Role of calcium and calmodulin in the interaction between islet cell secretion granules and plasma membranes

The effects of calcium and calmodulin on the binding of islet secretion granules to plasma membrane vesicles was studied. Free Ca++ (10(-7) to 10(-4) M) produced a concentration-dependent increase in the binding of inside-out. 125I-labeled plasma membrane vesicles to secretion granules; maximum stimulation occurred at 10(-5) M. By contrast, Ca++ had no effect on binding when either inside-out red blood cell or right-side-out islet plasma membrane vesicles were used. Calmodulin (1 microM), in the presence of 10(-4) M Ca++, increased the binding two-fold as compared to Ca++ alone, and trifluoperazine (TFP), and inhibitor of calmodulin, produced a concentration-dependent inhibition of the calmodulin effect. TFP also inhibited the binding produced by 10(-4) M Ca++ in the absence of exogenous calmodulin. These results suggest that Ca++ in the presence of calmodulin modulates the interaction between islet secretion granules and the cytoplasmic surface of the plasma membrane.