Influence of a Rapid Change of Temperature on the Mechanical Response of Mammalian Myocardium

AbstractEITccts of rapid changes of temperature of about 10o C on the mechanical response of electrically driven guinea-pig left atria suspended in a special chamber (Fig. 1) in Krebs-Henseleit medium have been studied.A simple step change to a higher temperature causes a typical immediate biphasic reaction of the amplitude followed by a progressive transition to a lower level steady state activity. A similar rapid cooling causes a less marked initial biphasic reaction followed by an ascending staircase to the higher steady state level (Fig. 2).In a quiet preparation cooling causes a slight increase of its resting length, warming a slight shortening, both often preceded by an initial deflection in the opposite direction.A short exposure (2 sec.) to a lower or a higher temperature causes besides the modification of the concomitant contraction a disturbance of the steady state dynamic equilibrium which needs several contraction cycles to be repaired.The staircase-like transitions are in fact a third kind of this form of adjustement of the heart muscle contraction to new conditions (rate of activity, input or output pressure, temperature) and seem to represent a general adaptation pattern specific to the heart muscle.A more detailed analysis of the immediate reaction shows that modifications of amplitude and of peak time appear in the contraction occurring during the rapid change of temperature and become more marked, the more the change precedes the contraction. These two modifications run almost parallel, that of the speed of contraction lags behind them, The rapid variation of amplitude seems to result from the primary variation of the duration of shortening.The 90 % relaxation time modification on cooling precedes those of amplitude and of peak time, but on warming is slightly biphasic and its decrease is delayed.