Potentiation of Contractility in the Heart Muscle of the Rat and Some Other Mammals

Abstract

THE steady-state amplitude of rhythmical myocardial contractions reached after the period of ‘staircase’ varies regularly with the frequency, following a typical curve in most animal species : it decreases at first, at very low frequencies, passes through a minimum (least favourable frequency), then at higher rates it increases up to a maximum (optimal frequency), to decrease again, at still higher rates, up to the limit of frequency which the preparation can follow¹. The shape of the curve is, however, much modified in the rat in comparison with that of other species, the initial decrease of contractility being very marked, while the following increase to the optimum is, on the contrary, only small or absent². In contrast to the classical staircase phenomenon, an inverted staircase (decrease of contraction) has been observed in the auricular or ventricular strips of the rat after a period of rest, or after the transition from a lower to a higher rate of stimulation³.