Phospholipase A activity was determined in homogenates and subcellular fractions of trypsin-dispersed cat adrenocortical cells. At pH 7.4 homogenate phospholipid hydrolysis was activated by added Ca2+ and inhibited by EGTA. Phospholipid degradation in the presence and absence of Synacthen was completely blocked by EGTA. Ca2+-dependent activation of a membrane-bound phospholipase may be a critical control mechanism for regulating the molecular changes taking place during stimulation by Synacthen.

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