Glucose-sensitive conductances in rat pancreatic β-cells: contribution to electrical activity

Abstract

The perforated patch technique was used to assess the relative contribution of K ATP channel activity, assessed from input conductance ( G input), and volume-sensitive anion channel activity to the induction of electrical activity in single isolated rat pancreatic β-cells by glucose, 2-ketoisocaproate and tolbutamide. In cells equilibrated in the absence of glucose, the membrane potential was −71 mV and G input 3.66 nS. Addition of 8 mM glucose resulted in depolarisation, electrical activity and a reduction in G input, reflecting an inhibition of K ATP channels. Cells equilibrated in 4 mM glucose had a membrane potential of −59 mV and a G input of 0.88 nS. In this case, a rise in glucose concentration to 8–20 mM again resulted in depolarisation and electrical activity, but caused a small increase in G input. 2-Ketoisocaproate also evoked electrical activity and an increase in G input, whereas electrical activity elicited by addition of tolbutamide was accompanied by reduced G input. Increasing the concentration of glucose from 4 to 8–20 mM generated a noisy inward current at −70 mV, reflecting activation of the volume-sensitive anion channel. The mean amplitude of this current was glucose-dependent within the range 4–20 mM. Addition of 2-ketoisocaproate or a 15% hypotonic solution elicited similar increases in inward current. In contrast, addition of tolbutamide failed to induce the inward current. It is concluded that K ATP channel activity is most sensitive to glucose within the range 0–4 mM. At higher glucose concentrations effective in generating electrical activity, activation of the volume-sensitive anion channel could contribute towards the nutrient-induced increase in G input.