Voltage clamp of cardiac muscle in a double sucrose gap. A feasibility study

A method of stabilizing the membrane potential of a small area of cardiac muscle membrane and the limitations of this method are described. Tiny bundles or strands, approximately 80 mum in diameter, of electrically interconnected fibers from the ventricles of rabbit hearts were used in a double sucrose gap. Current records associated with step changes in voltage were complicated by two capacitive surges of current of nodal and nonnodal origin and large "leakage" currents of nonnodal origin resulting mainly from the multifibered nature of the preparation and emphasized by the method. The transient, inward membrane currents in response to moderate depolarizing steps in command potential had the same duration as the upstroke of the action potential. In good runs, currents were smooth and free from notches. These initial currents behaved qualitatively like the initial sodium currents in squid axon and in other excitable membranes. A fraction of the initial sodium current persisted at least as long as 300 ms. The relationship between peak initial current and voltage was graded and linear in the positive direction. In the negative region the relationship was often very steep, indicating insufficient voltage control of all the membranes despite the squareness of the voltage record. Other indications of inadequacy of control could occur and thus even with this optimum preparation of cardiac muscle it was not feasible to analyze quantitatively either the initial or the prolonged sodium currents.