Relation between Intracellular Ca2+ Concentration and Contraction in Tetanized Myocytes of Rat and Mouse
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.
Key words1) ventricular myocyte 2) myofibrillar responsiveness to Ca2+ 3) tetanus
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