Lactate and Glycogen Metabolism in the Lizard Dipsosaurus Dorsalis following Exhaustive Exercise

Abstract

ABSTRACT We evaluated the metabolic mechanisms by which the iguanid lizard Dipsosaurus dorsalis deals with the lactate which accumulates during vigorous exercise. Fasted, cannulated lizards were run for 5 min on a treadmill at 40°C, which elevated whole-body lactate to 24 mmol l-1 and depleted hindlimb glycogen to 70% of resting levels. Oxygen consumption increased fivefold and respiratory exchange ratios approached 2·0. Exhausted animals were then injected intravenously with either [U-14C]lactate or [U-14C]glucose, and allowed to recover quietly on the treadmill at 40°C. After 2h, 79% of the accumulated lactate had been removed and hindlimb muscle glycogen stores had returned to pre-exercise levels. Although blood glucose remained unchanged at 8·6 ± 0·27 mmol l-1 throughout the recovery period, whole-body glucose increased significantly from 1·6 ±0·23 to 5·5 ± 0·38mmoll-1 (P<0·05). Based on isotope distribution, 50% of the lactate removed was used to synthesize glucose and glycogen, but only 16 % of the lactate was oxidized. Lactate oxidation accounted for about 40% of the post-exercise oxygen consumption. Lactate rather than glucose appeared to be the prevalent substrate for muscle glycogen synthesis under these conditions. These animals appear to employ a strategy of lactate removal which is different from that in mammals; favoring lactate-supported gluco-and glyconeogenesis and rapid muscle glycogen replenishment instead of rapid lactate removal via oxidative pathways.