The Importance of Calcium in Poststimulation Potentiation

Isotonic contractions recorded both before and during poststimulation potentiation in the toad isolated ventricle (Bufo marinus) revealed that the phenomenon of poststimulation potentiation was not altered by the presence or absence of the catechol amines, or by the specific amine antagonist, DCI. Similarly the inhibitors, sodium fluoride and sodium iodoacetate, were without effect. Changes in [Ca⁺⁺], [Mg⁺⁺], and [Na⁺] affected the degree of potentiation. High [Ca⁺⁺] as well as the cardiac glycosides abolished it, low [Na⁺] and the absence of Mg⁺⁺ depressed it. It has been shown that the percentage potentiation depends to some extent upon the total number of contractions occurring during the rapid stimulation phase. The amplitude of the contractions during this stage did not influence the degree of potentiation. These results are discussed in terms of Ca⁺⁺ accumulation or redistribution associated with an early phase of the membrane depolarization.