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II Physiologisches Institut, Universität des Saarlandes, 6650 Homburg/Saar, Germany

1. The single sucrose gap method was used to control the membrane potential of cat ventricular fibres.

2. Following the early rapid events (capacitive, Na and slow inward (si) current spikes) the membrane current on depolarization contained three time-dependent components which appeared attributable to the inactivation of I_si and the activation of two outward currents labelled I_K and I_x.

3. Tail currents were analysed with a view to confirming these conductance changes. At -60 mV the tail progressed from being predominantly inward in direction after short (30-50 msec) depolarizations to being predominantly outward after long (> 300 msec) depolarizations. Inward and outward components decayed exponentially with time constants independent of previous membrane history. The Q₁₀s were about 3.

4. Experiments with D600 and variations of the driving force identified the inward tail component ( 55 msec at -60 mV) as I_si. The major outward tail component ( 300 msec) appears to be carried primarily by potassium. A second outward tail component ( 3 sec) of much smaller amplitude than I_K was observed after long depolarizations and is tentatively labelled I_x.

5. Membrane currents at 0 mV can be described as the sum of three exponential processes: I_si inactivation ( 90 msec), I_K activation ( 370 msec) and I_x activation ( 3 sec). Conductance measurements (envelops of I_si and I_K tails) supported these time courses. I_si time constants increased from 50 msec at -40 mV to 120 msec at +40 mV. I_K time constants increased from 400 msec at -40 to about 520 msec at -25 mV before declining to 300 msec at +40 mV.

6. I_si amplitudes measured visually (difference between peak I_si and current level after 200-500 msec) were compared with those measured graphically (semilog plots, subtraction of I_K and I_x). As a consequence of the relative amplitudes and time courses of I_si and I_K, the shapes of the I_si voltage relations were not markedly different: visual estimates at 200 msec were in agreement with graphic estimates, visual estimates at 300 or 500 msec exceeded these by 15-30% between -20 and +20 mV.

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