Angiotensin effects on calcium and steroidogenesis in adrenal glomerulosa cells

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Corticotropin induces the expression of TREK-1 mRNA and K+ current in adrenocortical cells

Bovine adrenal zona fasciculata (AZF) cells express a two-pore/four-transmembrane segment bTREK-1 K+ channel that sets the resting potential and couples hormonal signals to depolarization-dependent Ca2+ entry and cortisol secretion. It was discovered that corticotropin (1-2000 pM) enhances the expression of bTREK-1 mRNA and membrane current in cultured AZF cells. Forskolin and 8-pcpt-cAMP mimicked corticotropin induction of bTREK-1 mRNA, but angiotensin II (AII) was ineffective. The induction of bTREK-1 mRNA by corticotropin was partially blocked by the A-kinase antagonist H-89. 8-(4-Chloro-phenylthio)-2-O-methyladenosine-3'-5'-cyclic monophosphate, a cAMP analog that activates cAMP-regulated guanine nucleotide exchange factors (Epac), failed to increase bTREK-1 mRNA. Corticotropin-stimulated increases in bTREK-1 mRNA were eliminated by inhibitors of protein synthesis or gene transcription. bTREK-1 current disappeared after 24 h in serum-supplemented medium, but in the presence of corticotropin, bTREK-1 expression was maintained for at least 48 h. The enhancement of bTREK-1 mRNA and ionic current contrasts with the corticotropin-induced down-regulation of the Kv1.4 voltage-gated K+ current and associated mRNA in AZF cells. These results demonstrate that corticotropin rapidly and potently induces the expression of bTREK-1 in AZF cells at the pretranslational level by a cAMP-dependent mechanism that is partially dependent on A-kinase but independent of Epac and Ca2+. They further indicate that prolonged stimulation of AZF cells by corticotropin, as occurs during long-term stress or disease, may produce pronounced changes in the expression of genes encoding ion channels, thereby reshaping the electrical properties of these cells to enhance or limit cortisol secretion.

Demonstration of an angiotensin II-induced negative feedback effect on aldosterone synthesis in isolated rat adrenal zona glomerulosa cells

Although both angiotensin II (Ang II) and potassium ion (K+) induce marked elevations of cytosolic free calcium concentration, [Ca2+]c, in adrenal zona glomerulosa cells-an effect which is thought to trigger aldosterone synthesis-Ang II is also known to reduce the sustained [Ca2+]c rise induced by K+. We have examined whether this effect of Ang II on the calcium messenger system is reflected at the level of the final biological response, aldosterone synthesis. In superfused isolated rat glomerulosa cells, K+ (8 mM) induced a sustained, 60-fold increase in aldosterone production. In contrast, the maximal response to Ang II (10 nM) amounted to only 10 times the basal production. When added subsequent to K+ stimulation, Ang II provoked an immediate and dramatic drop in aldosterone synthesis, to levels obtained with Ang II alone. Under conditions of maximal K+ stimulation, this effect depended upon Ang II concentration, while the well-known synergistic effect was observed with submaximal concentrations of both agonists. The inhibitory effect of Ang II could be reproduced with dioctanoylglycerol, a selective activator of protein kinase C. By contrast, the aldosterone response to adrenocorticotropic hormone (ACTH) was not affected by Ang II. At submaximal concentrations of ACTH, the steroidogenic effect of Ang II was even additive to that of ACTH. Thus, we have shown that, under conditions of maximal stimulation, Ang II exerts a profound inhibition of steroidogenesis in K(+)-stimulated rat adrenal glomerulosa cells. This counter-regulatory mechanism may ensure adequate levels of aldosterone production in vivo.

Lead increases aldosterone production by rat adrenal cells

Exposure to lead has been postulated to contribute to elevated blood pressure in humans and has been shown to raise blood pressure in animals. The mechanism of action of lead on blood pressure is unknown. We fed lead to rats in their drinking water and then examined the production of aldosterone by their adrenal cells in vitro. We also measured excretion of aldosterone and corticosterone by intact rats stimulated with corticotropin, with and without lead treatment. At a dose (273 ppm) that raised blood levels to 30 to 40 micrograms/dL, comparable to blood levels in exposed humans, lead induced increased aldosterone secretion in vitro and in vivo. The effect of lead was most evident when cells or animals were stimulated with aldosterone secretagogues. Experiments in vitro indicate that exposure to lead in vivo increases activity of one or more steps in the late pathway of aldosterone biosynthesis. The results suggest that the hypertensive effect of lead involves relative hyperaldosteronism and may be most evident when secretion of this hormone is stimulated.

Effect of osmolality on cytosolic free calcium and aldosterone secretion

Alterations in extracellular osmolality have powerful inverse effects on basal and potassium- and angiotensin-stimulated aldosterone secretion. With the use of bovine glomerulosa cells grown in primary culture, the effects of alterations in osmolality on cytosolic calcium concentration ([Ca2+]c), efflux and uptake of 45Ca2+, and aldosterone secretion were determined. Alterations in osmolality, independent of sodium concentration, have inverse effects on aldosterone secretion, which are correlated with simultaneous changes in [Ca2+]c measured using fura-2. Reductions in osmolality cause dose-dependent biphasic increases in [Ca2+]c different from the monophasic increases in [Ca2+]c produced by increases in potassium concentration. Like potassium- and angiotensin-stimulated increases in [Ca2+]c, hypotonically induced increases in [Ca2+]c are associated with an increase in 45Ca2+ efflux. Reductions in osmolality also increased the uptake of 45Ca2+, an effect apparent at 2 min and persistent for at least 30 min. In the absence of extracellular calcium, reductions in osmolality, as increases in potassium concentration but not angiotensin, fail to increase [Ca2+]c, efflux of 45Ca2+, or aldosterone secretion. In conclusion, osmolality-induced alterations in aldosterone secretion are associated with parallel changes in [Ca2+]c, effects caused by alteration in the influx of extracellular calcium. On the basis of these and previous studies, we hypothesize that osmolality affects calcium influx by activating voltage-dependent or stretch-activated calcium channels.

Platelet-derived growth factor is a potent inhibitor of angiotensin II-induced aldosterone synthesis

Platelet-derived growth factor (PDGF) is a potent mitogen for several cell types. In addition, PDGF has vasoconstrictive action and shares some signal transduction mechanisms with angiotensin II (AII). In the present study, we have examined the effects of PDGF on basal and AII-induced aldosterone synthesis in freshly isolated rat adrenal glomerulosa cells. Recombinant human PDGF-BB caused a dose-dependent inhibition of AII-induced aldosterone synthesis being effective at concentrations as low as 10(-12) M. We also investigated possible mechanisms of action of PDGF. We have previously reported that the 12-lipoxygenase (LO) pathway of arachidonic acid plays a key role in AII-induced aldosterone synthesis. We thus examined whether PDGF action is mediated by changes in 12-LO activation. PDGF, at the same doses that blocked AII-induced synthesis also significantly inhibited AII-induced increases in the 12-LO product, 12-hydroxyeicosatetraenoic acid (12-HETE) formation. Further, the addition of 12-HETE completely restored the stimulatory effect of AII during inhibition by PDGF. These results suggest that PDGF could act, at least in part, by inhibition of AII-induced 12-HETE formation. We also examined the role of diacylglycerol (DG) formation since we have previously reported that DG is the source of arachidonic acid for 12-HETE formation. We observed that both AII and PDGF stimulated [3H]arachidonic acid-labeled DG formation. However, PDGF did not alter AII-induced DG formation suggesting that PDGF action is not mediated by affecting AII-induced increases in DG.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide in trout ovarian follicles

An immunocytochemical study on atrial natriuretic peptide (ANP) was carried out in trout ovaries using an antiserum raised against human atrial natriuretic peptide (hANP). Weakly immunoreactive material was observed both in the ooplasm of the previtellogenic and the early vitellogenic oocytes and in the follicular cells of the late vitellogenic oocytes. Binding sites for hANP have been located using 125I-ANP and light-microscopic autoradiography. These studies provide evidence of specific binding of ANP-like material in both early and advanced vitellogenic oocytes. Autoradiograms indicated that the ANP-like binding sites were localized to a subpopulation of follicles. The presence of ANP-like immunoreactive material and its specific binding sites in the ovary suggest a role for atrial natriuretic peptide as a regulator of the fish ovarian function.

Sphingosine inhibits angiotensin-stimulated aldosterone synthesis

Sphingosine and other protein kinase C inhibitors were tested for their ability to inhibit aldosterone synthesis by bovine adrenal glomerulosa cells. Sphingosine inhibited angiotensin (AII)-stimulated aldosterone synthesis (IC50 of 5 microM). At doses that totally blocked steroidogenesis, sphingosine did not affect protein synthesis or [125I]AII binding to cells. Sphingosine also inhibited dibutyryl cyclic AMP (dbcAMP)-stimulated aldosterone synthesis. Sphingosine inhibited pregnenolone synthesis from cholesterol, but not the conversion of progesterone or 20 alpha-hydroxycholesterol to aldosterone. These results suggest that sphingosine inhibits steroidogenesis at a locus close to that where stimulation occurs by AII and dbcAMP. Other protein kinase C inhibitors were tested. Retinal, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), and staurosporine inhibited aldosterone synthesis stimulated by AII and dbcAMP. Retinal and H-7 also inhibited progesterone conversion to aldosterone, and retinal blocked [125I]AII binding. Staurosporine was more specific, inhibiting AII-stimulated aldosteronogenesis at concentrations which had little effect on conversion of progesterone to aldosterone. Because they inhibited dbcAMP stimulation, none of the inhibitors was sufficiently specific to use as a probe of the role of protein kinase C. The IC50 of sphingosine suggests that this or related products of lipid hydrolysis could act as endogenous regulators of adrenal cell function.

N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) does not modify the angiotensin II-stimulated calcium signal in cultured bovine glomerulosa cells

Studies were performed to determine if the sustained elevation in [Ca2+]c noted previously in glomerulosa cells in response to Ang II resulted from the presence of HEPES in the experimental medium. At confluence, primary cultures of bovine glomerulosa cells were maintained for 24-30 h in the presence of either 14 mM NaHCO3/5% CO2 or 25 mM HEPES/4 mM NaHCO3/air. During subsequent experimental periods, cells were incubated in the presence of the corresponding or reciprocal buffer, and the effects of Ang II on [Ca2+]c were monitored by fura 2 fluorescence. Increases in [Ca2+]c produced by Ang II in cells continuously maintained in either HCO3(-) - or HEPES-buffered media were similar, and with the same monolayer the nature of the Ang II-stimulated Ca2+ signal was independent of the buffer employed. Moreover, the Ang II-stimulated Ca2+ signal was not significantly affected by the removal of HCO3- from the superfusate. These results indicate that the sustained increase in [Ca2+]c is not an artifact introduced by the use of HEPES as an experimental buffer, but rather a normal component of the Ang II-stimulated Ca2+ signal.

Effects of angiotensin-II on inositol phosphate accumulation and calcium influx in bovine adrenal and Y-1 tumor adrenal cells

The present study compared the action of angiotensin II (A-II) in bovine adrenal fasciculata cells (BA) and Y-1 adrenal tumor cells which are sensitive and resistant respectively to its steroidogenic effect. In both models, A-II induced a time and dose-dependent inositol phosphate (Ins-Ps) accumulation and calcium influx. However, in Y-1 cells the Ins-Ps production was low and only Ins-P1 and Ins-P2 were accumulated. The calcium influx in BA cells was observed after 15 seconds and remained linear as long as the hormone was present, whereas in Y-1 cells calcium influx started prior to 15 seconds and reverted to basal values after 45 seconds. The effects of A-II on both cell types were specific since they were blocked by A-II antagonists. Taken together these results demonstrate the presence of functional A-II receptors in both cell types which are coupled to the two main intracellular messenger systems. Thus, the A-II steroidogenic refractoriness of Y-1 cells is probably related to some alteration(s) located beyond the calcium and/or protein kinase C A-II-messenger system.

Calcium dependence of osmolality-, potassium-, and angiotensin II-induced aldosterone secretion

Different calcium-dependent mechanisms may be involved in mediating stimulus-induced aldosterone secretion. Using isolated perfused canine adrenal glands, we determined the effect of reductions in extracellular [Ca2+] and of infusion of a voltage-dependent calcium channel antagonist, nifedipine, on aldosterone secretion induced by decreases in osmolality, by increases in [K+], or by infusion of angiotensin II (ANG II). Aldosterone secretion was stimulated to a similar level by reducing osmolality, by increasing [K+], or by infusing ANG II. After 50 min of stimulation, lowering [Ca2+] from 1.25 to 0.10 mM caused a marked and similar inhibition of osmolality- and [K+]-induced aldosterone secretion that was significantly greater than inhibition of ANG II-induced aldosterone secretion. Similarly, nifedipine at 3.3 X 10(-8) M caused marked and similar inhibition of osmolality- and [K+]-induced aldosterone secretion that was significantly greater than the inhibition of ANG II-induced aldosterone secretion. These data demonstrate that calcium-dependent processes are involved in osmolality-, [K+]-, and ANG II-induced aldosterone secretion. However, the calcium-dependent process(es) evoked by reductions in osmolality or increases in [K+] are considerably different from that evoked by ANG II. Osmolality and potassium appear to induce aldosterone secretion primarily by activating voltage-dependent calcium channels.

Angiotensin II and glucocorticoid release: direct effect at the adrenal level and modulation of the adrenocorticotropin-induced glucocorticoid release

Angiotensin II (A II) stimulates adrenal glomerulosa cells releasing mineralocorticoids; however, little is known about the A II effect on glucocorticoids output. The present study has been designed in order to see if A II could modify in vitro spontaneous and ACTH-induced corticosterone (B) release from both fasciculata-reticularis enriched and total adrenal cells. The results indicate that A II at 10(-12), 10(-11), 10(-10) and 10(-6) M concentrations did not modify basal B production and A II 10(-9), 10(-8) and 10(-7) M decreased basal B production from total adrenal cells. Whereas A II (10(-10)-10(-6) M) stimulated B release from fasciculata-reticularis enriched cells. On the one hand, 10(-8) M A II significantly decreased ACTH-elicited B release from total adrenal cells; effect completely abolished by saralasin (SAR, 10(-8) M), a specific A II receptor blocker. On the other hand, 10(-8) M A II did not modify ACTH-induced B release from fasciculata-reticularis enriched cells. Finally, 10(-10) to 10(-6) M A II and 22 pM ACTH stimulated aldosterone output from total adrenal cells; while, fasciculata-reticularis enriched cells did not secrete any measurable amount of aldosterone under basal condition and after incubation with A II. These data further suggest a regulatory role of A II in the release of glucocorticoids from the adrenal gland.

Angiotensin II-stimulated changes in calcium metabolism in cultured glomerulosa cells

Studies were performed to evaluate the relationships between the effects of angiotensin II on calcium metabolism and cytosolic free calcium concentration in primary monolayer cultures of bovine adrenal glomerulosa cells. As noted previously (Kramer (1988) Am. J. Physiol. (in press], angiotensin II produced rapid dose-dependent increases in cytosolic calcium characterized by both an initial transient component and a secondary sustained component. In the absence of extracellular calcium, angiotensin II produced an initial increase in cytosolic calcium comparable to that produced in the presence of calcium, but failed to maintain a sustained calcium signal. The initial, angiotensin-stimulated increase in cytosolic calcium was inhibited by dantrolene and 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8) in a concentration-dependent fashion. The onset of the angiotensin-stimulated calcium signal was accompanied by a dose-dependent increase in the rate of calcium efflux that achieved a maximum within 2-3 min and then declined to a level 2.5-3 times that from control cells. The initial rate of calcium influx was also increased about 2.5-fold by angiotensin II, an effect that was only apparent in cells that had been treated with the peptide for at least 5 min. These results indicate that the calcium signal produced by angiotensin II is initiated by the rapid mobilization of calcium from an intracellular site(s) and sustained by the continued uptake of extracellular calcium. Moreover, the kinetics of the calcium signal as well as the final, sustained calcium concentration achieved reflect the balance between intracellular calcium release, calcium influx and calcium efflux.

Diacylglycerol provides arachidonic acid for lipoxygenase products that mediate angiotensin II-induced aldosterone synthesis

The lipoxygenase products of arachidonic acid metabolism have been shown to be important mediators of stimulus secretion coupling in various endocrine tissues. We have recently shown that the 12-lipoxygenase product, 12-hydroxyeicosatetraenoic acid plays a key role as a new specific mediator of angiotensin II-induced aldosterone secretion in the adrenal. In view of the several pathways by which cellular arachidonate can be generated and the important role of diacylglycerol in angiotensin II-responses, we studied the role of diacylglycerol as the source of arachidonic acid for 12-hydroxyeicosatetraenoic formation. Treatment of normal human adrenal glomerulosa cells with the selective diacylglycerol-lipase inhibitor, RHC 80267, resulted in a dose-dependent inhibition of angiotensin II-induced aldosterone as well as 12-hydroxyeicosatetraenoic formation. These results suggest that AA derived from diacylglycerol is the precursor of 12-hydroxyeicosatetraenoic involved in angiotensin II-induced aldosterone secretion. These results reveal a new second messenger role for diacylglycerol in addition to activation of protein kinase C.

Angiotensin II causes sustained elevations in cytosolic calcium in glomerulosa cells

Studies were conducted to examine the effects of angiotensin II on cytosolic free calcium concentration in bovine adrenal glomerulosa cells maintained in primary culture. The calcium indicator, fura-2, and discontinuous dual-wavelength fluorescence spectroscopy were used to measure cytosolic free calcium in superfused adherent cell monolayers. Basal cytosolic free calcium concentration was 63.7 +/- 3.3 nM. The threshold concentration for angiotensin II-stimulated increases in cytosolic calcium was 10(-14)-10(-13) M, and maximal elevation of cytosolic calcium was produced by 10(-9) M angiotensin II. Angiotensin II (10(-13) M) produced a gradual increase in cytosolic calcium concentration that plateaued after 3-5 min of superfusion at a level approximately 1.2 times that of control cells. The calcium signal invoked by a maximal concentration (10(-9) M) of angiotensin II, in contrast, was characterized by an immediate, intense (approximately 8-fold) increase in cytosolic calcium concentration that decayed within 5 min to a lower, but sustained, level 2.5-3 times that of control cells. The calcium signals invoked by intermediate concentrations (10(-12)-10(-10) M) of angiotensin II exhibited dose-dependent increases in magnitude and a gradual transition in nature between those invoked by threshold and maximal concentrations of the peptide. The effect of angiotensin II to increase cytosolic calcium concentration was accompanied by an increase in aldosterone output. The increase in steroidogenesis was most closely correlated with the magnitude of the initial calcium signal. At high concentrations (10(-10) and 10(-9) M) of angiotensin II, there was a clear dissociation between aldosterone output and the magnitude of the sustained calcium signal.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium oscillations in single adrenal glomerulosa cells stimulated by angiotensin II

The cytosolic calcium (Ca2+i) response to angiotensin II (Ang II) was examined in single rat zona glomerulosa cells by monitoring fura-2 fluorescence with microspectrofluorimetry. Ang II concentrations ranged from 5 X 10(-12) to 5 X 10(-8) M. The mean peak Ca2+i increase was similar at all Ang II concentrations (205 +/- 11 nM), with a significant difference (P less than 0.05) found only between 5 X 10(-12) M (151 +/- 16 nM) and 5 X 10(-9) M (236 +/- 24 nM). Striking differences over the range of Ang II concentrations were found in the Ca2+i response kinetics. A dose-dependent delay of the onset of the Ca2+i response was observed ranging from 2.6 +/- 0.3 sec at 5 X 10(-8) M to 181 +/- 27 sec at 5 X 10(-12) M Ang II. After the delay, cells typically responded with an abrupt increase in Ca2+i, complete within 15 sec. At low Ang II concentrations (5 X 10(-11) and 5 X 10(-12) M), a complex response was often observed consisting of Ca2+i oscillations. Higher Ang II concentrations gave some evidence of Ca2+i oscillation, especially at 5 X 10(-10) M where oscillations appeared fused. Above 5 X 10(-10) M Ang II, the initial Ca2+i increase decayed to an apparent steady-state value 38-40% of the peak response within 5 min; 5 X 10(-10) M Ang II produced a smaller decline to 63% of the initial Ca2+i increase. In contrast to cell population studies, assessment of individual glomerulosa cells demonstrates (i) a dose-dependent delay prior to a rapid increase in Ca2+i; (ii) a similar peak increase at most Ang II concentrations; (iii) greater sensitivity of the Ca2+i response; and (iv) a complex oscillating Ca2+i response in the physiological range of Ang II.

Stimulus-secretion coupling in angiotensin-stimulated adrenal glomerulosa cells

Adrenal glomerulosa cell is a suitable model for a comparative study of signal transducing mechanisms since its secretory activity is regulated by at least three different mechanisms: the adenylate cyclase-cAMP system (for ACTH), the voltage-dependent Ca2+ channel (for K+ and perhaps for angiotensin II) and the inositol 1,4,5-trisphosphate-Ca2+ system (for angiotensin II and vasopressin). The role of inositol phosphates, extracellular Ca2+ and protein kinase C in the induction and sustaining of aldosterone production by cells exposed to angiotensin II is critically reviewed.

Dissociation of osmotic and ionic modulation of aldosterone secretion

Although other investigators have suggested that reductions in either Na or chloride concentration stimulate aldosterone secretion, we previously found that small reductions in NaCl (3-7 mM) that enhanced angiotensin II-(ANG II) and [K]- but not adrenocorticotropic hormone (ACTH)-stimulated aldosterone secretion are due to a change in osmolality. In the present study, aldosterone secretion by an isolated perfused canine adrenal gland was stimulated by low doses of ANG II or ACTH or by small increases in perfusate [K], and during this stimulation, replacing 25 mM NaCl with an isosmotic amount of mannitol enhanced aldosterone secretion induced by each of the above secretagogues. Choline chloride significantly enhanced ANG II-stimulated aldosterone secretion when used in place of 25 mM NaCl, but sodium methylsulfate did not. Large isosmotic reductions in [NaCl] failed to alter ACTH-stimulated cortisol secretion or the conversion of either exogenous corticosterone or 11-deoxycorticosterone to aldosterone. Thus, reductions in Na, but not in chloride concentration, specifically enhance the ability of the adrenal glomerulosa to secrete aldosterone in response to ANG II, K, and ACTH by an action on some site in the steroidogenic cascade that is sensitive to ANG II, potassium, and ACTH.

Angiotensin II causes a dual effect on potassium permeability in adrenal glomerulosa cells

In previous studies it was shown that angiotensin II causes a Ca-dependent increase in the K permeability of bovine adrenal glomerulosa cells [Am. J. Physiol. 250 (Endocrinol. Metab. 13): E125-E130, 1986]. Here we show that angiotensin II causes a significant and prolonged reduction in the 86Rb release immediately after the transient rise in 86Rb efflux. This inhibition was dose related. Apamin (100 nM) and tetraethylammonium (10 mM) completely abolished the initial transient rise in 86Rb efflux without affecting the latter sustained phase of reduced radioisotope release. On the contrary, the effect of angiotensin II on the second phase was absent when Ca was removed from the perifusion medium or replaced with Sr, but the effect on the early transient phase of 86Rb efflux was maintained in the absence of external Ca. An additional finding was the increased coefficient rate of 86Rb efflux that occurred when the cells were depolarized with 12 mM K. However, this effect was not observed when the inhibitory phase due to angiotensin II was fully developed and Ca was present in the external media. On the other hand, the biphasic effect of angiotensin II was still present in depolarized cells. These results suggest that angiotensin II may modulate membrane potential by changes in K permeability of the bovine adrenal glomerulosa cells.

Quantitative analysis of the cytosolic-free-Ca2+-dependency of aldosterone production in bovine adrenal glomerulosa cells. Different requirements for angiotensin II and K+

Angiotensin II (AII) and K+ raise the cytosolic free Ca2+ concentration [( Ca2+]i) and stimulate aldosterone production in isolated bovine adrenal glomerulosa cells. The mechanisms leading to an elevation of [Ca2+]i were analysed with the fluorescent Ca2+ probe quin 2. (1) Whereas [Ca2+]i rose transiently and returned to basal values within 5 min in response to AII, the effect of K+ was sustained for at least 15 min. (2) AII released Ca2+ from intracellular stores, whereas the [Ca2+]i response to K+ depended solely on extracellular [Ca2+]. (3) When added after K+ stimulation, AII provoked a dramatic decrease in [Ca2+]i to below the resting value. The role of [Ca2+]i in stimulating steroidogenesis was determined by manipulating the concentration of this cation. (4) In a cell superfusion system, the aldosterone response to AII is biphasic. Suppressing the transient [Ca2+]i elevation triggered by AII resulted in the disappearance of the initial secretory peak, but the final production rate was similar to that of control cells. (5) Normal basal [Ca2+]i levels were, however, necessary to maintain continuous AII-induced steroidogenesis. (6) When added after AII, the antagonist analogue [Sar1,Ala8]AII suppressed steroidogenesis without affecting [Ca2+]i levels. (7) In contrast, continuously elevated [Ca2+]i values were required for the initiation and the maintenance of K+-stimulated aldosterone production. These results demonstrate important differences in the mechanisms through which AII and K+ activate the Ca2+ messenger system. Moreover, functional correlations have shown that K+, but not AII, depends solely on a sustained [Ca2+]i response for its steroidogenic effect. However, the AII-induced effect is also a Ca2+-requiring process: the initial [Ca2+]i transient accelerates the onset of steroidogenesis, which is subsequently extremely sensitive to [Ca2+]i decreases below normal basal levels.

Plasma prorenin response to human chorionic gonadotropin in ovarian-hyperstimulated women: correlation with the number of ovarian follicles and steroid hormone concentrations

Plasma prorenin and active renin were measured before and after human chorionic gonadotropin (hCG) administration in two groups of patients undergoing ovarian stimulation for 4-6 days with follicle-stimulating hormone alone or in combination with luteinizing hormone, for in vitro fertilization. Baseline total plasma renin (prorenin plus active renin; n = 12) averaged 25 +/- 8 ng/ml per hr (mean +/- SD). Total renin did not change during ovarian stimulation but it increased to 46 +/- 16 ng/ml per hr (P less than 0.05) 1 or 2 days later, just before hCG administration. Thirty-six hours after hCG administration, just before laparoscopy and egg retrieval, total renin was 123 +/- 97 ng/ml per hr; a peak of 182 +/- 143 ng/ml per hr occurred 2-6 days later--i.e., during the luteal phase of the menstrual cycle. In eight of the patients who did not conceive, total renin returned to baseline 14 days after hCG administration. In four who conceived, a nadir was reached (57 +/- 13 ng/ml per hr) 8-12 days after hCG administration and then total renin increased again as the plasma beta hCG measurement began to rise. By day 16 it averaged 225 +/- 157 ng/ml per hr. In a second group of five patients active renin and prorenin were measured separately. Active renin comprised less than 20% of the total renin at all times. It was unchanged until day 4 after hCG administration and then increased significantly only when plasma progesterone was high. Thus, the initial response to hCG was entirely due to an increase in prorenin. A highly significant correlation was observed between the number of follicles and the total renin increases on the day of aspiration (r = 0.93, P less than 0.001) and at the peak (r = 0.89, P less than 0.001). After hCG administration, a temporal relationship was observed between the rise in total renin and plasma estradiol and progesterone levels. These results demonstrate that plasma prorenin increases markedly after administration of hCG and that the rise is directly related to the number of ovarian follicles and to plasma estrogen and progesterone levels. The findings suggest that prorenin is produced by the mature ovarian follicle and by the corpus luteum in response to gonadotropin stimulation.

The mechanism of angiotensin-induced desensitization of adrenal glomerulosa cells

Superfusion of isolated rat adrenal glomerulosa cells for 6 h with a medium containing 2.5 nM angiotensin II (AII) reduces the aldosterone response to AII, corticotropin and potassium. Here we report that under such conditions there is a decrease in the capacity of the cells to form inositol phosphates in response to a subsequent stimulation with AII. The capacity to convert corticosterone to aldosterone is also reduced by a prior exposure to AII. Superfusion with a high-potassium medium has no such an effect. Reduced phosphoinositide response may be responsible for the decreased aldosterone stimulation by AII, the inhibition of the late stage of aldosterone biosynthesis may account for the heterologous character of desensitization.

Role of calcium in stimulus-secretion coupling on isolated frog interrenal gland

The influence of extracellular calcium concentration on the steroidogenic response to ACTH and to the angiotensin II analogue [Sar1-Val5]AII has been studied in the frog, using a perfusion system technique. The release of corticosterone and aldosterone in the effluent medium was measured by specific radioimmunoassays. In calcium-free medium the stimulatory effect of ACTH (10(-9) M) was completely abolished whereas the response to dbcAMP (5 mM) was unchanged indicating that the role of calcium takes place before the formation of cAMP. Conversely, in the absence of calcium, angiotensin II (10(-7) M) was still able to stimulate corticosterone and aldosterone production. Addition of Co2+ (4 mM), a calcium antagonist, to the perfusion medium, inhibited partially the response of adrenal tissue to ACTH, dbcAMP and angiotensin. The voltage-dependent calcium channel blocker verapamil (10(-6) induced a dose-related inhibition of the corticotropic effect of ACTH. At the higher dose (10(-4) M), verapamil totally inhibited the stimulation of corticosterone and aldosterone production induced by ACTH. By contrast, at the same dose it did not alter the stimulatory effect of forskolin (2.4 X 10(-7)M) on corticosterone output, but significantly diminished forskolin-induced aldosterone response. Similarly, angiotensin-stimulated corticosterone production was slightly inhibited by 10(-4) M verapamil, whereas aldosterone response to angiotensin was totally abolished, indicating that verapamil may act intracellularly to block the conversion of corticosterone to aldosterone. Taken together, these results indicate that, in amphibians extracellular calcium is essential for the action of ACTH, either for the binding of the hormone to its receptor and/or for the transduction of the information from hormone-receptor complex to the adenylate cyclase moiety and that the mechanism of action of angiotensin does not involve calcium uptake by adrenocortical cells.