Gluconeogenesis is the principal mechanism of maintaining blood glucose levels within narrow limits during periods of fast, starvation, and exercise. Gluconeogenesis can also function to replenish depleted muscle glycogen stores after strenuous exercise. While lactate can serve as substrate for gluconeogenic replenishment of muscle glycogen following activity, most studies of mammals have indicated that the fate of most post-exercise lactate is oxidation to CO2 and H2O. Recent research has demonstrated that the major fraction of post-exercise lactate is gluconeogenically converted back to muscle glycogen in the lizard Dipsosaurus dorsalis. Even more striking was the observation that the site of gluconeogenesis was skeletal muscle rather than the liver, as is the case in mammals. In fact, lactate is the preferred substrate for glycogenesis in lizard muscle. In vivo and in vitro data demonstrate that red muscle has a greater capacity for glyconeogenesis than does white muscle, but that lactate incorporation into glycogen in both muscle types is stimulated by post-exercise levels of epinephrine. The proposed research will continue investigation of this strategy of lactate and glycogen metabolism through a side-by-side comparison of mouse and lizard muscle. The investigators' aim will be to determine the hormonal and biochemical mechanisms in muscle that result in lactate-supported oxidation in one case, and lactate-supported glyconeogenesis in the other. Experiments will investigate the regulation of muscle gluconeogenesis in vivo and in vitro by pH and by hormones, factors which change during recovery from exercise. Preliminary data indicate that activities of enzymes thought to regulate glycolytic/gluconeogenic flux change during the rest-to-recovery transition. The activities of enzymes such as PK and PEPCK, PFK and FDPase, and glycogen synthase and phosphorylase will be examined during this transition, and the effects that select hormones (insulin, epinephrine, corticosterone, and glucagon) have on these changes will be studied to determine possible mechanisms for hormonal regulation. The capacity and regulation of liver lactate metabolism under the same conditions will also be examined, as recent reports question whether or not lower vertebrate liver has any significant role in post-exercise metabolism. Nine experiments are described, the results of which, when taken together, will describe the process and regulation of lactate metabolism following exhaustive exercise in a vertebrate with gluconeogenic skeletal muscle, and contrast that to mouse muscle which lacks this gluconeogenic propensity. These studies are intended to describe in a more integrated fashion the metabolic function of """"""""gluconeogenic"""""""" skeletal muscle following vigorous activity, its enzymatic and hormonal regulation, and the reduced role that liver apparently plays in the recovery process.
