We describe a method for recording Na+ and K+ currents (INa and IK) from small, voltage‐clamped patches of sarcolemma by means of fire‐polished glass micropipettes of 7‐15 microns tip diameter. Recordings can be made successively from many areas of one fibre. On a given fibre, the amplitudes of INa and IK varied from point to point. Maximum Na+ current densities varied up to three‐fold over distances of 10‐30 microns, 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 microns X 90 microns section of sarcolemma. Based on the steepness of lateral gradients in channel density and the estimated survival time of a Na+ channel, it is calculated that at least half of the Na+ channels have a lateral diffusion coefficient of less than 2 X 10(‐12) cm2/s. This is three orders of magnitudes less than expected from their molecular size, and suggests that these channels are anchored in the sarcolemma.
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