Patch-clamp study of rubidium and potassium conductances in single cation channels from mammalian exocrine acini

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Abstract
Single-channel current recordings were carried out on excised inside-out patches of baso-lateral plasma membrane from exocrine acinar cells. The mouse pancreas and submandibular gland as well as the pig pancreas were investigated. In the mouse pancreas the voltage-insensitive Ca2+-activated cation channel was studied. Single-channel current-voltage (i/v) relationships were studied in symmetrical Rb+-rich solutions and in asymmetrical Rb+/Na+ and Na+/Rb+ solutions. In all cases the i/v relations were linear and had the same slope representing a single-channel conductance of about 33 pS which is identical to that previously obtained with symmetrical Na+ solutions or asymmetrical Na+/K+ solutions. In the mouse submandibular gland and the pig pancreas the voltage and Ca2+-activated K+ channel was studied. The outward currents observed after depolarization in the presence of quasi-physiological Na+/K+ gradients were immediately abolished when all the K+ in the bath fluid was replaced by Rb+ (bath fluid in contact with inside of plasma membrane). This effect was immediately and fully reversible upon return to the high K+ solution. The voltage and Ca2+-activated K+ channel was also studied in asymmetrical K+/Rb+ and Rb+/K+ solutions. In the first case inward (K+) currents could be observed but not outward (Rb+) currents, while in the other case inward (Rb+) currents could not be seen whereas outward (K+) currents were measured. The current-voltage relationships were approximately linear and the null potential was close to 0 mV in both situations. In contrast the null potential for current through the K+ channel in the presence of asymmetrical Na+/K+ or Li+/K+ solutions was about −70 mV and with reversed gradients about +60 mV. Outward K+ currents of reduced size (through the voltage and Ca2+-activated K+ channel) could be observed when the bath fluid contained 75 mM K+ and 75 mM Rb+, but not (in the same membrane patches) when 150 mM Rb+ and no K+ was present. It is concluded that the large voltage- and Ca2+-activated K+ channel has an extremely low Rb+ conductance. It is possible, however, that the permeability for Rb+ may be about the same as for K+. The voltage-insensitive Ca2+-activated cation channel does not discriminate between K+ and Rb+.