Cellular calcium as a determinant of action potential duration in rabbit myocardium

It has been suggested that the amount of calcium in the myocardial cell determines force production as well as the duration of the action potential. We have examined two factors proposed to determine the action potential duration: (I) the inotropic state of the muscle and (2) the preceding stimulation interval. The experiments were performed on isolated papillary muscles from rabbits. The preparations were stimulated to contract isometrically at a temperature of 37°C and intracellular recordings of action potentials were made. An increased inotropic state produced either by raised external calcium concentration or by increased stimulation frequency was associated with shortened action potentials, confirming factor (1) above. The dependence of the action potential duration on the preceding stimulation interval (factor (2) above) disappeared over a wide range of intervals when the preparation was exposed to 4 μM D600. The action potential duration was, however, still dependent on the preceding stimulation interval when the extracellular calcium concentration had been lowered from 2.0 mM to 1.0 and 0.5 mM respectively. The results are discussed in terms of a model which postulates an inverse relationship between the amount of calcium in a membrane compartment and the action potential duration.