Abstract:The effects of various inotropic interventions on post‐rest potentiation and its decay were investigated in isolated cardiac muscle. The inotropic interventions studied were: reduced extracellular Na+ and elevated extracellular Ca2+concentration; exposure to ouabain, monensin, isoprenaline, phenylephrine and cirazoline. Force of contraction (stimulation frequency 2 Hz) was measured isometrically in left atria and right ventricular strips of rat hearts. Maximum post‐rest potentiation was reached after 10 sec. of rest and amounted to 245±26% of pre‐rest control in ventricle and 192±15% in atria. Ca2+‐recirculation fraction was calculated from the decay of post‐rest potentiation after resumption of regular stimulation, it was 0.77±0.01 in 11 control ventricular strips. High concentrations of caffeine (3 mmol/1) completely abolished post‐rest potentiation in both tissues. The development of post‐rest potentiation was accelerated in the presence of most of the inotropic agents. However, with the exception of ouabain and only in atrial muscle, none of the inotropic interventions produced higher post‐rest contraction amplitudes than during controls. In rat heart muscle, the inotropic interventions studied are not any more effective in augmenting force of contraction than prolonged stimulation intervals. This suggests that (1) the distribution of Ca2+into the sarcoplasmic reticulum is at a maximum during post‐rest potentiation; (2) modifications of signal transduction pathways cannot further increase post‐rest potentiation; and therefore that (3) shifts in Ca2+distribution ac