Functional significance of metabolic responses to thermal acclimation in fish muscle.

Compensatory increases of the aerobic capacity of fish swimming muscle are frequently observed in response to cold acclimation. Such thermal compensation occurs both in fish that remain active in the cold and in fish that become dormant at cold temperatures. For cold-active fish, positive thermal compensation is best explained by conservation of the capacity for aerobic metabolic flux at low temperatures. The compensatory responses of cold-active species can be used to suggest the temperature range over which the activities of glycolytic and tricarboxylic acid cycle enzymes in a muscle, i.e., the muscle's "metabolic profile," can suffice. Analysis of the available data suggests that a 16 degrees C range of temperatures cannot be covered by one metabolic profile, even when the preferred temperatures are centered between the acclimation temperatures. For cold-inactive species that remain normoxic during winter dormancy, the compensatory metabolic modifications may facilitate lipid catabolism. Alternately, an increased aerobic capacity may be adaptive during the relatively cold periods that precede and follow winter dormancy. For goldfish and carp that encounter hypoxia and anoxia during winter dormancy, increased mitochondrial abundance could facilitate ethanol production during anoxia and the diffusion of oxygen to mitochondria during hypoxia. Finally, metabolic modifications during natural acclimatization indicate both thermal compensation and direct thermal effects and suggest that thermal compensation may be masked by reproductive and feeding activities.