The long-lasting cardiac action potentials are related to pressure generation in the heart

Vitor Rodrigues Silva and José Guilherme Chaui-Berlinck

Both the cardiac and the skeletal are striated muscles. However, the action potential (A.P.) in these two types of muscle is quite different. The A.P. in skeletal musculature has a short duration, less than 40 ms in total, forming a spike in a graph over time, while the cardiac A.P. has a long phase-2, forming a plateau with a duration of 200 ms, and the A.P. as a whole lasting more than 300 ms. Two non-excludable explanations are usually suggested for such differences: (1) to avoid cardiac tetanus, and (2) to minimize the risk of electrical re-entries. For such explanations to be sufficient, we should expect that the duration of cardiac A.P. would increase with the increase in heart rate, as this is when an eventual tetany could impose itself. However, this is the opposite of what is observed from both the phylogenetic and the individual's point of view, with the increase in heart rate being accompanied by a decrease in the duration of cardiac A.P.. For a skeletal muscle to perform external work, there must be simultaneous contraction of several fibers. This is obtained through the temporal and spatial summations of the motor units. The heart has no neural motor control. Thus, our hypothesis was that the long duration of cardiac A.P. fulfilled these two summations simultaneously, allowing the last activated cells to find the first-activated cells still in contraction. To test the hypothesis, a cellular automaton model consisting of a specialized conducting system and muscle cells with electromechanical coupling, involving a 2D camera, was developed. The results show that the effective pressure exerted by the organ is related to the A.P. duration, and that the force developed in relation to the A.P. time has a profile similar to that observed experimentally in isolated ventricular cells. To access the full text click on: https://doi.org/10.20935/AL3598