Unique calcium dependencies of the activating mechanism of the early and late aldosterone biosynthetic pathways in the rat

Third-generation Mineralocorticoid Receptor Antagonists: Why Do We Need a Fourth?

The first mineralocorticoid receptor (MR) antagonist, spironolactone, was developed almost 60 years ago to treat primary aldosteronism and pathological edema. Its use waned in part because of its lack of selectivity. Subsequently, knowledge of the scope of MR function was expanded along with clinical evidence of the therapeutic importance of MR antagonists to prevent the ravages of inappropriate MR activation. Forty-two years elapsed between the first and MR-selective second generation of MR antagonists. Fifteen years later, despite serious shortcomings of the existing antagonists, a third-generation antagonist has yet to be marketed. Progress has been slowed by the lack of appreciation of the large variety of cell types that express the MR and its diverse cell-type-specific actions, and also its unique complex interaction actions at the molecular level. New MR antagonists should preferentially target the inflammatory and fibrotic effects of MR and perhaps its excitatory effects on sympathetic nervous system, but not the renal tubular epithelium or neurons of the cortex and hippocampus. This review briefly describes efforts to develop a third-generation MR antagonist and why fourth generation antagonists and selective agonists based on structural determinants of tissue and ligand-specific MR activation should be contemplated.

Role of voltage-gated calcium channels in potassium-stimulated aldosterone secretion from rat adrenal zona glomerulosa cells

The mineralocorticoid aldosterone plays an important role in the regulation of plasma electrolyte homeostasis. Exposure of acutely isolated rat adrenal zona glomerulosa cells to elevated K(+) activates voltage-gated calcium channels and initiates a calcium-dependent increase in aldosterone synthesis. We developed a novel 96-well format aldosterone secretion assay to rapidly evaluate the effect of known T- and L-type calcium channel antagonists on K(+)-stimulated aldosterone secretion and better define the role of voltage-gated calcium channels in this process. Reported T-type antagonists, mibefradil and Ni(2+), and selected L-type antagonist dihydropyridines, inhibited K(+)-stimulated aldosterone synthesis. Dihydropyridine-mediated inhibition occurred at concentrations which had no effect on rat alpha1H T-type Ca(2+) currents. In contrast, below 10 microM, the L-type antagonists verapamil and diltiazem showed only minimal inhibitory effects. To examine the selectivity of the calcium channel antagonist-mediated inhibition, we established an aldosterone secretion assay in which 8Br-cAMP stimulates aldosterone secretion independent of extracellular calcium. Mibefradil remained inhibitory in this assay, while the dihydropyridines had only limited effects. Taken together, these data demonstrate a role for the L-type calcium channel in K(+)-stimulated aldosterone secretion. Further, they confirm the need for selective T-type calcium channel antagonists to better address the role of T-type channels in K(+)-stimulated aldosterone secretion.

Insights into the mechanism by which inhibition of Na,K-ATPase stimulates aldosterone production

Inhibition of Na,K-adenosine triphosphatase (Na,K-ATPase) activity by ouabain has been shown to increase the release of aldosterone from rat glomerulosa cells, but the mechanism by which this elevation of aldosterone production occurs has not been established. Small changes in membrane potential can significantly affect aldosterone release. Consequently, inhibition of Na,K-ATPase in glomerulosa cells may stimulate aldosterone production by membrane depolarization. If so, ouabain-stimulated production should be dependent on calcium influx through voltage-gated calcium channels. It has previously been shown that ouabain induces a moderately rapid increase in cytosolic calcium in rat glomerulosa cells. Therefore, in this study, we test whether ouabain stimulates aldosterone production with a time course consistent with early membrane depolarization as suggested by the previously reported early increase in cytosolic calcium. To study the time course of aldosterone production, we developed a perfusion technique that allows an examination of the initial effects of ouabain on aldosterone production. The results show that ouabain rapidly stimulates aldosterone production. Continuous perfusion with 0.25 or 1 mmol/L ouabain induced a brisk, robust increase in aldosterone production, followed by a decrease to near baseline over 60 minutes. Ouabain-stimulated aldosterone production was dependent on the presence of extracellular calcium and calcium influx through voltage-gated calcium channels. Our results support the hypothesis that the inhibition of Na,K-ATPase in rat adrenal glomerulosa cells immediately depolarizes the membrane potential and opens voltage-gated calcium channels.

Rapid modulation of renal and adrenal responsiveness to angiotensin II

Reciprocal changes in adrenal and vascular responsiveness to angiotensin II (Ang II) are part of the normal adaptation to shifts in salt intake. When dietary salt intake is abruptly reduced from high to low, enhancement in aldosterone secretion requires several days to develop. Once established it is not known how quickly the enhancement is reversed with salt repletion. We investigated the time course and relative contributions of salt, volume expansion, or both to this process by studying 15 normotensive subjects; 5 were studied during both high-salt and low-salt balance, and 10 were studied only in low-salt balance. For rapid volume expansion to reverse low-salt balance, 5 subjects received in random order an infusion of normal saline or dextran. The adrenal glomerulosa and renal vascular responses to Ang II were assessed after each volume expansion maneuver. Saline and dextran infusions suppressed plasma renin activity and aldosterone equally, although dextran acted more slowly. Both also increased renal perfusion and renal vascular and pressor responses to Ang II, which in 3 to 7 hours became identical to responses seen during high-salt intake ("modulation"). Saline infusion also blunted adrenal responsiveness to Ang II during that same interval. Despite suppression of the renin-angiotensin system by dextran infusion, aldosterone responsiveness to Ang II remained enhanced. These observations suggest that the renal and vascular responses to Ang II are modulated rapidly by the effects of volume expansion per se.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldosterone secretion and the mechanism of potassium adaptation in rats

In order to better understand the cellular mechanism of potassium (K+) adaptation, the sensitivity of aldosterone secretion to acute changes in extracellular K+ concentration was studied in freshly dissected adrenal capsules of rats adapted to diets of high or low K+ content, of rats adapted to low or high sodium (Na+) diets, and also of control rats. In control tissues, the aldosterone secretion from the capsules of an individual animal averages 0.44 +/- 0.05 nmol/h, increasing 4.4-fold between 2 and 8 mM K+ but decreasing between 8 and 10 mM K+. Although a high K+ diet increases aldosterone secretion by only 34% at 4 mM K+, the rate of secretion increases 3.7-fold more steeply than control as the K+ concentration increases. This change is equivalent to a parallel 3.1-fold increase in the effective number of T- and L-type calcium (Ca2+) channels, accompanied by a 1.3-fold increase in the K(+)-insensitive rate of aldosterone secretion. In contrast, after Na+ restriction, aldosterone secretion is about 3 times the control rate for all K+ concentrations tested, equivalent to an increase in the basal rate and the effective number of L-channels. Thus, the alteration in the number of effective T-channels is specific to diets of increased K+ content, not simply an effect of increased secretory capacity. After a low K+ diet, aldosterone secretion is 18% of control at 4 mM K+ and changes little with the K+ concentration, consistent with a 94% to 96% decrease in the effective number of T- and L-channels plus a 77% decrease in the K(+)-insensitive rate of aldosterone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Dantrolene inhibits adrenal steroidogenesis by a mechanism independent of effects on stored calcium release

The muscle relaxant dantrolene has been widely used in signal transduction studies as an inhibitor of intracellular calcium release. However, in vivo studies have shown that the drug may inhibit steroidogenesis by a mechanism which is distinct from its effects on calcium mobilization. Using freshly isolated cells and mitochondria from the outermost regions of bovine adrenal cortex we have shown that dantrolene (0.2 mM) significantly inhibits steroid synthesis stimulated by either angiotensin II (AII) or by addition of various precursors. Our results suggest that dantrolene inhibits the rate-limiting steps of adrenocortical steroidogenesis, i.e. the intramitochondrial conversion of cholesterol to pregnenolone (for both aldosterone and cortisol) and the conversion of corticosterone to aldosterone (for aldosterone), by a mechanism independent from its known effects on calcium release. A possible alternative mechanism may involve direct inhibition of cytochrome P450-dependent hydroxylation reactions.

Evidence that angiotensin II decreases mitochondrial calcium in the glomerulosa cell

The present studies were performed using primary monolayer cultures of bovine glomerulosa cells to determine whether the elevation in cytosolic calcium concentration produced by angiotensin II was accompanied by an elevation in mitochondrial calcium. Exchangeable mitochondria calcium content was assessed indirectly by measuring the changes in cytosolic calcium concentration and calcium efflux produced by the mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Total mitochondrial calcium content was also assessed directly by atomic absorption spectroscopy. CCCP had a direct effect to promote calcium release from an oligomycin/antimycin-sensitive (mitochondrial) calcium pool in permeabilized cells. In intact cells, CCCP caused rapid reductions in cellular ATP content and the ratio of ATP to ADP. Still, its effects on calcium dynamics were exerted primarily at the mitochondrial level as evidenced by inhibition with ruthenium red, but not dantrolene. As expected, angiotensin II produced a rapid increase in calcium efflux and an equally rapid and sustained increase in cytosolic calcium concentration. Nonetheless, CCCP-stimulated elevations in cytosolic calcium concentration and calcium efflux were reduced by angiotensin II in a concentration-dependent manner. Total mitochondrial calcium content was also lower in angiotensin-treated than in control cells. These results indicate that angiotensin II causes a net decrease in mitochondrial calcium stores. On the basis of these data, it is proposed that alterations in calcium metabolism initiated by angiotensin II are exerted not only at the membrane and cytosolic levels but also at the level of the mitochondria. Changes in mitochondrial calcium dynamics may directly contribute to the regulation of mitochondrial steroidogenic enzymes by angiotensin II.

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.

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.

Effect of atrial natriuretic peptide on ACTH, dibutyryl cAMP, angiotensin II and potassium-stimulated aldosterone secretion by rat adrenal glomerulosa cells

We examined the effect of rat atrial natriuretic peptide (ANP) on ACTH, dibutyryl cAMP, angiotensin II and potassium-stimulated aldosterone secretion by dispersed rat adrenal glomerulosa cells. ANP inhibited ACTH, angiotensin II and potassium-stimulated aldosterone secretion with IC50's between 0.15-0.20 nM. Inhibition by 10 nM ANP could not be overcome with higher concentrations of these stimuli. ANP shifted the dibutyryl cAMP dose-response curve slightly to the right but did not blunt the maximal aldosterone secretory response. The sites of ANP inhibition in the aldosterone biosynthetic pathway for these stimuli were also examined. ANP inhibited activation of the cholesterol desmolase (CD) enzyme complex by ACTH, angiotensin II and potassium. Activation of the corticosterone methyl oxidase (CMO) enzyme complex by potassium was inhibited by ANP, however, activation by ACTH was not blocked. We concluded that: 1) ANP is a potent inhibitor of ACTH, angiotensin II and potassium-stimulated aldosterone secretion; 2) inhibition of ACTH stimulation is primarily due to lower cAMP levels and; 3) inhibition of angiotensin II and potassium stimulation reflects a block in the activating mechanism of the CMO and/or CD enzyme complexes, whereas CD but not CMO activation by ACTH is inhibited by ANP.