Abstract
1. A single sucrose-gap voltage-clamp technique was used to control the membrane potential and to measure current in rabbit sino-atrial (SA) strips. K+ activity in the extracellular space was simultaneously measured using K+-selective micro-electrodes. 2. Using double-barrelled K+ selective micro-electrodes it was possible to measure the time course of accumulation or depletion of K+ accompanying a single action potential without complications arising from mechanical or electrical artifacts. 3. K+ activity in the extracellular space increased during the action potential and then decreased to base-line levels during the diastolic depolarization phase. Single beat accumulations of 0.1-0.4 M could be measured. 4. The magnitude of accumulation or depletion of K+ depended upon the membrane potential such that K+ accumulated at potentials positive to -50 mV (K+ efflux greater than K+ uptake) and was depleted from the extracellular space at potentials negative to -50 mV (K+ efflux less than K+ uptake). 5. The rate of K+ depletion was fairly constant during the time course of a clamp step within the range of diastolic depolarization (-55 to -75 mV) even though the accompanying membrane current showed marked time-dependent kinetics. 6. The total membrane conductance measured during the time course of the diastolic depolarization or during the time course of activation of time-dependent 'pace-maker' current remained fairly constant or increased. 7. No reversal potential for the time-dependent 'pace-maker' current could be measured at EK in solutions containing 2.7, 5.4 and 8.1 mM-K+. 8. These results do not support the turn-off a K+ conductance as the primary mechanisms for the generation of the pace-maker potential in SA nodal tissue; rather the results are more consistent with the idea that activation of an inward current, with large positive equilibrium potential, is responsible for pace-making activity.
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