Postrest contractions as an example for the regulation of the course of isometric twitch

Postrest contractions (PRC) of isometrically contracting rabbit papillary muscles are used to analyse how the shape of the twitch can be explained by existing models for the accompanying Ca-regulation. By the change of the extracellular Ca concentration (Cae) potentiated or depressed PRC were produced. The course of the twitch is separated by mechanical indices commonly used. The results fit some biochemical and electro-physiological findings, as the regulatory role of the sarcoplasmic reticulum and the Na-Ca exchange, but do not reflect others. The sharp separation between an early and late phase of the contraction and relaxation, respectively, is limited by the overlapping of different Ca regulating processes.