Rubidium and sodium permeability of the ATP‐sensitive K+ channel in single rat pancreatic beta‐cells.

Publisher Website
Google Scholar
Abstract
1. The patch‐clamp method has been used to study the selectivity of single ATP‐sensitive potassium channels in excised membrane patches from dissociated rat pancreatic beta‐cells. 2. In symmetrical K+ concentrations the current‐voltage relation of this channel showed slight inward rectification. The K+ permeability coefficient was 1.05 x 10(‐13) cm3/s ([K+]o = 140 mM; 20 degrees C) in the inside‐out patch and somewhat smaller when measured under the same conditions in the outside‐out configuration (0.86 x 10(‐13) cm3/s). 3. When intracellular Rb+ replaced K+, inward K+ currents were unaffected but the outward currents carried by Rb+ were substantially smaller. The extent of the reduction in the outward currents depended on the internal Rb+/K+ ratio and increased as [Rb+]i was raised. Both inward and outward Rb+ currents were blocked by 1 mM‐ATP. No currents were measurable in symmetrical 140 mM‐Rb+ solutions. 4. With 140 mM [K+]o and 140 mM [Rb+]i the single‐channel current‐voltage relation reversed at +8 mV and the potential at which the variance of the ATP‐sensitive current was least was shifted by 6 mV to more positive potentials. These data suggest a PRb/PK ratio of around 0.7. 5. Outward Rb+ currents were reduced at all potentials in inside‐out patches exposed to 107 mM‐Rb+ solution intracellularly and 5 mM‐K+ externally. 6. Partial replacement of external K+ with Rb+ substantially reduced the inward currents recorded from outside‐out patches and also decreased outward K+ currents. 7. In outside‐out patches, the addition of 1 mM‐Rb+ to the external solution produced a block of inward K+ currents that initially increased and then decreased again with hyperpolarization. This suggests that the Rb+ block of K+ currents is voltage dependent and that Rb+ acts as a permeant blocker of K+ currents. 8. The sodium permeability of the channel, relative to that of potassium, was 0.39 for internal Na+ and 0.007 for external Na+ ions. 9. We conclude that Rb+ serves as an acceptable tracer for K+ in efflux studies when changes in K+ flux through ATP‐sensitive K+ channels are of interest but that the magnitude of such fluxes will be considerably underestimated.