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.
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