Use of the Ca-shortening curve to estimate the myofilament responsiveness to Ca2+ in tetanized rat ventricular myocytes

We previously estimated the myofilament responsiveness to CA(2+) in isolated intact ventricular myocytes, using the steady-state relationship between cytosolic CA(2+) concentration ([CA(2+)](i)) and cell-shortening during tetanus (Ca-L trajectory). This method was useful and easy; however, it could not be used for a high dose of CA sensitizer because the instantaneous plots after the application of CA sensitizer did not make a fixed point of shortening (we used 5% shortening). Therefore we must produce another method to investigate CA(2+) responsiveness. For an estimation of a wider range of the Ca-L trajectory, we fitted the Ca-L trajectory data with the Hill equation to construct the Ca-shortening curve. To fit this curve, we measured the maximal shortening, which was on average 31.6%. The value of [CA(2+)](i) to produce the half-maximal shortening (CA(50)) was dose-dependently decreased by EMD57033 (sensitization). Either isoproterenol or 3-isobutyl-1-methylxanthine increased CA(50) (desensitization) with a concomitant increase in intracellular c-AMP. EMD57439, a selective PDE-III inhibitor, did not significantly increase the c-AMP concentration and produced little change in CA(50). These results are in agreement with previous reports with skinned or intact multicellular preparations. The Ca-shortening curve constructed in intact cardiac myocytes can be used to estimate the myofibrillar responsiveness to CA(2+) in a wide range of [CA(2+)](i).