The effects of cyclopiazonic acid, a specific inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, on membrane currents and contraction were investigated under voltage clamped conditions on frog atrial trabeculae using the double mannitol-gap technique. In ringer solution, cyclopiazonic acid (10 microM) decreased the phasic contraction at all levels of depolarization without significant change in the time to peak of the contraction and time constant of relaxation. In a low-sodium the amplitude and the current/voltage relationship of L-type and T-type Ca2+ currents were not modified while their inactivation phase was markedly slowed by cyclopiazonic acid. At all levels of depolarization the tonic contraction was increased by cyclopiazonic acid. In the presence of cyclopiazonic acid, no significant variation in diastolic level of intracellular calcium ([Ca2+]i) was found while the maximum amplitude of the Ca2+ transient associated with an action potential was reduced and its time course slowed. All the reversible changes observed could be interpreted by assuming that, in frog atria, sarcoplasmic reticulum was present and cyclopiazonic acid inhibits the sarcoplasmic reticulum Ca(2+)-ATPase. It is proposed that under normal conditions relaxation is due mainly to Na+/Ca2+ exchange. The increase in [Ca2+]i during the contraction phase is due to ICa that provokes a fast release of Ca2+ from the sarcoplasmic reticulum and to an influx of Ca2+ provided by reverse Na+/Ca2+ exchange.