransport. The findings do not support the macula densa hypothesis. The stimulation by the diuretic was prevented and reversed by thiols such as cysteine and dithiothreitol, which also prevented and reversed the stimulation of renin secretion by the nondiuretic sulfhydryl reagent P-chloromercuriphenyl-sulfonate (PCMPS). These results suggest that EA stimulates renin secretion in vitro via reversible chemical reactions with specific membrane sulfhydryl groups that may have no functional role in the Na(+)-K(+)-2Cl- cotransport.

Mechanism for low renin in blacks: studies in hypophysectomised rat model.

Black people have a lower plasma renin activity (PRA) than is appropriate for the level of blood pressure, but the mechanism remains unknown. Studies in our laboratory, using the hypophysectomised (Hypox) rat model, have provided a partial explanation of the inappropriately low PRA with respect to BP. Kidneys were isolated and perfused and renin secretion responsiveness studied with isoproterenol (Iso) infusion, calcium (Ca) depletion, and pressure reduction; an enriched preparation of juxtaglomerular (JG) cells was prepared for determination of cellular renin content (CRC); and preparations of isolated renin granules (IRG) and plasma membrane vesicles (PMV) from the purified JG cells were used to assess the storage and compartmentalisation of renin. Renin secretion was lower in Hypox than in normal and sodium (Na) deprived rats. On the other hand, CRC, IRG, and PMV were identical (statistically) in Hypox and Na deprived rats. Despite identical content and storage, kidneys from Hypox rats secreted significantly less renin in response to Iso, Ca depletion, and low pressure. One interesting observation is that upon stimulation, PMV of Hypox rats stored a much larger percentage of renin than normal or Na deprived rats, suggesting that the PMV may play a role as a renin sink in the low PRA levels observed in the Hypox rats. Since black people have renin profiles and responsiveness similar to those in Hypox rats, this model may be useful in studying the mechanisms responsible for their lower PRA.

Involvement of calmodulin in mediating inhibitory action of intracellular Ca2+ on renin secretion.

This study sought to elucidate further the cellular mechanism(s) involved in the control of renin secretion by Ca2+. The rate of renin secretion in vitro by rabbit and dog renal cortical slices was inversely related to medium Ca2+ concentration. The inverse relationship was observed only when the cell membrane permeability to Ca2+ was increased by K+ depolarization, suggesting that the Ca2+ concentration in the juxtaglomerular cell modulates renin secretion. From this relationship, renin secretion appears to turn on at intracellular Ca2+ concentrations between 10(-8) and 10(-7) M. Calmidazolium, a potent calmodulin antagonist, markedly stimulated basal renin secretion in a concentration-dependent manner. Pretreatment of slices with calmidazolium blocked the inhibition of renin secretion by high-K+ medium. Calmidazolium and several other calmodulin antagonists (W-7, W-13, and trifluoperazine) partly or fully reversed the inhibition of renin secretion previously inhibited by high-K+ medium in the order of their potencies as calmodulin antagonists. Indeed, W-5, a biologically inactive structural analogue of W-7, was without effect. These results support the hypothesis that renin secretion is inversely related to intracellular Ca2+ and that Ca2+ inhibits renin secretion by a calmodulin-dependent process.

Interrelationship of blood flow, juxtaglomerular cells, and hypertension: role of physical equilibrium and Ca.

Recent experimental evidence has provided important clues as to the role of electrolytes, particularly Ca, in the regulation of blood flow, renin secretion, and blood pressure. The smooth muscle cells of arterioles in general and the juxtaglomerular cells in the renal afferent arterioles have been shown to have Ca channels sensitive to voltage, hormones, and stretch. This paper reviews a model that utilizes these features along with a fundamental law of physics to point to some plausible explanations for some interesting experimental observations on renal blood flow, renin secretion, and hypertension. The chief features of the model are that in the steady state the arteriole must achieve a stable physical equilibrium in which the forces tending to distend the vessel (transmural pressure) counterbalance the forces tending to prevent distension (wall tension); the wall tension consists of a passive and an active component, the latter of which is sensitive to stretch of the vessel; and stretch activates the opening of stretch-sensitive Ca permeability channels that promote the influx of Ca to trigger active tension development. Thus Ca is the signal that couples stretch to contraction. This latter feature is the so-called myogenic response. Altered equilibrium may be initiated either by a rise in perfusion or tissue pressure to alter the distending force or by a rise in cytosolic Ca to increase active tension development and the constricting force. Several factors may initiate disequilibrium, some of which are discussed. Equilibrium is soon reestablished, however, at a new steady state. The model predicts curves for renal blood flow autoregulation and renin secretion in response to changes in renal perfusion pressure, tissue pressure, extracellular Ca, and blockers and promoters of Ca influx and Ca efflux. These predictions agree well with existing experimental evidence and suggest new experiments. The model provides a theoretical basis for explaining the steady-state blood pressure profile observed in renovascular hypertension and perhaps in other forms of hypertension as well. The model also provides a theoretical basis for understanding the volume-vasoconstriction approach used by some workers and the autoregulation approach used by others in explaining the mechanisms of hypertension.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of renin secretion by Ca2+ and cyclic AMP through two parallel mechanisms.

The cellular mechanism of action of cyclic AMP (cAMP) mediating the beta-adrenergic stimulation of renin secretion was studied, with special reference to its interactions with the inhibitory pathway of renin secretion by Ca2+ calmodulin. Forskolin, a potent stimulator of adenyl cyclase that bypasses the hormone-receptor interactions, stimulated renin secretion in vitro from rabbit renal cortical slices in a concentration-dependent manner. Renin secretion stimulated by submaximal concentration of forskolin was partly or completely antagonized or blocked by raising intracellular Ca2+ concentration by incubating slices in a high-K+ depolarizing medium, but renin secretion stimulated by the maximal effective concentration of forskolin was not inhibited by Ca2+. In addition, the maximal effective concentration of forskolin (10(-5) M) increased renin secretion by a fixed amount independent of medium (by inference, intracellular) Ca2+ concentration in the range of 10(-8) to 10(-6) M in a high-K+ medium. Furthermore, the degree of stimulation of renin secretion by forskolin was greater with greater removal of the inhibitory effect of Ca2+ calmodulin pathway on renin secretion with use of potent calmodulin antagonists, suggesting that the stimulatory effect of cAMP on renin secretion may be maximal in the absence of the inhibitory influence of Ca2+. These results are consistent with the hypothesis that cAMP (by inference, the beta-adrenergic stimulus) stimulates renin secretion through a cellular mechanism independent of that through the Ca2+ -calmodulin pathway.

Forskolin and calcium: interactions in the control of renin secretion and perfusate flow in the isolated rat kidney.

Forskolin (activator of adenyl cyclase), high concentrations of K+ and high renal perfusion pressure (manoeuvres known to increase Ca2+ permeability), and calmidazolium (the specific blocker of calmodulin) were used to investigate the mechanisms whereby adenosine 3',5'-phosphate (cyclic AMP) and Ca2+ interact to control renin secretion and perfusate flow in the isolated perfused rat kidney. Forskolin stimulated renin secretion and caused vasodilation in a dose-dependent manner in medium containing 5 mM-Ca2+. Reducing the Ca2+ concentration to 1.25 mM did not affect the renin stimulatory response but blunted the vasodilation. High K+ concentration reversed the forskolin-induced renin secretion and vasodilation. Conversely, forskolin reversed the high K+-induced renin inhibition of renin secretion and vasoconstriction. These effects of forskolin and high K+ were absent when Ca2+ was withheld from the perfusion medium. High renal perfusion pressure also reversed the forskolin-induced renin secretion. Calmidazolium prevented the inhibition mediated by high K+ and high perfusion pressure and thereby restored the forskolin-induced stimulation. Calmidazolium also caused a prompt and marked vasoconstriction. The calmidazolium-induced stimulation of renin secretion was Ca2+-dependent since the drug was ineffective in the absence of Ca2+. On the other hand, the prompt and potent vasoconstriction was present even in the Ca2+-free medium. These results support the hypothesis that cyclic AMP stimulates renin secretion by a mechanism which involves a lowering of membrane permeability to Ca2+ in addition to lowering cytosolic Ca2+ concentration. High K+ and high renal perfusion pressure inhibit renin secretion by raising the membrane permeability to Ca2+, thereby raising the intracellular Ca2+ concentration which then inhibits renin secretion by a calmodulin-dependent process. A further general conclusion from these studies is that membrane permeability to Ca2+ and cellular Ca2+ concentration are of central importance in the control of renin secretion and renal blood flow.

Steady-state autoregulation of renal blood flow: a myogenic model.

This paper presents a model of myogenic control of renal blood flow based on the proposition that steady-state flow occurs when the distending and constricting forces acting on the afferent arteriole are equal. The distending force is represented by the Laplace relationship. The opposing force is governed by the properties of the arterioles and has two components--a purely passive component and an "active" component resulting from vascular smooth muscle contraction. Within the myogenic model, vascular smooth muscle contraction is initiated by "stretch"-induced changes in calcium permeability. Terms are developed describing the effect of stretch on calcium permeability, intracellular calcium, and contractile activity. The model is adapted to describe the myogenic control of blood flow in the dog kidney. Sigmoidal relationships between stretch and calcium permeability and between intracellular calcium and muscle tension seem to account for the shape of the autoregulatory curve. The model predicts a shifting of the autoregulatory pressure-flow curve upward and to the right in response to increased tissue hydrostatic pressure. The model is also exquisitely sensitive to changes in the parameters governing intracellular calcium. These predictions agree well with experimental observations.

Renin-angiotensin system in hypophysectomized rats. I. Control of blood pressure.

The mechanisms whereby the pituitary gland maintains arterial pressure were investigated in rats. The arterial pressure in hypophysectomized rats was 30 mmHg below normal. Saralasin or captopril caused a further fall of 25 and 30 mmHg, respectively, suggesting that the renin-angiotensin system plays a role in blood pressure maintenance in hypophysectomized rats. Growth hormone administration to hypophysectomized rats increased the arterial pressure, but pretreatment with captopril prevented the effect. Plasma renin activity and basal renin secretion (in vitro) was normal in hypophysectomized rats despite a twofold greater renal renin content. Secretory responsiveness to isoproterenol and calcium omission was lower in hypophysectomized rats. It is concluded that the renin-angiotensin system plays a role in maintaining arterial blood pressure in hypophysectomized rats although the responsiveness of the system may be decreased.

Cellular mechanisms of renin secretion.

Renin secretion from the juxtaglomerular cell is controlled by numerous receptors, humoral agents, and ions. Recently, a stretch receptor hypothesis has been advanced to suggest that all of these diverse factors control renin secretion by a mechanism initiated by a fall in cytoplasmic Ca2+. This fall in Ca2+ may be achieved by lowering Ca2+ influx, raising Ca2+ efflux, or sequestering Ca2+ into cellular organelles and binding sites. The increased renin secretion observed with low arterial pressure, beta-adrenergic agonists, parathyroid hormone, glucagon, cyclic AMP, prostaglandins, low Ca2+ and Ca2+ ionophore, high Mg2+, and Na+ and Cl- may be explained in this context. On the other hand, the decreased renin secretion observed with high pressure, alpha-adrenergic agonists, some prostaglandins, angiotensin, vasopressin, and high K+ may be explained by a rise in cytoplasmic Ca2+ mediated by an opposite sequence of events. Recent observations suggest that the fall in cytoplasmic Ca2+ sets in motion the transport of renin from its site of storage (granules) or synthesis into the cytoplasmic space and finally across the plasma membrane. Thus although renin is stored in granules, its secretion occurs by a process quite different from exocytosis.