POTASSIUM AND RUBIDIUM PERMEABILITY AND POTASSIUM CONDUCTANCE OF THE -CELL MEMBRANE IN MOUSE ISLETS OF LANGERHANS

¹ Department of Biophysics, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ

The efflux of ⁴²K⁺ and ⁸⁶Rb⁺ has been studied in collagenase-isolated normal mouse islets of Langerhans. In double-labelled experiments, the ratio of efflux rate constants of Rb⁺ and K⁺ (k_Rb/k_K) was 0·80 in 5 mM-K⁺ and 0 glucose. The ratio was unaffected by glucose concentrations up to 22·2 mM. In the presence of 50 mM-K⁺, 0 glucose, the ratio increased to 0·91 and in 50 mM-Rb+, 0 glucose and 0 K⁺, the ratio was 1·06. With these limitations, the results indicate that ⁸⁶Rb⁺ is an acceptable isotope for K⁺. Using the Goldman model, K⁺ and Rb⁺ permeability coefficients and K⁺ slope and chord conductances were computed. The permeabilities decreased in glucose and in high K⁺ or high Rb⁺. In the case of high external K⁺, the K⁺ conductances increased. Also there may be more than one type of K⁺ channel with differing selectivities to K⁺ and Rb⁺. The addition of glucose in the presence of 50 mM-K⁺ had no further effect on Rb⁺ permeability. It is suggested that there are about ten small K⁺ channels open in the resting -cell and that progressive closure of these channels is involved in the depolarization of the cell membrane that initiates spike activity.