Roles of lactate and catecholamines in the energetics of brief locomotion in an ectothermic vertebrate.

We have investigated the magnitude and duration of excess post-exercise oxygen consumption (EPOC) in a lizard following a single bout of vigorous exercise of 5-60 s, common activity durations for many ectothermic vertebrates. Desert iguanas (Dipsosaurus dorsalis) were run for 5 s, 15 s, 30 s, or 60 s. Oxygen consumption (VO2) increased from 0.16 ml O2 g(-1) h(-1) at rest to 1.3-1.6 ml O2 g(-1) h(-1) during 5-60 s of running. EPOC duration increased with activity duration, ranging from 35-63 min. EPOC volume, the excess oxygen consumed post-exercise, doubled from 0.13 ml O2 g(-1) following 5 s of activity to 0.25 ml O2 g(-1) after 60 s. EPOC represented 91-98% of the total metabolic expense of the activity. EPOC durations were always shorter than the period required for lactate removal, illustrating that these two processes are not causally related. Alpha- and beta-adrenergic receptor blockade by phentolamine and propranolol had no effect on resting VO2 but depressed excess post-exercise oxygen consumption volumes 2540%. The extent of catechol stimulation post-exercise may be motivation or stimulus dependent. The data indicate that metabolic elevations post-exercise represent the majority of activity costs in lizards. The study suggests that EPOC of ectothermic vertebrates is sensitive to exercise duration and catecholamine release post-activity, even when activity periods are less than 60 s in duration.

Activity before exercise influences recovery metabolism in the lizard Dipsosaurus dorsalis.

During recovery from even a brief period of exercise, metabolic rate remains elevated above resting levels for extended periods. The intensity and duration of exercise as well as body temperature and hormone levels can influence this excess post-exercise oxygen consumption (EPOC). We examined the influence of activity before exercise (ABE), commonly termed warm-up in endotherms, on EPOC in the desert iguana Dipsosaurus dorsalis. The rate of oxygen consumption and blood lactate levels were measured in 11 female D. dorsalis (mass 41.1 +/- 3.0 g; mean +/- s.e.m.) during rest, after two types of ABE and after 5 min of exhaustive exercise followed by 60 min of recovery. ABE was either single (15 s of maximal activity followed by a 27 min pause) or intermittent (twelve 15 s periods of exercise separated by 2 min pauses). Our results indicate that both single and intermittent ABE reduced recovery metabolic rate. EPOC volumes decreased from 0.261 to 0.156 ml of oxygen consumed during 60 min of recovery when lizards were subjected to intermittent ABE. The average cost of activity (net V(O2) during exercise and 60 min of recovery per distance traveled) was almost 40 % greater in lizards that exercised without any prior activity than in lizards that underwent ABE. Blood lactate levels and removal rates were greatest in animals that underwent ABE. These findings may be of particular importance for terrestrial ectotherms that typically use burst locomotion and have a small aerobic scope and a long recovery period.

The roles of acidosis and lactate in the behavioral hypothermia of exhausted lizards.

We conducted this study to determine whether two of the physiological changes associated with non-sustainable exercise, elevated blood lactate levels and decreased arterial pH, contribute to the behavioral hypothermia exhibited by exhausted lizards. Dipsosaurus dorsalis were placed in a thermal gradient and their body temperatures were recorded from 08:00 to 14:00 h. At 14:00 h, animals were subjected to different experimental regimens. In the exercise (E) regimen, animals at 40 degrees C were forced to exercise maximally for 5 min on a treadmill. In the lactate (L) regimen, animals were infused with 11.5 ml kg-1 of 250-500 mmol l-1 sodium lactate. In the osmolarity control (O) regimen, animals were injected with 11.5 ml kg-1 of 500 mmol l-1 NaCl, and in the injection control (I) regimen, animals were injected with 11.5 ml kg-1 of 150 mmol l-1 NaCl. In the hypercapnia (H) regimen, the thermal gradient was flushed with a gas mixture containing 10 % CO2, 21 % O2 and 69 % N2, a treatment that lowers the arterial pH of D. dorsalis to a value comparable with that imposed by exhaustive exercise. A group of control (C) animals was left undisturbed in the thermal gradient for 24 h. Animals in all experimental groups were returned to the thermal gradient, and their cloacal temperatures were monitored until 08:00 h the following morning. The mean cloacal temperature of E animals underwent a significant decrease of 4-7 degrees C, relative to control animals, which persisted for 7 h. The mean cloacal temperatures of animals subjected to 2 h of regimen H also decreased by 3.5-9 degrees C and remained depressed for 12 h following the beginning of the treatment. L, O and I animals did not undergo a significant change in body temperature following treatment, and their mean body temperatures did not differ from those of C animals at any time during the experiment. The results of this study suggest that the metabolic acidosis, but not the elevated blood lactate level, that follows exhausting exercise might play a role in the behavioral hypothermia that follows exhausting exercise in D. dorsalis.

The influence of corticosterone and glucagon on metabolic recovery from exhaustive exercise in the desert iguana Dipsosaurus dorsalis.

The skeletal muscles of ectothermic vertebrates possess an elevated glyconeogenic capacity that is responsible for a major portion of lactate removal and glycogen resynthesis following exercise. In lizards, changes in plasma hormone levels and the influence of differing hormone levels on muscle metabolism postexercise are poorly understood. We measured the effects of 5 min of exhaustive exercise on plasma levels of glucagon and corticosterone in the desert iguana Dipsosaurus dorsalis. We also determined the extent to which these hormones influence, or are influenced by, postexercise plasma lactate concentrations postexercise. Exercise resulted in the accumulation of 20 mM blood lactate, while plasma glucose levels remained stable throughout 90 min of recovery. Plasma glucagon was elevated sevenfold during 5 min of exercise and returned to resting levels within 45 min of recovery. Glucagon stimulated lactate incorporation into glycogen in isolated red muscle fiber bundles. Plasma corticosterone was also elevated to three times normal resting values, but only after 45 min of recovery. Blocking corticosterone elevation with metyrapone did not alter the kinetics of plasma lactate removal. In lizards, the dramatic rise in plasma glucagon occurs at the same time as previously reported elevated skeletal muscle glyconeogenesis and elevated glucagon stimulates lactate removal in vitro, strongly suggesting a role for glucagon in postexercise skeletal muscle metabolism.

Alterations in key gluconeogenic regulators with age and endurance training.

The purpose of the present investigation was to examine changes in six potential regulators of hepatic gluconeogenesis with normal aging and endurance training: fructose 2,6-bisphosphate (F 2,6-P2), mitochondrial and cytosolic phosphoenolpyruvate carboxykinase (PEPCK) activity, PEPCK mRNA, and pyruvate carboxylase and malate dehydrogenase activity. Young (4 months), middle-aged (12 months), and old (22 months) male-Fischer 344 rats (N = 66) were divided into trained and sedentary groups. Trained animals were run 1 h/d, 5 d/wk for 10 weeks at treadmill speeds of 75% age-specific maximal running capacity. Animals were killed at rest, and the right main lobe of the liver was removed. F 2,6-P2 levels were significantly greater in old compared with young animals regardless of training condition (119% and 80% increase in old trained and untrained animals, respectively). No changes were found with training. Rates of cytosolic PEPCK activity declined significantly with age in both trained (1.3 +/- 0.1, 1.0 +/- 0.1, and 0.7 +/- 0.1 mumol/g/min in young, middle-aged, and old, respectively) and untrained (1.3 +/- 0.1, 1.1 +/- 0.1, and 0.8 +/- 0.2 mumol/g/min) groups. Training did not result in any significant differences between age groups. PEPCK gene expression (mRNA) determined by Northern blot analysis decreased 30% in trained and untrained old animals compared to the young counterparts; again, training had no effect in any age group. No significant differences were found in pyruvate carboxylase, mitochondrial PEPCK, or malate dehydrogenase activity with either age or training. These results suggest that previous age-related declines found in hepatic gluconeogenic capacity can be attributed, in part, to changes in F 2,6-P2, cytosolic PEPCK activity, and PEPCK mRNA, but not to alterations in the activities of mitochondrial PEPCK, malate dehydrogenase, or pyruvate carboxylase. Since training had no effect on any regulator studied, the factors responsible for attenuation in the age-related decline in gluconeogenesis with training remain to be determined.

Regulation of skeletal muscle metabolism in the lizard Dipsosaurus dorsalis by fructose-2,6-bisphosphate.

Changes in liver and skeletal muscle fructose-2,6-bisphosphate (Fru-2,6-P2) concentrations were compared during fasting, exercise, and recovery in the lizard Dipsosaurus dorsalis and in outbred mice (Mus musculus). We present the first correlative evidence that suggests that a decrease in the content of Fru-2,6-P2 may mediate elevated gluconeogenesis in lizard skeletal muscle. Contents of Fru-2,6-P2 in lizard gastrocnemius and red and white iliofibularis (IF) were significantly lower (as much as 55% in white IF) during recovery from exhaustive exercise than at rest. Recovery from exhaustive exercise had no significant effect on Fru-2,6-P2 concentrations in any mouse muscle examined. Fasting significantly depressed lizard and mouse liver Fru-2,6-P2 contents and decreased lizard red IF by over 84% from the fed condition. Lizard red and white muscle fiber bundles incubated in 20 mM lactate had significantly lower Fru-2,6-P2 (94 and 61% depression, respectively) than those incubated in 8.5 mM glucose. These results are consistent with the hypothesis that Fru-2,6-P2 acts as a signal for controlling gluconeogenesis in lizard skeletal muscle.

Sensitivity of Metabolic Rate, Growth, and Fecundity of Tadpole Shrimp Triops longicaudatus to Environmental Variation.

The influence of fluctuations of ambient oxygen tensions and temperature on the rate of oxygen consumption (VO2) was determined for the tadpole shrimp, Triops longicaudatus. VO2 was oxygen dependent up to 185 torr PO2, and Q10 for oxygen consumption between 20° and 30°C was 1.9. From these results it was estimated that oxygen consumption increases more than 1100 µl · g-1 · h-1 in T. longicaudatus for typical diurnal changes in temperature and oxygen in desert ephemeral pools. Elevated VO2 may be coupled with increased growth rate and fecundity, because these characteristics were highly sensitive to changes in ambient temperature and oxygen tension. Depressing mean daily temperatures by 2.3°C significantly decreased body mass, whereas hyperoxia (200 torr) significantly increased growth compared to that of animals raised under hypoxic conditions (70 torr). Fecundity was dependent on animal mass and ambient oxygen tension. Thus, for a 22-day season, one T. longicaudatus female could produce 30 more eggs per 10 torr increase in oxygen tension and 43 more eggs per 1°C change in mean daily temperature. These results indicate that there are selective pressures for metabolic sensitivity to the high temperature-high oxygen conditions of the ephemeral environments inhabited by T. longicaudatus.

Acid-base status of a pulmonate land snail (Helix aspersa) and a prosobranch amphibious snail (Pomacea bridgesi) during dormancy.

Changes in metabolism and acid-base status were compared during dormancy in the pulmonate land snail Helix aspersa and a prosobranch amphibious snail Pomacea bridgesi. The typical condition of higher blood PCO2 and bicarbonate levels for air-breathing versus water-breathing vertebrates was shown for the two snail species. When exposed to dry air for 24 hr, both species depressed oxygen uptake by about 65%. In Pomacea, hypercapnia (increase in hemolymph PCO2 from 5.5 to 18 torr) resulting from dormancy produced no significant change in pH due to large increases in bicarbonate (over 17 mmol/l). In Helix, on the other hand, hypercapnia (increase in hemolymph PCO2 from 13 to 18 torr) resulting from dormancy produced a significant decrease in pH and a less than 7 mmol/l increase in bicarbonate. Pre-existing high levels of bicarbonate in Helix may prevent compensation of hypercapnia resulting from dormancy, similar to the case described for air-breathing vertebrates. Complete compensation of respiratory acidosis during the first 24 hr of dormancy in Pomacea suggests that metabolic rate suppression is independent of pH.