The effect of 4-aminopyridine on the frequency-force relationship of isolated left atria of guinea-pigs.

The effect of 4-aminopyridine (4-AP) on the frequency-force relationship of isolated electrically-driven left atria of guinea-pigs was studied. 4-AP increased atrial contractility between the optimum and pessimum frequencies, but not the rested state contractions. The compound increased the time to peak force and the relaxation time. Results obtained indicate that 4-AP-induced positive inotropic effect is different from that induced by noradrenaline, and supports the view that the compound exerts a direct action on the myocardium to increase contractility. Release of endogenous noradrenaline by 4-AP cannot, however, be excluded.