Membrane currents in cat myocardium: separation of inward and outward components

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Abstract
The single sucrose gap method was used to control the membrane potential of cat ventricular fibers. Following the early rapid events (capacitive, Na and slow inward (si) current spikes) the membrane current on depolarization contained 3 time-dependent [.tau.] components which appeared attributable to the inactivation of Isi and the activation of 2 outward currents labeled IK and Ix. Tail currents were analyzed with a view to confirming these conductance changes. At -60 mV the tail progressed from being predominantly inward in direction after short (30-50 ms) depolarizations to being predominantly outward after long (> 300 ms) depolarizations. Inward and outward components decayed exponentially with time constants independent of previous membrane history. The Q10 were about 3. Experiments with D600 and variations of the driving force identified the inward tail component (.tau. .apprxeq. 55 ms at -60 mV) as Isi. The major outward tail component (.tau. .apprxeq. 300 ms) appears to be carried primarily by K. A second outward tail component (.tau. .apprxeq. 3 s) of much smaller amplitude than IK was observed after long depolarizations and is tentatively labeled Ix. Membrane currents at 0 mV can be described as the sum of three exponential processes: Isi inactivation (.tau. .apprxeq. ms), IK activation (.tau. .apprxeq. 370 ms) and Ix activation (.tau. .apprxeq. 3 s). Conductance measurements (envelops of Isi and IK tails) supported these time courses. Isi time constants increased from 50 ms at -40 mV to 120 msec at +40 mV. IK time constants increased from 400 ms at -40 to about 520 ms at -25 mV before declining to 300 ms at +40 mV. Isi amplitudes measured visually (difference between peak Isi and current level after 200-500 ms) were compared with those measured graphically (semilog plots, subtraction of IK and Ix). As a consequence of the relative amplitudes and time courses of Isi and IK, the shapes of the Isi voltage relations were not markedly different: visual estimates at 200 ms were in agreement with graphic estimates, visual estimates at 300 or 500 ms exceeded these by 15-30% between -20 and +20 mV.