CALMODULIN IS THE DIVALENT-CATION RECEPTOR FOR RAPID ENDOCYTOSIS, BUTNOT EXOCYTOSIS, IN ADRENAL CHROMAFFIN CELLS

Exocytosis and the ensuing rapid endocytosis in adrenal chromaffin cel ls are both Ca2+-dependent phenomena but differ in their divalent cati on specificity, implying distinct Ca2+ receptors for the two processes . To ascertain whether calmodulin is the Ca2+ receptor for either proc ess, we blocked its function by introducing calmodulin-binding peptide s or anti-calmodulin antibodies into these cells. Exo/endocytosis was followed by measurement of cell membrane capacitance. Rapid endocytosi s, but not exocytosis, was abolished by these treatments, indicating t hat calmodulin is the Ca2+ receptor for rapid endocytosis but is not i nvolved in exocytosis. The principal calmodulin target is not protein phosphatase-as, as antagonism of this enzyme did not inhibit but accel erated rapid endocytosis. Calmodulin may thus regulate both the rate a nd extent of rapid endocytosis by distinct pathways.