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

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.

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.

The phosphatidylinositide-Ca2+ hypothesis does not apply to the steroidogenic action of corticotropin

The hypothesis that ACTH (corticotropin) stimulates steroidogenesis by a mechanism that involves breakdown of polyphosphoinositides and increase in intracellular Ca2+ (called here the 'phosphatidylinositide-Ca2+ mechanism') was tested in Y-1 adrenal-tumour cells and in bovine fasciculata cells, by using incorporation of 32P and myo-[3H]inositol to study phospholipid metabolism, and quin-2 and fura 2 to measure intracellular Ca2+. As a positive control, we repeated experiments showing that angiotensin II stimulates glomerulosa cells by way of the phosphatidylinositide-Ca2+ mechanism, by using the same methods. With Y-1 and fasciculata cells, no change was observed in the incorporation of either of the labelled precursors into any phosphatidylinositide or into any of three major phosphoinositols, i.e. inositol phosphate, bisphosphate and trisphosphate. Moreover, no change in mass of any of these compounds was seen. No change was observed in the concentration of intracellular Ca2+ in Y-1 or fasciculata cells on addition of ACTH, by using either quin-2 or fura 2. By contrast, decreased incorporation of 32P into phosphatidylinositol bisphosphate and an increase in intracellular Ca2+ were seen when glomerulosa cells were treated with angiotensin II. It is concluded that the phosphatidylinositide-Ca2+ mechanism is not used by Y-1 adrenal or bovine fasciculata cells in the steroidogenic response to ACTH unless the mechanism is radically different from that seen with all other hormones so far tested in which this mechanism occurs.

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)