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

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⁺.