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1. A voltage-clamp technique was developed for stable recording of small currents in guinea-pig ventricular muscle. Small cylindrical preparations were impaled with three micro-electrodes, one for measuring the feed-back potential and two for injecting current. 2. 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 micron. 3. When the clamp current was injected symmetrically into each half of the preparation via two electronic current pumps a spatially homogeneous clamp could be achieved in preparations with a diameter of less than or equal to 250 micron and a length of less than or equal to 2 lambda. 4. 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 microF/cm2 a specific membrane resistance (Rm) of 6.7 +/- 1.8 k omega cm2 was obtained in Tyrode solution containing 3 mM-K. 5. The total surface area was calculated from the measured capacity of the preparation assuming a Cm of 1 microF/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. 6. 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. 7. The influence of the extracellular potassium concentration ([K]o) on Rm was studied in the range 1.5-6 mM-[K]o. Rm was found to depend on [K]o less than predicted by the constant field theory.
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