Mechanics of rat myocardium revisited: investigations of ultra-thin cardiac muscles under high energy demand.

Disregarding the influence of thickness on elevated strength of isolated preparations inevitably leads to erroneous tension-frequency relations, especially in the range of high frequencies. Thus, much of the confusion in interpreting the atypical negative staircase phenomenon of the rat heart is due to this. In view of the fact that the rat has become the preferred laboratory animal in cardiological research, it was imperative to reinvestigate force-frequency relations using ultra-thin preparations of the rat right ventricle (d less than 0.1 mm). Contrary to popular opinion, it could be demonstrated that the rat myocardium shows a positive staircase in the range of physiological heart rates. An increase in tension is still attainable even at frequencies up to 600 min-1. The interval-strength relations exhibit a minimum at frequencies of 60-120 min-1, being shifted to higher frequencies with increasing diameter, vanishing completely for thick preparations (d greater than 1.0 mm). At high extracellular Ca++ concentration the positive staircase even of ultra-thin muscles is flattened. However, it can be reinforced when the strength, and thus the energy expenditure, is reduced by lowering the extension. The same is true for contractions under hypoxia. From these findings it seems probable that many investigations on isolated heart muscles of the rat, as well as other species, are objectionable when done under high energy demand, as diffusion problems will certainly limit any rise in contractility.