Effect of temperature and caffeine on the interval-strength relationship of mammalian ventricular myocardium

In isolated cat heart papillary muscle electrically driven at the basic rate of 30 per min the effect of change in temperature (37 degrees C vs. 30 degrees C) and caffeine upon post-extrasystolic potentiation (PEP) phenomenon was studied. At 30 degrees C the basic tension developed by the regular contraction increased an average of (s +/- sm) 23 +/- 5.2% as compared to 37 degrees C. Furthermore at 30 degrees C, the curve relating the magnitude of PEP to the interval (msec) between regular and premature stimulus was significantly shifted upwards and to the right as compared to the curve obtained at 37 degrees C. The time persistence effect of PEP or at 30 degrees C. In fact, up to 5 min the corrected curve relating percentage increase of the first potentiated contraction with predetermined intervening rest period (pause) was significantly shifted upwards at the lower temperature. Caffeine (2.7 and 5.4 mM/l at 37 degrees C) produced a significant increase (s +/- sm) + 51.5 +/- 6.8% of the basic tension developed by the regular contractions as compared to the control period in the same muscles. However, caffeine decreased significantly both the relative magnitude (%) and the time persistence (corrected values) of PEP appearing on the first contraction after a predetermined interveinin pause. In control experiments in the same muscles caffeine decreased the tension developed by the first contraction following any equivalent pause during rhythmic stimulation. This relative decrease in tension of the first contraction reached a greater significance as the pause was lengthened to the limits of the rested state contraction (5 min). The results are discussed regarding the excitation-contraction coupling and calcium inactivation as the main mechanisms for positive inotropic effect during PEP. The effect of lowering the temperature could be related to the lengthening of the active state, slowing of Ca++ inactivation mechanism and/or greater stiffness of the serial elastic element. The effect of caffeine is probably related to an increase of the intracellular Ca displacement to the contractile site.