Lateral distribution of sodium and potassium channels in frog skeletal muscle: measurements with a patch‐clamp technique.

Abstract

A method for recording Na+ and K+ currents (INa and IK) from small, voltage-clamped patches of [frog] sarcolemma by means of fire-polished glass micropipettes of 7-15 .mu.m tip diameter is described. Recordings can be made successively from many areas of one fiber. On a given fiber, the amplitudes of INa and IK varied from point to point. Maximum Na+ current densities varied up to 3-fold over distances of 10-30 .mu.m, typically between 4 and 12 mA/cm2. K+ currents showed somewhat less lateral variation. Local densities of INa and IK showed no correlation. Apparently, the density of Na+ (and to a lesser extent K+) channels varies laterally. A contour map of Na+ channel density is constructed for a 20 .mu.m .times. 90 .mu.m section of sarcolemma. Based on the steepness of lateral gradients in channel density and the estimated survival time of a Na+ channel, at least half of the Na+ channels have a lateral diffusion coefficient of less than 2 .times. 10-2 cm2/s. This is 3 orders of magnitudes less than expected from their molecular size, and suggests that these channels are anchored in the sarcolemma.