The control of 86Rb efflux from rat isolated pancreatic islets by the sulphonylureas tolbutamide and glibenclamide.

Abstract

The efflux of 86Rb from rat isolated pancreatic islets preloaded with the isotope and perifused in vitro, has been used to monitor the effects of sulphonylureas on the potassium permeability, Pk, of pancreatic beta-cells. Tolbutamide (5 microM to 5 mM) had a dual effect, causing initially a decrease in 86Rb efflux (the 'on' response) which was rapidly superseded on drug removal by a large phasic increase in 86Rb efflux (the 'off' response). Each kinetic response had a different dose-dependency: the 'on' response was half-maximal at tolbutamide concentrations of 0.02 mM, maximal at 0.2 mM and decreased by concentrations greater than 0.2 mM whereas the 'off' response was half-maximal at 0.07 mM, maximal at 0.7 mM, with further increases in concentration (up to 5 mM) causing no further change in magnitude. Analysis of the time- and concentration-dependency of tolbutamide action, by presenting increasing concentrations (0 to 1.4 mM) of tolbutamide as a ramp or step function, established a critical dependence of the kinetics of 86Rb efflux during and after exposure to tolbutamide upon the initial rate of increase of the tolbutamide concentration rather than its final steady state. In the presence of quinine (10 microM), D600 (50 microM), or tetraethylammonium (20 mM), the secondary increase in 86Rb following tolbutamide (0.7 mM) removal was totally inhibited. Co2+ (2.56 mM) not only blocked the secondary 'off' response but also potentiated the initial 'on' response of tolbutamide. Glibenclamide produced a rapid decrease in 86Rb efflux but at a much lower concentration (10 microM) than tolbutamide and with no 'off' response apparent over a wide range of concentration (1 to 100 microM); moreover the decrease in 86Rb efflux was sustained and only slowly reversible. It is concluded that tolbutamide has two opposing actions on islet beta-cell 86Rb efflux, and therefore PK: (i) a tendency to increase a calcium-sensitive PK by stimulating calcium entry into the cell and (ii) a decrease in PK that may be due to a direct effect on the calcium-sensitive PK itself. The more sustained pharmacological action of glibenclamide is explained by the longer-lasting decrease in PK that it produces.