Delay in diastolic restitution of full myocardial contractility in guinea pig atrium

In an isolated perfused left atrium of guinea pig, pressure amplitudes were recorded at 37 degrees C and a stimulation rate of 240/min with interposed variable resting pauses. After prolonged intervals the preparation develops maximum pressure, which does not depend on perfusion time or on any positive inotropic intervention. The latter is found to shorten the time needed for restitution of full contractility. Nifedipine (5 X 10(-7)M) exerts its negative inotropic effect mainly by a delay in restitution time; acetylcholine (2 X 10(-7)M), by a concomitant reduction of all pressure amplitudes over the whole range of stimulus intervals. The latter effect is similar to that of lowering [Ca2+]o.