A study of the effect of the rate of stimulation on the transient outward current in sheep cardiac Purkinje fibres.

Abstract

The transient outward current, Ito, of sheep Purkinje fibers was recorded using the 2 microelectrode voltage clamp technique. Ito is strongly rate-dependent; the magnitude of Ito activated during a test voltage clamp pulse after a train of action potentials is less at higher rates of stimulation. After an increase or decrease in rate there is an abrupt change in Ito in the 1st response followed by slower changes over the next several hundred responses. When a preparation is rested after repetitive activity Ito recovers in 2 phases; there is an initial rapid, approximately exponential phase of recovery in the first 10 s which is probably due to reactivation; this is followed by a slower phase of recovery lasting several hundred seconds. Curves showing the time course of reactivation of Ito were obtained at different rates. At high rates, the curves approach smaller values of Ito and the steady-state control values of the current occur on the shoulder of the curves, i.e., before reactivation is complete. It is proposed that the reduction of Ito at high rates is due to 2 factors: incomplete reactivation which accounts for the rapid changes of Ito and a 2nd unknown factor which accounts for the slower changes in the current. Inspection of current-voltage relationships for Ito suggests that the reduction of Ito at high rates is mainly due to a decrease of conductance rather than to a reduction of the reversal potential. Replacement of the Ca in the bathing solution by Sr does not abolish Ito in sheep Purkinje fibers, suggesting that the current is distinct from the transient outward current in calf Purkinje fibers.