Relation between Intracellular Ca²⁺ Concentration and Contraction in Tetanized Myocytes of Rat and Mouse

Summary

To investigate the characteristics of Ca²⁺ responsiveness of myofibrils in isolated myocytes of rats and mice, we used the relation between intracellular Ca²⁺ concentration ([Ca²⁺]_i) and cell shortening during tetanus. Single ventricular myocytes were isolated from rat and mouse using enzymatic dispersion technique. Isolated myocytes were loaded with fura-2 AM, and the resultant fluorescence ratio signals excited at 340 nm and 380 nm wave length [F(340)/F(380)] were measured simultaneously with cell length. To produce tetanus, myocytes were treated with thapsigargin (0.2 μM) and the repetitive electrical stimulation of 10 Hz was applied to the myocytes. An instantaneous plot of the fluorescence ratio signal versus cell length (R-L trajectory) during tetanus was constructed. The R-L trajectory was extended without a substantial shift by an increase in the extracellular Ca²⁺ concentration in rat and mouse ventricular myocytes. Myofibrillar responsiveness to Ca²⁺ of mouse ventricular myocytes seems to be less than that of rat ventricular myocytes. In both rat and mouse ventricular myocytes, the trajectory was shifted rightward by the non-selective phosphodiesterase inhibitor, 3-isobutyl-1-methylxantine (IBMX) (desensitization of the myofibrils to Ca²⁺) and was shifted leftward by the Ca²⁺ sensitizing thiadiazinone derivative, EMD57033 (sensitization of the myofibrils to Ca²⁺). In mouse ventricular myocytes, β-adrenergic stimulant, isoproterenol shifted the R-L trajectory to the right (desensitization of the myofibrils to Ca²⁺). These results suggest that the R-L trajectory is a useful method to estimate the myofibrillar responsiveness to Ca²⁺ in mammalian ventricular myocytes, and that mouse myocytes might have different characteristics of Ca²⁺ responsiveness compared to that of rat myocytes.