Aspects of angiotensin action in the adrenal. Key roles for calcium and phosphatidyl inositol

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.

The effects of ACTH, serotonin, K+ and angiotensin analogues on 32P incorporation into phospholipids of the rat adrenal cortex: basis for an assay method using zona glomerulosa cells

The effects of various concentrations of serotonin, ACTH, K+, angiotensin II (AII), angiotensin III (AIII) and [Sar1]angiotensin II (SAII) on steroidogenesis and the incorporation of 32P (after preincubation to near equilibrium with the ATP pool) into phosphatidylinositol (PI), phosphatidic acid (PA) and phosphatidylcholine (PC) in a preparation of capsular cells from rat adrenals, consisting of 95% zona glomerulosa (z.g.) and 5% zona fasciculata plus reticularis (z.f.r.) cells, were investigated. Serotonin and ACTH stimulated steroidogenesis in the usual manner but had little or no effect on 32P incorporation into any of the three phospholipids. However, AII, AIII and SAII stimulated steroidogenesis and also 32P incorporation into PA and PI (maximally to about 280% of control values) but not into PC. These results taken together with other data on effects on the cAMP output and Ca2+ fluxes of z.g. cells suggest that stimulation by ACTH and serotonin is mediated by cAMP as second messenger. However, the angiotensins probably act through Ca2+, with associated changes in phospholipid metabolism. The 32P incorporation into PA as a function of lg concentration of AII was linear and showed a reasonable index of precision (0.36 +/- 0.03, eight experiments, 0.23 +/- 0.02 for a further eight experiments) and correlation with steroidogenesis. The corresponding incorporation into PI showed a maximum effect and a much poorer index of precision (1.02 +/- 0.30 (4.69 +/- 3.7] over the same full range of AII concentration used. The effects of AIII and SAII showed similar characteristics for 32P incorporation into both PA and PI, but, as for stimulation of steroidogenesis, at higher concentrations for AIII than for AII. The effects of different doses of AII, AIII and ACTH on the corticosterone output and 32P incorporation into PA, PI and PC of a preparation of cells, consisting of more than 98% z.f.r. cells, from rat decapsulated adrenals were also studied. ACTH, at low doses, which nevertheless markedly stimulated corticosterone output, had a small (maximally to about 125% of control values) but significant effect on 32P incorporation into PA, PI and PC. The maximum effect was usually at about 10(-10) M ACTH and was not significant at 10(-8) M.