PETERSEN, A.M.*; GLEESON, T.T.; Univ. of Colorado, Boulder: Characterization of metabolic recovery from activity in Rana catesbeiana

The dynamics of lactate transport and glycogen synthesis during activity and recovery were examined in 15& deg;C acclimated Rana catesbeiana. Frogs were injected with ¹⁴ C- lactate prior to activity. Average resting blood pH was 8.08. and fell to 7.65 immediately following 2 minutes of hopping. After 4 hours of recovery, blood pH returned to 7.98 but remained significantly different from resting values, suggesting incomplete recovery over 4 hours. Blood lactate levels rose from a resting value of 5.64 mM to a peak of 22.53 mM at 60 minutes of recovery. Although lactate is completely cleared in the gastrocnemius muscle by 4 hours of recovery, plasma lactate levels remain elevated by 2-3x over the same course of recovery. This incomplete recovery from activity in terms of plasma lactate and acid-base balance may indicate inhibition of lactate flux into muscle at 15& deg;C . A similar pattern of recovery has been reported only in significantly colder, hypoxic frogs. Heart lactate levels increased 5x following exercise and remained elevated significantly following 4 hours of recovery. The frogs oxidized approximately 3% and 5% of the injected labeled lactate following 4 hours of rest and 4 hours of recovery, respectively. A significant amount of label was recovered in the form of glycogen in the liver and muscle, demonstrating the gluconeogenic capabilities of these tissues. Labeled lactate was also recovered in brain tissue. These findings are in keeping with previous studies on fish and reptiles that demonstrate anaerobic fueling of exercise, with relatively low lactate oxidation and high rates of lactate incorporation into glycogen during recovery. Future studies will investigate the effects of ecologically relevant temperature fluctuations on key mediators of post-activity metabolism such as insulin, extracellular and intracellular pH, and metabolite production and clearance.