[Changes in the efficiency coefficient of the heart and diaphragm during cold adaptation]

The heart and diaphragm efficiency seems to be an actively regulated variable. It alters in prolonged adaptation (raises in adaptation to hypoxia; diminishes in cold adaptation which means an increase in heat production per unit of muscle's contractile activity). Enhancement of noradrenaline calorigenic effect on skeletal muscles during cold adaptation is corroborated in an isolated diaphragm. Diminishing of the heart and diaphragm efficiency in thyrotoxicosis suggests participation of thyroid hormones in formation of adaptive decrease of muscular contraction efficiency in cold adaptation. The heart efficiency raises in hypothermia and, probably, in limitation of the myocardium oxygen supply. Possible mechanisms of changes of muscular contraction efficiency are discussed.