Calcium-binding proteins and insulin release

Calcium and cAMP are interdependent regulators of glucose-induced insulin release. In the present study we investigated the importance of cAMP and calcium-binding proteins for biphasic insulin secretion by assessing the effects of two phenothiazines known to block such proteins, trifluoroperazine (TFP) and promethazine (PMZ). In isolated rat islets, during 60-min incubations with 16.7 mmol/l glucose both agents inhibited the insulin response with ID50 values of 15 μmol/l for TFP and 5 μmol/l for PMZ. Both agents decreased the maximal insulin response without gross changes in the islet sensitivity to glucose. TFP (15 μmol/l), whereas inducing 50% inhibition of second-phase insulin release, totally suppressed the cAMP response to glucose and the accompanying first-phase insulin secretion (5-min incubations); these effects of TFP could be partially reversed by isobutyl methylxanthine (IBMX). In contrast, 5 μmol/l PMZ, which produced 60% inhibition of second-phase insulin release, had no effect on first-phase insulin and cAMP responses to glucose. Furthermore, IBMX did not modify the inhibitory effect of PMZ on second-phase insulin secretion. The following is concluded: 1. TFP acts preferentially on first-phase insulin release and inhibits cAMP formation; this suggests that calmodulin plays a major role in mediating the initial glucose effect on secretion via stimulation of cAMP. 2. The islet probably contains calcium-sensitive proteins other than calmodulin, since the low concentrations of PMZ shown to inhibit second-phase insulin release lack effects on calmodulin. Synexin could be such a protein. 3. PMZ had no effect on cAMP generation and first-phase insulin release; it is speculated that synexin-like proteins may mediate the glucose effect on second-phase release by increasing the responsiveness of the islet to calcium/cAMP.