Alterations in excitation of mammalian myocardium as a function of chronic loading and their implications in the mechanical events.

It is shown that hypertrophied rat myocardium exhibits marked prolongation of its action potential during non-essential changes both in the resting potential, as well as in the amplitude of the action potential, and during a moderate but nonsignificant reduction in the maximum upstroke velocity. When examining action potentials of various myocardial regions of the same heart, it can be demonstrated that high chronic loading involves broad action potentials both in rat and cat hearts. After depression of the fast inward current either by TTX or by partial depolarization under high extracellular K+ concentrations, the prolongation of the Ca2+ mediated action potentials of myocardial cells under high chronic loading is still manifest. The broadening of the action potential is certainly responsible to some extent for the prolonged activity of the corresponding myocardial cells, which is expressed in the significant increase in the isometric peak time and in the augmentation of isometric force.