Intracellular Calcium and Myocardial Contractility

Postextrasystolic potentiation was studied in the isolated perfused rabbit heart to determine its effect on intracellular calcium, sodium, and potassium concentration, 45 Ca uptake, and calcium distribution in mitochondria and microsomes. Paired stimulation at 80/min augmented myocardial contractility with a gradually developing increase in total intracellular calcium and sodium. A greater 45 Ca uptake was also observed and this was proportionate to the increase of intracellular calcium so that no change in exchangeable calcium occurred. A small but statistically insignificant increment was observed in mitochondrial and microsomal calcium. An increase of the rate of single stimuli to 160/min produced a minimal change in myocardial contractility compared to paired stimulation at 80/min, no change in total intracellular calcium, and a greater calcium uptake so that there was a significant increment in exchangeable calcium. Although these experiments do not identify the cellular processes responsible for the positive inotropic response to paired stimulation, they suggest that an alteration of myocardial calcium metabolism is involved. Furthermore, it is apparent that this alteration is distinct from that occurring during a comparable increase in the rate of delivery of single stimuli to the heart.