22Na+ Efflux from Normal and ob/ob Mouse Islets of Langerhans
Previous studies have produced differing results for the effect of glucose on 22Na+ efflux from islets. Interpretation of 22Na+ efflux data is difficult due to compartmentalization and the high concentration of Na+ in the extracellular space. An attempt has been made to resolve the problem using a triple-label method, with tracer loading in the perifusion chamber. This method was used to compare 22Na+ efflux rate in normal and ob/ob mouse islets. 3H-sucrose was used as an extracellular space marker and 86Rb+ as a ‘control’ isotope, representing K+, and present predominantly in the intracellular compartment, in addition to 22Na+. 3H-sucrose efflux showed two rate constants: one due to the chamber (about 2 min-1; confirmed by blank chamber experiments) and one due to the islet extracellular space (about 0.3 min-1). The efflux of 22Na+ and 86Rb+ could each be described by three rate constants, the first two being equivalent to the sucrose rate constants and the third being due to efflux across the cell membrane. In the absence of glucose, the 22Na+ rate constant was 0.06 min-1 and the 86Rb+ rate constant was 0.03 min-1. Glucose had no significant effect on 22Na+ efflux from normal or ob/ob islets but decreased 86Rb+ efflux as expected. Permeabilities (Pj) of Na+ and Rb+ were calculated from steady-state efflux data and membrane potentials.