The effect of changes in potassium concentration on the maximal steroidogenic response of purified zona glomerulosa cells to angiotensin II

Role of cAMP in the effects of K+ on the steroidogenesis of zona glomerulosa cells

1. Investigations of the role of cAMP in the stimulation of the steroidogenesis of zona glomerulosa (ZG) cells by increased extracellular K+ concentration are reviewed. 2. Possible reasons for discrepancies in the results of different investigators on whether K+ increases the cAMP content or output of ZG tissue or dispersed cells are discussed. 3. The concentration of cAMP in the incubating media of ZG tissue or cells, rather than their cAMP content, seems to respond more sensitively to stimulation by extracellular K+, as was also found for adrenocorticotropic hormone stimulation of zona fasciculata-reticularis cells. 4. The addition of the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (IBMX) to incubations with the aim of increasing the sensitivity of the response in cAMP to extracellular K+ in ZG cells may give rise to effects, probably nonspecific, which actually inhibit the measured response. 5. The immediate stimulation in the steroidogenesis of ZG cells with raised extracellular K+ is probably mostly due to the direct effect of increases in cytoplasmic Ca2+ (arising from increases in Ca2+ influx) on mitochondrial processes. However, increases in cAMP may prolong the stimulation of steroidogenesis by increased extracellular K+. This increased cAMP is probably due to stimulation of adenylyl cyclase activity. 6. It has been concluded that the increase in Ca2+ influx output after rises in the extracellular K+ concentration of ZG cells is responsible for most of the increase in cAMP. 7. According to one group of investigators, there is weak stimulation of phospholipase C (PLC) activity after increasing the extracellular K+ concentration of rat ZG cells. 8. If there is such a stimulation of PLC activity, it seems that the action of increased extracellular K+ can potentially involve all known mechanisms for the stimulation of steroidogenesis in endocrine cells. The common primary event is probably the increase in Ca2+ influx. The relative importance of these various potential mechanisms may depend on the particular in vitro conditions used.

A compartmental model of acute stimulation of aldosterone secretion in vivo by potassium and ANG II

Conscious sheep with an adrenal autotransplant were used to study the relationship between acute change in K+ concentration and acute change in aldosterone secretion rate (ASR). The kinetics of K+ within the adrenal circulation were shown to be consistent with distribution within a single compartment. In contrast, change in ASR during and after KCl infusion was consistent with a two-compartment model, where ASR was a sigmoidal function of concentration of K+ (or other unidentified agent) in the second, or "effect," compartment. Prior work on the relationship between acute change in angiotensin II (ANG II) concentration and acute change in ASR was extended by investigating the interaction of K+ and ANG II in the control of ASR. The results of this study indicate that acute change in ANG II concentration may cause an inhibition of the usual response of the adrenal to acute change in K+ concentration.

Dopamine inhibition of potassium-stimulated aldosterone biosynthesis in bovine adrenal zona glomerulosa cells

Dopamine inhibits angiotensin II-stimulated aldosterone production by an effect on the late phase of biosynthesis. This study was undertaken to investigate the effect of dopamine on potassium-stimulated aldosterone biosynthesis in adrenal glomerulosa cells in vitro. As potassium concentrations were increased from 0 to 12 mM, aldosterone production increased up to 6 mM potassium, but not beyond this concentration. Dopamine (10(-5)M) inhibited the aldosterone response to potassium. The effect of potassium on pregnenolone accumulation (the early phase of aldosterone biosynthesis) was assessed in cells treated with trilostane which inhibits the conversion of pregnenolone onward to aldosterone. Increasing potassium concentrations up to 12 mM gave increasing pregnenolone accumulation; however dopamine did not influence this effect. The potassium stimulated conversion of corticosterone to aldosterone, an index of activity in the late phase of aldosterone biosynthesis, was assessed using aminoglutethimide to prevent cholesterol side-chain cleavage. Significantly more corticosterone was converted to aldosterone at 6 mM potassium than at 0 or 12 mM; dopamine inhibited the conversion of corticosterone to aldosterone at 6 mM potassium. These data indicate that dopamine inhibits potassium-stimulated aldosterone production by an effect restricted to the late phase of the aldosterone biosynthetic pathway similar to its previously established effect on angiotensin II-stimulated aldosterone biosynthesis.

The mechanism of the effect of K+ on the steroidogenesis of rat zona glomerulosa cells of the adrenal cortex: role of cyclic AMP

The effects of various concentrations of extracellular K+ (3.6-13 mM) on the steroid (corticosterone and aldosterone) and cyclic AMP outputs of capsular cells (95% zona glomerulosa) of the rat adrenal cortex were studied at different concentrations of extracellular Ca2+. Small amounts of EGTA (50 microM) were added to reduce the free Ca2+ concentrations effectively to zero at the lowest possible total Ca2+ concentration. At a total extracellular concentration of 2.5 mM Ca2+, in 27 experiments the mean values of the steroid and cAMP outputs showed a maximum at 8.4 mM K+. The increase in steroid and cAMP outputs at 5.9, 8.4 and 13 mM K+ compared with that at 3.6 mM were highly significant (p less than 0.01). The overall correlation of either corticosterone or aldosterone with cAMP outputs was also highly significant and was even better from 3.6 to 8.4 mM K+. Lowering the effective free concentration of Ca2+ to zero decreased the steroid and cAMP outputs significantly at all K+ concentrations, and no output was then significantly higher than at 3.6 mM. With the pooled data on outputs at all total Ca2+ (2.5, 0.5, 0.25, 0.10, 0.05 and 0.0 mM) and K+ (3.6, 5.9, 8.4 and 13 mM) concentrations, the correlation of either steroid with cAMP outputs was highly significant (but again optimally from 3.6 to 8.4 mM K+). Nifedipine (10(-6) to 10(-4) M) was added to the incubations with the aim of specifically inhibiting Ca2+ influx at total extracellular Ca2+ concentrations of 2.5, 1.25 and 0.25 mM and with the usual K+ concentrations. The cAMP outputs were reduced at all K+ concentrations above 3.6 mM K+. The effect was highly significant at 10(-4) M nifedipine and a total Ca2+ of 1.25 mM, which with the incubation conditions used, corresponds to the free Ca2+ concentrations in vivo. These results indicate that cAMP plays a significant role in the stimulation of steroid output by K+ particularly between 3.6 and 8.4 mM K+. In this range of K+ concentrations the stimulation of cAMP seems to be controlled by increases in Ca2+ influx. The correlation of steroid and cAMP output at the higher K+ concentrations (between 8.4 and 13 mM K) and at the various total Ca2+ concentrations is less significant. Also, with all concentrations of added nifedipine there is an 'anomalous' increase in steroid output at 13 mM K+ and at total Ca2+ concentrations of 2.5 and 1.25 mM. However, at the same K+ concentrations and at 0.25 mM Ca2+, nifedipine decreases steroid outputs.(ABSTRACT TRUNCATED AT 400 WORDS)

The relationship between tissue preparation and function; methods for the study of control of aldosterone secretion: a review

The study of the control of aldosterone synthesis and secretion by the rat adrenal gland has over the past thirty years involved the application of many different in vivo and in vitro techniques. In this review the relationship between the data that each of these methods has produced is compared. There are striking differences in overall steroid production rates, and in the qualitative nature of the steroid profile which the various methods produce. In particular, aldosterone is secreted at higher rates in vivo, and when whole tissue preparations are used in vitro, than in incubations of isolated glomerulosa cells. In addition, while corticosterone is a major product of glomerulosa tissue in vitro, the available evidence suggests that it is not a major glomerulosa product in vivo.

Effect of calcium on corticosteroid secretion by isolated frog interrenal gland

The direct effect of extracellular calcium concentrations on corticosteroidogenesis has been examined in the frog, using a perifusion system technique. The release of corticosterone and aldosterone in the effluent medium was monitored by specific radioimmunoassays. Increasing concentrations of Ca2+ (from 2 to 15 mM) gave rise to a dose-related stimulation of corticosteroid release, whereas the increment of either Na+ or K+ concentrations did not modify steroid production. Iterative administration of a moderate concentration of calcium (6 mM) led to a reproducible stimulation of steroid secretion whereas the same dose infused during 6 h induced a transient rise in corticosteroid secretion followed by a plateau. The direct effect of Ca2+ on steroidogenesis was confirmed by the dose-dependent stimulation of steroid secretion induced by the calcium ionophore A 23187. Perifusion with a calcium-free medium or blockade of Ca2+ channels by 4 mM Co2+ both resulted in a significant decrease in steroid production. Conversely, the administration of verapamil (up to 10(-4) M) did not affect steroidogenesis. These results provide evidence that extracellular calcium ions are required for basal production of corticosteroids in amphibians and that Ca2+ influx does not occur through voltage-dependent channels. Since, in the frog, blood Ca2+ concentrations vary in a rather large range, these results suggest that circulating Ca2+ levels may regulate corticosteroid production in these animals.

Primary tissue culture of human adrenocortical Conn's adenomata. Bromocriptine as a possible agonist-antagonist of angiotensin at the cellular level

Angiotensin II-induced the hypertrophy of the cytoplasmic compartment and significantly increased (5-3H)uridine incorporation into RNA species by Conn's human adult adenomatous cells in primary tissue culture. On its own, bromocriptine, while enlarging only the nucleolar compartment, also intensely stimulated the incorporation of (5-3H)uridine into RNA species by the cultured adrenocortical adenomatous cells. However, an equimolar mixture of angiotensin II and bromocriptine was totally ineffective, eliciting no change in cellular morphometry or isotope incorporation with respect to the control specimens run in parallel. The present findings support the view that bromocriptine can influence the metabolism of Conn's cells directly at the cellular level by acting as an agonist-antagonist of angiotensin.

The primary tissue culture of human adrenocortical Conn's adenomata. I. The synergistic stimulation of adenomatous cell growth by purine cyclic nucleotides and by ACTH1-24 and angiotensin II

Primary cultures of dissociated parenchymal cells from Conn's adenomata causing primary hyperaldosteronism were successfully set up by a method previously used with normal adult human and rat adrenocortical tissue. In such cultures the adenomatous cells largely prevailed (making up 87% of the whole cell population), could survive for lengthy terms (at least up to 30 days), and were endowed with a spontaneous, discrete capability to proliferate. The de novo RNA- and DNA-synthetic and mitotic activities of Conn's cells were markedly stimulated in cultures exposed between 16 and 21 to daily doses of exogenous cyclic AMP, either alone or in equimolar association with cyclic GMP. A significantly weaker, though still prominent enhancement of adenomatous cell growth was elicited also by daily administrations of an equimolar mixture of ACTH1-24 and angiotensin II. In contrast, little stimulation or inhibition of growth or no effect at all could be observed when cyclic GMP, ACTH1-24, and angiotensin II were respectively administered, each by itself.

Angiotensin alters 45Ca2+ fluxes in bovine adrenal glomerulosa cells

Angiotensin II stimulated 45Ca2+ release from bovine adrenal glomerulosa cells. It also decreased the influx of 45Ca2+ into glomerulosa cells. The effects were observed within 2 min of hormone addition and were blocked by Saralasin a competitive inhibitor of angiotensin. Des-Phe8-angiotensin II, a biologically inert analog, was inactive in this system. Angiotensin II also inhibited the influx of 133Ba2+ and 54Mn2+, whereas 51Cr6+ and 57Co2+ were unaffected. Alterations in 45Ca2+ fluxes were seen with concentrations of angiotensin that stimulate aldosterone biosynthesis in bovine glomerulosa cell preparations. These results suggest that calcium plays a key role in angiotensin-stimulated aldosteronogenesis.

Effect of potassium chloride on plasma renin activity and plasma aldosterone during sodium restriction in normal man

The present study was designed to evaluate the spectrum of responses of PRA and plasma aldosterone (PAldo) to a range of oral potassium intakes (0 to 300 mEq of potassium chloride per day) in 20 normal human subjects receiving an electrolyte-free diet. Potassium exhibited a dose-dependent natriuretic effect. The results of the PRA studies indicate that normal dietary amounts (50 mEq/day) of potassium chloride do not prevent the increase in PRA with absolute sodium deprivation and that PRA is maximally stimulated on 150 mEq of potassium chloride per day. The rise in PRA is directly correlated with serum potassium concentration. The results of the PAldo studies indicate that potassium chloride deprivation attenuates PAldo increases due to sodium deprivation and that PAldo is maximally stimulated on 150mEq of potassium chloride per day. The rise in PAldo is directly correlated with serum potassium concentration and with PRA. The administration of 300 mEq/day of potassium chloride caused significant hyperkalemia and blunted both PRA and PAldo increases. Our results suggest that potassium chloride has an important role in the regulation of PRA and PAldo, and that only following potassium chloride deprivation is the PRA/PAldo response dissociated.