Effect of Epinephrine on the Mechanical and Phosphorylase Activity of Normo-and Hypothermic Hearts

The effect of temperature on the action of epinephrine and on the percentage of the enzyme phosphorylase present in the a form in isolated rat (homeothermic) and toad (poi-kilothermic) hearts was investigated. Rat hearts responded to cold per se with reduced amplitude of contraction and bradycardia. The percentage of the phosphorylase enzyme which was in the a form declined as the perfusion temperature fell. Mechanical activity ceased when rat hearts were perfused at temperatures below 10°C. Epinephrine failed to exert a positive inotropic effect when added to rat hearts perfused at 12°C; at this temperature epinephrine increased the phosphorylase a activity to only 42% compared with 66% at 35°C. The response of toad hearts to cold per se differed from that of rat hearts. At 2°C the contractile amplitude of toad hearts was maximal and the phosphorylase a activity was well maintained. Stimulation enhanced, DCI and preliminary reserpinisation reduced the proportion of the enzyme which was in the a form in 2°C toad hearts. It is suggested that poikilothermic hearts may respond to cold by releasing their endogenous catecholamines and that this may be indirectly responsible for the ability of poikilothermic cardiac tissue to withstand low temperatures.