INHIBITION OF L-TYPE CALCIUM CHANNELS BY INTERNAL GTP [GAMMA-S] IN MOUSE PANCREATIC-BETA CELLS

Pretreatment of pancreatic beta-cells with pertussis toxin resulted in a 30% increase in peak whole-cell Ca2+ currents recorded in the absence of exogenous intracellular guanine nucleotides. Intracellular application of 90-mu-M GTP[gamma-S], by liberation from a caged precursor, resulted in 40% reduction of the peak Ca2+ current irrespective of whether the current was carried by Ca2+ or Ba2+. Effects on the delayed outward K+ current were small and restricted to a transient Ca2+-dependent K+ current component. Inhibition by GTP[gamma-S] of the Ca2+ current was not mimicked by standard GTP and could not be prevented either by pretreatment with pertussis toxin or by inclusion of GDP[beta-S] or cyclic AMP in the intracellular medium. The inhibitory effect of GTP[gamma-S] could be counteracted by a prepulse to a large depolarizing voltage. A similar effect of a depolarizing prepulse was observed in control cells with no exogenous guanine nucleotides. These observations indicate that inhibition of beta-cell Ca2+ current by G protein activation results from direct interaction with the channel and does not involve second-messenger systems. Our findings also suggest that the beta-cell Ca2+ current is subject to resting inhibition by G proteins.