Measurement of transmembrane potential and current in cardiac muscle: a new voltage clamp method.

Abstract

A single sucrose gap voltage clamp technique was developed to correct for artifacts of leakage current and extracellular resistance making possible improved measurement of membrane current and membrane potential in [Rana pipiens] cardiac muscle. A 4th compartment termed guard gap was added to the sucrose gap. The guard gap is maintained at the same potential as the Ringer pool, so that no extracellular leakage current can flow into the Ringer pool. Comparison of experimental results with the predictions of an idealized cable model indicates that the guard gap is effective in trapping leakage current. The slow charging of membrane capacitance during due to extracellular series resistance was accelerated by applying a prepulse of the command potential past the final voltage clamp value. A 2nd technique, termed chopped current pulse clamp, was used to compensate for the extracellular resistance throughout the voltage clamp step. The applied current was turned on and off at a frequency of 0.5-2 kHz. The membrane potential sampled during the zero current phase was fed back through the clamp loop. With either of these compensation techniques, the voltage and current traces settle to effectively constant values within 2-4 ms after initiation of a hyperpolarizing voltage clamp step from rest. The membrane conductance measured by the prepulse and chopped current-pulse technique are equal and confirm a higher conductance at rest than during the plateau of the action potential. The instantaneous current-voltage relation of the membrane is linear during the plateau of the frog ventricular action potential.