The passive electrical properties of guinea‐pig ventricular muscle as examined with a voltage‐clamp technique.

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Abstract
A voltage-clamp technique was developed for stable recording of small currents in guinea-pig ventricular muscle. Small cylindrical preparations were impaled with 3 micro-electrodes, 1 for measuring the feed-back potential and 2 for injecting current. The longitudinal potential profile resulting from current injection at one point was measured. It agreed well with the theoretical predictions for a linear cable which is sealed at both ends (healing over), with a length constant (.lambda. of 580 .+-. 145 .mu.m). When the clamp current was injected symmetrically into each half of the preparation via 2 electronic current pumps a spatially homogeneous clamp could be achieved in preparations with a diameter of .ltoreq. 250 .mu.m and a length of .ltoreq. 2 .lambda.. The membrane capacity and the membrane resistance of the preparations at the resting potential were measured with small voltage-clamp pulses. Assuming a specific membrane capacity (Cm) of 1 .mu.F/cm2 a specific membrane resistance (Rm) of 6.7 .+-. 1.8 k.OMEGA. cm2 was obtained in Tyrode solution containing 3 mM-K. The total surface area was calculated from the measured capacity of the preparation assuming a Cm of 1 .mu.F/cm2. The total cellular volume was estimated from optical measurement of the external dimensions of the preparation assuming an extracellular space of 25%. From these data the average surface/volume ratio of individual cells was calculated to be 7200 cm2/cm3. From the measured electrical constants the specific resistance of the intracellular space (Ri) was calculated to be 200-250 .OMEGA. cm. With small constant current pulses a membrane time constant of 6.6 .+-. 1.3 ms was measured. The influence of the extracellular K concentration ([K]o) on Rm was studied in the range 1.5-6 mM-[K]o. Rm depended on [K]o less than predicted by the constant field theory.

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