Effect of acetylcholine on time-independent currents in sheep cardiac Purkinje fibers.

Voltage-clamp experiments were carried out in sheep Purkinje fibers in order to find an explanation for the prolongation of the action potential, the positive shift of the plateau, the hyperpolarization of the maximum diastolic potential and the increase in rate of diastolic depolarization, occurring in the presence of acetylcholine (Ach). In the presence of Ach the instantaneous current-voltage relation is shifted in the inward direction for potentials positive to −75 mV, while the opposite shift is obtained for more negative potentials; the results suggest a decrease in background conductance. The contribution of K, Cl, Na and Ca to the Ach sensitive current was studied by varying K concentration or adding 20 mmol·l Cs, by omitting Cl or Na, and by changing the Ca concentration. In 20 mmol·l Cs the apparent reversal potential of the Ach sensitive current is −50 mV, as compared to −75 mV in normal Tyrode. The component of the Ach sensitive current, which is suppressed by Cs, shows inward going rectification. In different K concentrations the reversal potential of the Ach sensitive current is changed; the shift obeys the theoretical change in equilibrium potential of K. The results are consistent with a decrease in K background current by Ach (inward and outward rectifier). In Cl free media the Ach sensitive current is not decreased excluding a major contribution of Cl ions. The Ach effect also persists in Na free media; the reversal potential of the Ach sensitive current is slightly shifted in the hyperpolarizing direction. These results indicate that active electrogenic pumps (Na or Na−Ca) do not play an important role; they are in accord with a reduction in inward Na background current by Ach. The shift of the current-voltage relation by Ach was greater the lower the Ca concentration; the mechanism is not clear. The inward shift of the current at −40 mV was dependent on the Ach concentration. Half-maximum effect was obtained at 3·10 mol·l Ach; the Hill coefficient was 1.12. It is concluded that Ach interacts in a one to one reaction with a muscarinic receptor and reduces the background current mainly carried by K (inward and outward rectifier), and less by Na (and probably Ca). [ABSTRACT FROM AUTHOR]