The influence of temperature on the time course of the mechanical activity in rabbit papillary muscle.

The influence of temperature on the time course of the mechanical activity was investigated in isolated papillary muscles of the rabbit. By using the "damped release" technique activity curves were determined in terms of velocity of shortening of the contractile unit at a constant length. At any of the temperatures studied, the time course of the mechanical activity exhibited a slow onset and did not form a distinct plateau. At low contraction frequencies and normal (2 mM) calcium concentration lowering the temperature from 32.5 to 26.5 degrees C did not markedly affect the rate of development of the activity but increased the peak amplitude and the total duration of the activity as well as the time from the stimulus to 90% of the maximum. It also resulted in a slower decay phase. At higher contraction frequencies and/or increased calcium concentration the rate of development of the activity was reduced by decreasing the temperature from 32.5 degrees C to 26.5 degrees C, whereas the maximal activity reached similar levels at the two temperatures. It is concluded that an alteration in temperature affects several different steps in myocardial excitation-contraction coupling, the net change in maximum amplitude of the mechanical activity being influenced by both extracellular calcium concentration and contraction frequency.