The effect of calcium on potassium-induced depolarization of adrenal glomerulosa cells

Saline suppression testing-induced hypocalcemia and implications for clinical interpretations

BACKGROUND

Extracellular calcium critically regulates physiologic aldosterone production. Moreover, abnormal calcium flux and signaling are involved in the pathogenesis of the majority of primary aldosteronism cases.

METHODS

We investigated the influence of the saline suppression test (SST) on calcium homeostasis in prospectively recruited participants (n = 86).

RESULTS

During SST, 100% of participants had decreases in serum calcium, with 48% developing frank hypocalcemia. Serum calcium declined from 2.30 ± 0.08 mmol/L to 2.13 ± 0.08 mmol/L (P < .001) with parallel increases in parathyroid hormone from 6.06 ± 2.39 pmol/L to 8.13 ± 2.42 pmol/L (P < .001). In contrast, serum potassium and bicarbonate did not change, whereas eGFR increased and serum glucose decreased (P < .001). Lower body surface area (translating to greater effective circulating volume expansion during SST) was associated with greater reductions in (β = .33, P = .001), and absolutely lower, serum calcium levels (β = .25, P = .001). When evaluating clinically-relevant diagnostic thresholds, participants with post-SST aldosterone levels <138 pmol/L had lower post-SST calcium and 25-hydroxyvitamin D levels (P < .05), and higher post-SST parathyroid hormone levels (P < .05) compared with those with post-SST aldosterone levels >277 pmol/L.

CONCLUSION

SST uniformly decreases serum calcium, which is likely to be due to the combination of variable dilution, increased renal clearance, and vitamin D status. These acute reductions in bioavailable calcium are associated with lower post-SST aldosterone. Given the critical role of extracellular calcium in regulating aldosterone production, these findings warrant renewed inquiry into the validity of SST interpretations for excluding primary aldosteronism.

Okadaic acid inhibits angiotensin II, adrenocorticotropin and potassium-dependent aldosterone secretion

The present study was designed to assess the effect of okadaic acid (OA), a protein phosphatase inhibitor, on aldosterone secretion in response to angiotensin II (AII), adrenocorticotropin (ACTH) and rises in external potassium concentration (K+). AII (10nM) caused a 20-fold increase in aldosterone production and OA reduced this response by 45%. ACTH (10nM) caused an 8.6-fold increase in aldosterone secretion and OA reduced this by 83%. Increasing K+ concentration from 3 to 12mM caused a 13-fold increase in aldosterone production, which OA inhibited by 36%. These results suggest that protein phosphatases participate in the control of adrenal steroid production, even though ACTH, AII and K+ act via different intracellular messenger systems.

Evidence suggesting that the angiotensin II-sensitive intracellular Ca2+ pool is reloaded from the external space in adrenal glomerulosa cells

Adrenal glomerulosa cells prelabeled with 45Ca2+ and perifused for 10 min with 10 nM angiotensin II (AII) in a dynamic perifusion system show a biphasic response with an initial transient increase in 45Ca2+ efflux, followed by a sustained phase of increased 45Ca2+ efflux. When labeled adrenal golmerulosa cells were treated with 10 nM AII for three consecutive periods of 5 min, the transient increase in 45Ca2+ efflux was observed only in the first period. However, when 40Ca2+ was measured in the perifusate using a Ca2+-sensitive electrode coupled to the perifusion system, a transient increase in 40Ca2+ efflux was observed in each period of AII treatment. Exposing the cells to AII for 1 min, the amount of 40Ca2+ effluxed out of the cells was 58.3 +/- 8.4 nmol/10(8) cells. In contrast, when the cells were exposed to an increase in the external potassium (K+) concentration of 4 to 12 mM during 1 min of perifusion, the amount of 40Ca2+ effluxed was 16 +/- 5 nmol/10(8) cells. These results indicate that AII induces an increase in the Ca2+ concentration in a local domain outside of the plasma membrane. This Ca2+ comes from AII-induced intracellular Ca2+ depletion and may play a role in refilling intracellular Ca2+ stores.

Is ouabain produced by the adrenal gland?

1. Ouabain or a related stereoisomer, termed endogenous ouabain, has been identified in adrenal cortex tissue and culture medium from adrenocortical cells. 2. Angiotensin II and adrenocorticotropin, the main activators of aldosterone secretion from adrenal glomerulosa cells appear to increase the production of this compound. 3. The purpose of this review is to briefly discuss recent available experimental evidence suggesting that endogenous ouabain is secreted by the zona glomerulosa of the adrenal gland.

Recent progress in understanding aldosterone secretion

1. The synthesis and secretion of aldosterone in the adrenal zona glomerulosa in physiologic conditions is controlled by adrenocorticotropin (ACTH), angiotensin II (AII), and extracellular (K+). 2. ACTH effects on aldosterone output are explained by cyclic AMP-(cAMP)- and Ca(2+)-dependent mechanisms. 3. All effects on aldosterone secretion are initiated by an increase in Ca2+ influx through hormone-operated Ca2+ channels and G-protein- and phospholipase C-(PLC) dependent hydrolysis of phosphoinositides leading to the generation of inositol 1,4,5 trisphosphate (IP3) and DAG that induce intracellular Ca2+ release and PKC activation, respectively. 4. ACTH increases DAG formation with marginal or undetectable IP3 generation. The effect of ACTH on DAG levels is discussed. 5. The requirement of external Ca2+ in PLC activation and aldosterone secretion also is discussed.

Effects of dihydropyridines on aldosterone secretion in adrenal capsule preparations from pregnant rats

The primary aim of this study was to determine when sensitivity in the aldosterone response to extracellular potassium (K+) decreases during pregnancy. Second, it tested the hypothesis that calcium channel alterations occur in the adrenal cortex during pregnancy. The decreased sensitivity to K+, observed at 22 days of gestation, was not evident at 15 days and between 18 and 36 h postpartum. Increases in extracellular calcium concentration heightened sensitivity to K+ in adrenal capsule preparations derived from nonpregnant rats but had no effect in pregnant animals. The influence of nifedipine and BAY K 8644 (blocker and activator, respectively, of voltage-operated calcium channels) on the aldosterone response to K+ and to adrenocorticotropic hormone (ACTH) was studied. Sensitivity to K+ in nonpregnant rats decreased in the presence of nifedipine and became similar to that in pregnant rats. Responses to ACTH were not affected by nifedipine. BAY K 8644 produced a larger increase in sensitivity in adrenal capsule preparations from pregnant than from nonpregnant rats, leading to superposition of the two dose-response curves to K+. These results indicate that voltage-operated calcium channels involved in aldosterone secretion are functionally impaired during pregnancy.

Evidence for a tonic inhibitory role of nifedipine-sensitive calcium channels in aldosterone biosynthesis

This study investigated the effects of the calcium channel blockers nifedipine (a dihydropyridine) and verapamil (a papaverine derivative), on aldosterone production utilizing isolation of the early and late phases of aldosterone biosynthesis. Pregnenolone production (the early phase of aldosterone biosynthesis) was assessed in trilostane-treated bovine glomerulosa cells, used to inhibit the conversion of pregnenolone onwards to aldosterone. Conversion of exogenous corticosterone to aldosterone, an index of late phase activity, was assessed using aminoglutethimide to inhibit endogenous aldosterone production. Low concentrations of nifedipine, 10(-11)-10(-9) M, stimulated basal total aldosterone biosynthesis by enhancing the late phase although the early phase was inhibited. In the presence of 12 mM potassium (K+), which is less effective in stimulating aldosterone production than lower K+ concentrations, aldosterone production was enhanced by nifedipine, 10(-8) M, by an effect on the late phase. At K+ 6 and 8 mM, nifedipine, 10(-4) M, inhibited the early phase. Nifedipine 10(-5) inhibited angiotensin II (AII)-stimulated total aldosterone biosynthesis by independent effects on the early and late phases. Verapamil, 10(-4) M, inhibited total and early phase aldosterone production at K+, 4 mM and inhibited both phases at K+, 8 mM, stimulation was not observed using verapamil. Verapamil, 10(-4) M, also inhibited AII-stimulated aldosterone production. Basal and AII-stimulated pregnenolone production were inhibited by verapamil, 10(-4) M (basal) and 10(-6) M (AII-stimulated). These studies using nifedipine have revealed subtle calcium-dependent mechanisms involved in the tonic inhibition of activity in the late phase of aldosterone biosynthesis and the reversal of the inhibitory effect of high K+ concentrations also on the late phase. In addition, the data reported indicate that both AII and K+ independently enhance activity in the early and late phases of aldosterone production by calcium-dependent mechanisms.

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.

Angiotensin II increases diacylglycerol in calf adrenal glomerulosa cells by activating de novo phospholipid synthesis

Effects of angiotensin II (AII) on diacylglycerol (DAG) synthesis were examined in calf adrenal glomerulosa cells. AII provoked rapid increases in [3H]glycerol-labeling and content of DAG. Effects on [3H]glycerol-labeling of DAG were observed both in cells prelabeled with [3H]glycerol for 60 minutes, and when AII and [3H]glycerol were added simultaneously. Increases in [3H] DAG labeling were associated with increases in total glycerolipid labeling, and in simultaneous addition experiments, were preceded by increased [3H] phosphatidic acid (PA) labeling. Labeling of glycerol-3-PO4, on the other hand, was not increased by AII, suggesting that increases in lipid labeling were not due to prior increases in precursor specific activity. ACTH, which does not increase the hydrolysis of inositol-phospholipids appreciably in this tissue, provoked increases in content and [3H]glycerol-labeling of DAG, which were only slightly less than those provoked by AII. Thus, part of the AII-induced increase in DAG may also be derived from sources other than inositol-phospholipids. Moreover, AII-induced increases in DAG appear to be at least partly derived from increased de novo synthesis of PA.

Ca channels in adrenal glomerulosa cells: K+ and angiotensin II increase T-type Ca channel current

Ca channel currents were studied in freshly dispersed bovine adrenal glomerulosa cells to better understand the control of aldosterone secretion by extracellular K concentration (Ko) and angiotensin II (AII). The whole-cell variation of the patch voltage clamp technique was used. Two types of Ca channels were found. One type is similar to the "T-type" Ca channels found in many excitable cells. These channels deactivate slowly (tau approximately equal to 7 ms at -75 mV) and inactivate rapidly during strong depolarizations. The second channel type activates and inactivates at more positive potentials than the T-type Ca channels and deactivates rapidly. These channels are similar to the "L-type" Ca channels found in muscle and nerve. Our studies provide three reasons for concluding that T-type Ca channels have an important role in mediating stimulus-secretion coupling in response to high K+ or AII: (i) aldosterone secretion and steady-state current through T-type Ca channels are biphasic functions of Ko and both increase in parallel for Ko = 2-10 mM; (ii) nitrendipine blocks the T-type Ca channels and the stimulation of aldosterone secretion by high K+ or AII with similar potency; (iii) AII increases Ca entry through the T-type Ca channels.

Spontaneous and ACTH-induced interrenal steroidogenesis in juvenile coho salmon (Oncorhynchus kisutch): effects of monovalent ions and osmolality in vitro

We determined the in vitro effects of changes in extracellular monovalent ion levels and osmotic pressure on the spontaneous and adrenocorticotropin (ACTH)-stimulated interrenal activity of coho salmon (Oncorhynchus kisutch). We used a perifusion system of incubation and monitored interrenal activity by measuring the effluent cortisol content with a radioimmunoassay. An increase in the medium osmolality with mannitol, from 206 to 290 or 353 mosmol, caused an increase in the spontaneous release of cortisol only slightly (compared with the much greater increase induced by porcine-ACTH). A similar minor increase was observed when NaCl was elevated from 130 to 180 mM. On the other hand, the spontaneous release of cortisol was not affected by increasing the KCl level from 3.2 to 9.6 mM, but was clearly increased when KCl was raised from 3.2 mM to a supraphysiological level of 27.2 mM. Ionic or osmolality changes, within the physiological range observed in coho salmon plasma, did not affect the characteristics of interrenal secretion of cortisol in response to porcine-ACTH. If our results with interrenal cells in vitro are representative of the basic functioning of the cells in vivo, then one would have to conclude that changes in concentrations of plasma monovalent ions or in osmotic pressure may not play a significant physiological role in the regulation of interrenal steroidogenesis or corticosteroid release in coho salmon.

Diazepam inhibits potassium-induced aldosterone secretion in adrenal glomerulosa cell

In an attempt to elucidate a possible role of peripheral benzodiazepine receptor in adrenal glomerulosa cell, effect of diazepam on potassium-induced aldosterone secretion was studied using isolated bovine adrenal glomerulosa cell. Diazepam inhibited aldosterone secretion stimulated by 8mM potassium in a dose dependent manner. The ID50 was approximately 14 nM. Although diazepam inhibited potassium action effectively, forskolin-induced aldosterone secretion was not affected by diazepam. These results indicate that peripheral benzodiazepine receptor may have an active role in regulating aldosterone secretion. The voltage dependent calcium channel may be a possible site of benzodiazepine action in this tissue.

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)

Effect of angiotensin II, ATP, and ionophore A23187 on potassium efflux in adrenal glomerulosa cells

Angiotensin II stimulus on perifused bovine adrenal glomerulosa cells elicited an increase in 86Rb efflux from cells previously equilibrated with the radioisotope. When 45Ca fluxes were measured under similar conditions, it was observed that Ca and Rb effluxes occurred within the first 30 s of the addition of the hormone and were independent of the presence of external Ca. The 86Rb efflux due to angiotensin II was inhibited by quinine and apamin. The hypothesis that the angiotensin II response is a consequence of an increase in the K permeability of the glomerulosa cell membrane triggered by an increase in cytosolic Ca is supported by the finding that the divalent cation ionophore A23187 also initiated 86Rb or K loss (as measured by an external K electrode). This increased K conductance was also seen with 10(-4) M ATP. Quinine and apamin greatly reduced the effect of ATP or A23187 on 86Rb or K release in adrenal glomerulosa cells. The results suggest that Ca-dependent K channels or carriers are present in the membranes of bovine adrenal glomerulosa cells and are sensitive to hormonal stimulus.

Mechanism of inhibitory action of TMB-8 [8-(NN-diethylamino)octyl-3,4,5-trimethoxybenzoate] on aldosterone secretion in adrenal glomerulosa cells

The mechanism of 8-(NN-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) action was evaluated in isolated adrenal glomerulosa cells. TMB-8 inhibits both angiotensin II- and K+-stimulated aldosterone secretion in a dose-dependent manner. The ID50 for angiotensin II- and K+-stimulated aldosterone secretion is 46 and 28 microM, respectively. In spite of the fact that 100 microM-TMB-8 inhibits angiotensin II-stimulated aldosterone secretion almost completely, TMB-8 (100 microM) does not inhibit angiotensin II-induced 45Ca2+ efflux from prelabelled cells nor does it affect inositol 1,4,5-trisphosphate-induced calcium release from non-mitochondrial pool(s) in saponin-permeabilized cells. TMB-8 has no inhibitory effect on A23187-induced aldosterone secretion, but 12-O-tetradecanoylphorbol 13-acetate-induced aldosterone secretion is completely abolished. TMB-8 effectively inhibits both angiotensin II- and K+-induced increases in calcium influx but has no effect on A23187-induced calcium influx. TMB-8 inhibits the activity of protein kinase C dose-dependently. These results indicate that TMB-8 inhibits aldosterone secretion without inhibiting mobilization of calcium from an intracellular pool. The inhibitory effect of TMB-8 is due largely to an inhibition of plasma membrane calcium influx, but this drug also inhibits the activity of protein kinase C directly.

Intracellular calcium and adenosine 3',5'-cyclic monophosphate as mediators of potassium-induced aldosterone secretion

We compared the action of K+ on aldosterone secretion from isolated bovine adrenal glomerulosa cells with that of ionophore A23187. Addition of either 50 nM-A23187 or 8 mM-K+ to perifused cells induces a similar initial aldosterone-secretory responses, and a similar sustained increases in Ca2+ entry. However, K+-induced secretion is more sustained than is A23187-induced secretion, even though each agonist appears to act by increasing Ca2+ entry into the cells. When [3H]inositol-labelled cells are stimulated by 8 mM-K+, a small decrease in phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] is observed. This decrease is not accompanied by an increase in inositol trisphosphate (InsP3) concentration. Also, if [3H]arachidonic acid-labelled cells are exposed to 8 mM-K+, there is no increase in [3H]diacylglycerol production. When [3H]inositol-labelled cells are stimulated by 50 nM-A23187, a small decrease in PtdIns(4,5)P2 is observed. This decrease is not accompanied by an increase in InsP3. The cyclic AMP content of K+-treated cells was approximately twice that in A23187-treated cells. If cells are perifused simultaneously with 50 nM-forskolin and 50 nM-A23187, the initial aldosterone-secretory response is similar to that induced by A23187 alone, and the response is sustained rather than transient, and is similar to that seen during perifusion of cells with 8 mM-K+. This dose of forskolin (50 nM) causes an elevation of cyclic AMP concentration in A23187-treated cells, to a value similar to that in K+-treated cells. These results indicate that, in K+-treated cells, a rise in cyclic AMP content serves as a positive sensitivity modulator of the Ca2+ message, and plays a key role in mediating the sustained aldosterone-secretory response.

Effects of ANG II and K+ on Ca efflux and aldosterone production in adrenal glomerulosa cells

A comparison was made of the effects of angiotensin II (ANG II) and K+ on aldosterone secretion and calcium efflux from porcine adrenal glomerulosa cells. In the presence of 1.25 mM Ca2+ in the perifusion medium, both 1 x 10(-9) M ANG II and 12 mM K+ caused a 3-fold increase in rate of aldosterone secretion. ANG II caused a 3.5-fold increase in the fractional efflux ratios of radiocalcium from cells prelabeled with 45Ca, but K+ caused only a 1.5-fold increase. When the perifusate contained no Ca2+ or the Ca2+ was replaced by 0.6 mM Sr2+, the effects of K+ on both 45Ca efflux and aldosterone production were abolished. On the other hand, ANG II still caused approximately the same increase in the fractional efflux ratio of radiocalcium and 25-50% of the normal increase in aldosterone production rate. When the perifusate contained 1.25 mM calcium and 25 microM dantrolene, K+ produced the same 1.5-fold increase in calcium efflux and the same maximal rate of aldosterone production as seen in the absence of dantrolene. In contrast, dantrolene greatly inhibited ANG II-induced increase in the fractional efflux ratio of calcium, 1.5-fold compared with 3.5-fold in the absence of dantrolene. Likewise, dantrolene delayed the onset of the ANG II-induced increase in aldosterone secretion and reduced the maximal rate of secretion to 50%. Cells incubated with both dantrolene and media containing no Ca2+ or containing Sr2+ in place of Ca2+ showed no response to ANG II either in terms of calcium efflux or aldosterone production.(ABSTRACT TRUNCATED AT 250 WORDS)

Dihydropyridine calcium agonist and antagonist effects on aldosterone secretion

The effects of the dihydropyridine calcium antagonists nimodipine, nitrendipine, and nisoldipine were studied on K+-induced and angiotensin II (ANG II)-induced aldosterone secretion from isolated calf adrenal glomerulosa cells. These drugs were more effective inhibitors of K+-induced secretion than ANG II-induced secretion. However, when the ANG II-induced release of intracellular calcium was blocked by prior treatment of cells with dantrolene, then nitrendipine was equally as effective in blocking ANG II- as K+-induced secretion. On the other hand, the dihydropyridine agonist, BAY K8644, was found to enhance K+-induced secretion to a greater extent than ANG II-induced secretion when the latter was studied in either the absence or presence of dantrolene. It is concluded that K+ and ANG II mobilize calcium from different sources for stimulus-response coupling in these cells and that there is no difference in the sensitivity to nitrendipine of the plasma membrane calcium channels operated by these two different agonists.

The effects of extracellular K+ and angiotensin II on cytosolic Ca++ and steroidogenesis in adrenal glomerulosa cells

We evaluated changes in cytosolic calcium concentration (Ca++) and steroidogenesis in rat adrenal glomerulosa cells (GC) stimulated with potassium (K+) or angiotensin II (AII). Cytosolic Ca++ concentration was determined using the Ca++-sensitive, fluorescent dye QUIN 2. Raising extracellular K+ increased cytosolic Ca++ from 267 +/- 23 nM at 3.7 mM K+ to a maximum of 377 +/- 40 nM at 8.7 mM K+ (p less than 0.01, N = 23). AII also increased cytosolic Ca++ from 238 +/- 20 nM to a maximum of 427 +/- 42 nM at 10(-7) M (p less than 0.01, N = 16). In parallel studies, K+ and AII stimulated aldosterone secretion from QUIN 2-loaded GC at concentrations similar to those which raised cytosolic Ca++. QUIN 2-loaded cells were as responsive steroidogenically as unloaded cells and showed trypan blue exclusion of 98% suggesting that QUIN 2 did not compromise cellular viability. These results provide direct support for a role of cytosolic Ca++ as a second messenger during stimulation of aldosterone secretion by both K+ and AII.

Aldosterone secretion, measurements of membrane potential and intracellular potassium activity in the isolated adrenal zone glomerulosa

Cell membrane potential and intracellular potassium activity (microelectrodes filled with ion-sensitive liquid ion exchanger) were measured in the zona glomerulosa of superfused hemi-adrenals of rats kept on different diets. Simultaneously, samples of the superfusate were collected and analyzed by radioimmunoassay for aldosterone content. Cell membrane potential and intracellular potassium activity were not influenced by high sodium, low sodium or high potassium diet. However, aldosterone secretion significantly changed. These results suggest that membrane potential and intracellular potassium activity per se may not be linked to changes in aldosterone secretion.