The effect of heart rate on the membrane currents of isolated sheep Purkinje fibres.

1. The effect of the rate of stimulation on the membrane currents of sheep Purkinje fibres at 37 degrees C has been examined. 2. In the diastolic range of potentials, the pacemaker current if was unchanged at different stimulus rates. 3. On going from 6 to 60 min-1 the effect on the background current was similar to that of a decrease in the bathing K+ concentration, because there was a decrease in outward current in the plateau range of potentials and an increase in outward current in the diastolic range of potentials. At rates above 60 min-1 there was extra outward background current at all potentials. Partial block of the Na+-K+ pump by ouabain reduced these changes in background current. 4. On going from 6 to 60 min-1 there was an increase in the slow inward current isi, but at rates above 60 min-1 there was usually a decrease in isi. The decrease in isi at rates greater than 60 min-1 was in part the result of insufficient time between action potentials for complete recovery of isi from inactivation. 5. The effect of these rate-dependent changes in membrane current on electrical activity has been considered. 6. The increase in the pacemaker potential at high rates is likely to be the consequence of the decrease in membrane conductance at diastolic potentials as a result of the changes in background current. 7. The increase in the maximum diastolic potential at high rates is likely to be the result of the extra outward background current at diastolic potentials. 8. The prolongation of the action potential on going from 6 to 60 min-1 is likely to be the result of the increase of isi, as well as the decrease in outward background current at plateau potentials. 9. The shortening of the action potential above 60 min-1 is likely to be the result of the decrease in isi although the extra outward background current may also contribute.