Calcium-dependent protein kinase activity and protein phosphorylation in zones of the adrenal cortex

The guinea pig adrenal cortex is composed of two chromatically distinct concentric zones. The steroidogenic response to ACTH by the two zones is likewise distinct: ACTH stimulates cholesterol side-chain cleavage activity in the outermost zone, but fails to do so in the inner zone. This despite the fact that adenylate cyclase activation by ACTH and cAMP formation are similar for the two zones. To further examine this model, protein kinase activity and protein phosphorylation have been examined. It was found that the cAMP-dependent, Ca2+/phospholipid-dependent, and Ca2+/calmodulin-dependent protein kinase activities were significantly higher in the outer zone than in the inner zone by 70, 60 and 800%, respectively. Although the physiological meaning of a zonal difference in protein kinase activity is not as yet clear, the marked difference in Ca2+/calmodulin-dependent protein kinase activity between the inner and outer zones correlates well with the marked difference in steroidogenesis that exists between the two zones. Of the Ca2+/calmodulin-dependent protein kinases known to exist, there is preliminary evidence to suggest the presence of kinase III in the guinea pig adrenal cortex. Protein phosphorylation induced by the three kinase systems in the two adrenocortical zones revealed notable differences in phosphoprotein patterns. In addition, it was found that exogenous calmodulin was phosphorylated and that the kinase responsible for this was more active in the inner zone.