[Relationship between isometric and isotonic contractile responses of the mammalian myocardium].

Effects of 6 and 12 mM of caffeine, 10(-5) M of verapamil, variation in stimulation frequency between 0.1 and 0.7 Hz, cooling to 15 degrees C after elevation of perfusate Ca2+ to 15 mM on isometric (IM) and isotonic (IT) contractions of guinea pig papillary muscles were studied. The first and the last interventions mentioned above were found to slow down relaxation phase and increase the IT/IM response amplitude ratio by 50-70%. Under the action of both interventions the maximum of IT response appears after the maximum of IM response with delay of 50-100 msec. A close correlation (r = 0.89) between changes of IT/IM and delay of IT peak was revealed. Different sensitivity of IM and IT responses to inotropic interventions could not be explained by the change of membrane electrical activity because the AP did not change after a transition from IM to IT mode. An analysis of experimental data suggests that the delay between IT and IM peaks and the ratio of their amplitudes depend on the rate of calcium uptake within the myocardial cell.