Studies on the stimulation by Ca2+ and the inhibition by ADP of steroid 11beta-hydroxylation in adrenal mitochondria

Suppression of aldosterone synthesis and secretion by ca(2+) channel antagonists

Aldosterone, a specific mineralocorticoid receptor (MR) agonist and a key player in the development of hypertension, is synthesized as a final product of renin-angiotensin-aldosterone system. Hypertension can be generally treated by negating the effects of angiotensin II through the use of angiotensin-converting enzyme inhibitors (ACE-Is) or angiotensin II type 1 receptor antagonists (ARBs). However, the efficacy of angiotensin II blockade by such drugs is sometimes diminished by the so-called "aldosterone breakthrough" effect, by which ACE-Is or ARBs (renin-angiotensin system (RAS) inhibitors) gradually lose their effectiveness against hypertension due to the overproduction of aldosterone, known as primary aldosteronism. Although MR antagonists are used to antagonize the effects of aldosterone, these drugs may, however, give rise to life-threatening adverse actions, such as hyperkalemia, particularly when used in conjunction with RAS inhibitors. Recently, several groups have reported that some dihydropyridine Ca(2+) channel blockers (CCBs) have inhibitory actions on aldosterone production in in vitro and in the clinical setting. Therefore, the use of such dihydropyridine CCBs to treat aldosterone-related hypertension may prove beneficial to circumvent such therapeutic problems. In this paper, we discuss the mechanism of action of CCBs on aldosterone production and clinical perspectives for CCB use to inhibit MR activity in hypertensive patients.

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.

The stimulation by calcium and its inhibition by ADP of cholesterol side-chain cleavage activity in adrenal mitochondria

Cholesterol side-chain cleavage activity (cholesterol SCC) in a mitochondrial preparation is increased by calcium in a sigmoidal manner. A 5-10-fold increase is obtained and an effect may be seen at 20 microM CaCl2. ADP inhibits the stimulation by calcium with a shift of the sigmoid curve to the right and 10 microM ADP results in a 4-fold increase in the amount of CaCl2 required to obtain one-half the maximal stimulation value. The inhibition is specific for ADP and inhibition by ATP is due to the formation of ADP. The characteristics of the calcium-ADP modulation are such that a suitable ADP-inhibited level of cholesterol sidechain cleavage activity will be stimulated by a given increment of calcium to a greater extent than in the absence of the added ADP. Steroid 11 beta-hydroxylation is also stimulated in a sigmoidal manner by calcium and this stimulation is inhibited by ADP. The 11 beta-hydroxylation, however, is less sensitive to calcium and ADP so that changes in cholesterol SCC are obtained at concentration of calcium and ADP where minimal effects on 11 beta-hydroxylation are found. Calmodulin-like activity is present in the mitochondrial preparation. No evidence, however, for a role for calmodulin in the calcium-ADP effects could be obtained, but the possibility of its involvement cannot be excluded. The calcium-ADP modulations are of a magnitude and take place at sufficiently low concentrations to suggest a physiological role in the regulation of mitochondrial steroidogenesis.