Phenytoin Inhibition of Insulin Release: Studies on the Involvement of Ca²⁺ Fluxes in Rat Pancreatic Islets Free

The mechanism by which phenytoin inhibits insulin release was studied. Insulin release and ⁴⁵Ca²⁺ efflux were measured during perifusion of collagenase isolated rat islets after 2-day maintenance in tissue culture in the presence of a trace amount of ⁴⁵Ca²⁺. Islets maintained in the absence of the isotope were used to measure ⁴⁵Ca²⁺ uptake over 5 min. Glucose (16.7 mM) induced a biphasic release of insulin, which was accompanied by a biphasic increase in the rate of ⁴⁵Ca²⁺ efflux above basal. Phenytoin (80 μM) added during second phase rapidly inhibited insulin release. In contrast, phenytoin added together with glucose failed to affect first phase, but reduced second phase release by 64%. Phenytoin failed to affect basal ⁴⁵Ca²⁺ efflux in the presence or absence of Ca²⁺, nor did it interfere with the inhibitory effect of high glucose on Ca²⁺ efflux. The drug did not affect basal Ca²⁺ uptake, but significantly reduced glucose-induced Ca²⁺ uptake. Glucose utilization was not inhibited by phenytoin. It is suggested that phenytoin inhibits glucose-stimulated insulin release by interfering with Ca²⁺ uptake via voltage-dependent Ca²⁺ channels. The pattern of inhibition of insulin release appears to favor this conclusion as the second phase is more dependent on the stimulation of Ca²⁺ uptake than is the first phase.