[Time dependence of Ca supply in rat and rabbit myocardium]

The time dependence of Ca supply in rat (R) and rabbit myocardium (K) for the activation was analyzed by the action potential area up to 50% of repolarization (APA50) of the action potential (AP) during the electrical restitution as well as by extra contractions (EC) during the mechanical restitution and subsequent postextrastimulatory contraction (PEC) or potentiation (PEP) in isometrically contracting papillary muscle at different extracellular Ca concentrations (Cae). In K a considerable increase in APA50 of the EC-AP during the electrical restitution within the interval of rhythmical contraction (T) strongly correlates with the subsequent PEP. In R both the alteration of the APA50 and the PEP are smaller than in K. Compared with K in R the mechanical restitution is delayed, but accelerated by rising Cae as demonstrated by increasing EC/T. The course of mechanical restitution and the low PEP are regarded to be connected with the fact that the sarcoplasmic reticulum is the main source of activator Ca in R. Differences in both electrical and mechanical restitution and, especially the higher PEP, agree with the sarcolemma as the main source of activator Ca in K.