Transition from the polikilotherm to the homeotherm: possible role of sodium transport and thyroid hormone.

The evolutionary transition from poikilothermy to homeothermy required the recruitment of complex mechanisms that provided both higher rates of heat production and regulation of heat loss in the presence of wide variations in ambient temperature. Two pathways that have been considered to be contributors to the transition to homeothermy are thyroid-regulated thermogenesis and nonshivering thermogenesis in the adaptation to cold. These pathways may yield higher rates of heat production in homeotherms by two processes that need not be mutually exclusive: a) hydrolysis of ATP and consequent generation of the Pi acceptor--ADP, or b) decreased coupling of mitochondrial oxidative phosphorylation. A significant fraction of both thyroid and catecholamine thermogenesis has been ascribed to enhanced energy expenditure in active transmembrane Na+ transport (i.e., the Na+ pump). By extension these findings implicate the Na+ pump as a heat source in the evolution of the homeotherms. The thermogenic response to thyroid hormone is both rapid and profound in homeotherms (mammals), and more slowly evident and probably lesser in magnitude in poikilotherms (amphibia and reptiles). One crucial pathway in the evolution to homeothermy may be nonshivering thermogenesis in response to cold. In mice, most of the elevated rate of oxygen consumption of liver and skeletal muscle elicited by cold exposure is ouabain-sensitive. Thus, the Na+ pump appears to be an important heat source in the transition to homeothermy. The underlying biochemical mechanisms that mediate the contribution of the Na+ pump to heat production, including responsiveness to thyroid hormone and catecholamines, however, may not be unique to the homeotherms.