Critical evaluation of isometric indexes of relaxation in rat and cat papillary muscles and toad ventricular strips

The effects of increasing calcium concentration and isoproterenol on myocardial relaxation were compared in cat and rat papillary muscles and toad ventricular strips. Isoproterenol effects were studied at equivalent positions of the calcium contractility response curve in the three species: at the plateau and at 25% and 50% of maximum. Calcium concentration was increased from the 25% point to the plateau of the calcium contractility curve for each species. Contractility was characterized by maximal velocity of tension development (+Ṫ). To study the relaxation phase, the following parameters were measured: maximal velocity of relaxation, (-Ṫ), the relationship between +Ṫ and -Ṫ (+Ṫ/-Ṫ), time to peak tension (TTP), time to half relaxation (t 1/2), relaxation time (R_t), total twitch duration (T_t) and the time constant of isometric tension decline from the time of -Ṫ (Tau). In rat muscles, t 1/2, R_t and T_t at all calcium levels, and Tau and +Ṫ/-Ṫ at the 50% and 100% of the calcium contractility curve, behaved as 'lusitropic': they significantly decreased with isoproterenol and not with calcium. Similar results were obtained in cat muscles except that at the 50% point of the calcium contractility curve, +Ṫ/-Ṫ was not significantly decreased. The behaviour of toad ventricle was different from that of mammalian heart: +Ṫ/-Ṫ and Tau appeared as lusitropic at all levels of the calcium curve, whereas only at the plateau some of the 'time' parameters were able to discriminate the isoproterenol lusitropic effect. These differences may reflect the strong dependence in amphibian heart of the time course of the action potential duration, not observed in mammalian ventricle.