STUDIES ON THE INTRACELLULAR MECHANISM OF ACTION OF ALPHA-MELANOCYTE-STIMULATING HORMONE ON RAT ADRENAL ZONA GLOMERULOSA

The intracellular mechanisms of action of alpha-MSH in rat adrenocortical cells were examined. When rat adrenal capsule (largely glomerulosa) cells were stimulated with a range of concentrations of alpha-MSH there was significant stimulation of aldosterone secretion at 10(-10) mol/l, although cyclic AMP was not increased until high concentrations of alpha-MSH were used (10(-6) mol/l and above). However, cells incubated with ACTH showed an increase in aldosterone secretion at 10(-11) mol/l and levels of cyclic AMP were elevated at 10(-9) mol ACTH/l. When rat adrenal whole capsules were incubated with alpha-MSH, membrane-bound protein kinase C (PKC) activity was increased and cytosolic enzyme activity decreased, showing PKC activation. Stimulation with angiotensin II also induced translocation of PKC activity, but ACTH did not. When [H-3]inositol-loaded glomerulosa cells were stimulated with alpha-MSH there was significant generation of [H-3]inositol trisphosphate (IP3) at concentrations of alpha-MSH which stimulated secretion of aldosterone. Significantly increased levels of [H-3]IP3 were also measured when loaded cells were exposed to angiotensin II. ACTH did not cause any significant stimulation of [H-3]IP3 production at any concentration used. These results indicate that activation of PKC and phospholipase C is important in modulating the steroidogenic effect of alpha-MSH.