Some limitations of the double sucrose gap, and its use in a study of the slow outward current in mammalian ventricular muscle.

1. A double sucrose gap method to clamp small bundles (diameter 0.8-1.2 mm) of sheep or calf ventricular fibres is described.2. Comparison between micro-electrode recordings from the central gap and the externally recorded potentials showed good agreement between the time course and amplitude of the action potentials. The rapid sodium inward current was not controlled on depolarizing clamp steps. On repolarization, control was obtained within 20 msec. The method is regarded as only suitable for a study of slow currents.3. During clamps of several seconds duration slow changes in outward current can be demonstrated. The potential, where the instantaneous current-voltage relationship crosses the voltage axis, shifted in a positive direction as the clamp duration was increased (clamp amplitude constant), and did not alter much if the clamp amplitude was increased while the duration remained constant. For these reasons it is concluded that K ions accumulate round the cells.4. A comparison between the instantaneous current-voltage relationship in various K solutions and after a depolarizing clamp, showed that an increase in external K could not exactly mimic the changes during a clamp. Because of this, a conductance change, unrelated to accumulation, is also postulated.5. To measure the influence of the slowly increasing outward current during an action potential, slowly increasing inward current (to reduce net outward current) was applied during an action potential and the prolongation of the action potential measured. In the same experiment the increase in outward current during a clamp in a solution with tetrodotoxin and Mn was measured. From these experiments it is concluded that the slow increase in outward current does not contribute much to repolarization in sheep or calf.6. Equations are derived for the voltage distribution and leakage current in the double sucrose gap. From the equations it is possible to calculate an optimal gap width. Too small a gap causes the membrane current to be swamped by the leakage current. Too wide a gap gives unequal potential distribution.