Changes in carbohydrate metabolism during acclimation to a moderate hyperthermic environment in rats.

We studied the influence of heat acclimation (1 to 48 h and 4 to 60 d at 35 +/- 1 degrees C) on certain hepatic carbohydrate-related enzymes and substrates in rats. The results showed a decrease of liver glycogen content and GPho-ase a activity during the period of short-term exposure, followed by normalization to the control level and stabilization to the new level in the period of long-term heat acclimation. Conversely, G-6-P-ase and F-1,6-BP-ase activities increased during the short-term period, followed by a decrease and stabilization to a new, lower level in the prolonged acclimation. The blood glucose level decreased during whole period of acclimation, whereas intermediate substrates increased during the short-term and stabilized at a new, higher level during prolonged acclimation. The time-dependent changes of duration of heat acclimation could be summarized in three phases: short-term heat exposure (1 to 24 h) with intensive glycogenolysis and gluconeogenesis to glucose; a period with temporary changes (24 h to 7 d) with tendency of normalization to control level, and prolonged heat acclimation (7 d to 60 d), which favors both direct and indirect glycogen synthesis.