Inhibition of L-type calcium channels by internal GTP [γS] in mouse pancreatic β cells

Abstract

Pretreatment of pancreatic β 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 μM GTP[γS], 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[γS] 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[βS] or cyclic AMP in the intracellular medium. The inhibitory effect of GTP[γS] 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 β 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 β cell Ca²⁺ current is subject to resting inhibition by G proteins.