ABSTRACT There is a wide interspecific range in plasma glucose levels in teleosts from less than 0.5 to greater than 10 mmol l −1 . Here we assessed how glucose availability influences glucose metabolism in hearts of Atlantic cod ( Gadus morhua ), rainbow trout ( Oncorhynchus mykiss ), lumpfish ( Cyclopterus lumpus ) and short-horned sculpin ( Myoxocephalus scorpius ) under normoxic and hypoxic conditions. These species had plasma glucose levels of 5.1, 4.8, 0.9 and 0.5 mmol l −1 , respectively. Rates of glucose metabolism and lactate production were determined in isolated hearts perfused with medium containing physiological levels of glucose. Under normoxic conditions there was no significant difference in rates of either glucose metabolism (average 15 nmol g −1 min −1 ) or lactate production (average 30 nmol g −1 min −1 ) across species. Under hypoxia (12% of air saturation) there were significant increases in rates of glucose metabolism and lactate production in hearts from Atlantic cod (glucose-130; lactate-663 nmol g −1 min −1 ) and rainbow trout (glucose-103; lactate-774 nmol g −1 min −1 ); however, there was no change in rate of glucose metabolism in hearts from either lumpfish or short-horned sculpin and only increases in lactate production to rates much lower than the other species. Furthermore, Atlantic cod hearts perfused with medium containing low non-physiological levels of glucose (0.5 mmol l −1 ) had the same rates of glucose metabolism under normoxic and hypoxic treatments. Anaerobic metabolism supported by extracellular glucose is compromised in fish with low levels of plasma glucose, which in turn may decrease performance under oxygen-limiting conditions at the whole-animal level.

ABSTRACT Fish exhibit a wide range of species-specific blood glucose levels. How this relates to glucose utilization is yet to be fully realized. Here, we assessed glucose transport and metabolism in myocytes isolated from Atlantic cod ( Gadus morhua ) and short-horned sculpin ( Myoxocephalus scorpius ), species with blood glucose levels of 3.7 and 0.57 mmol l −1 , respectively. Glucose metabolism was assessed by the production of 3 H 2 O from [2- 3 H]glucose. Glucose metabolism was 3.5- to 6-fold higher by myocytes from Atlantic cod than by those from short-horned sculpin at the same level of extracellular glucose. In Atlantic cod myocytes, glucose metabolism displayed what appears to be a saturable component with respect to extracellular glucose, and cytochalasin B inhibited glucose metabolism. These features revealed a facilitated glucose diffusion mechanism that accounts for between 30% and 55% of glucose entry at physiological levels of extracellular glucose. Facilitated glucose diffusion appears to be minimal in myocytes for short-horned sculpin. Glucose entry by simple diffusion occurs in both cell types with the same linear relationship between glucose metabolism and extracellular glucose concentration, presumably due to similarities in membrane composition. Oxygen consumption by myocytes incubated in medium containing physiological levels of extracellular glucose (Atlantic cod 5 mmol l −1 , short-horned sculpin 0.5 mmol l −1 ) was similar in the two species and was not decreased by cytochalasin B, suggesting that these cells have the capability of oxidizing alternative on-board metabolic fuels. Cells produced lactate at low rates but glycogen levels did not change during the incubation period. In cells from both species, glucose utilization assessed by both simple chemical analysis of glucose disappearance from the medium and 3 H 2 O production was half the rate of lactate production and as such extracellular glucose was not available for oxidative metabolism. Overall, extracellular glucose makes only a minor contribution to ATP production but a sustained glycolysis may be necessary to support Ca 2+ transport mechanisms at either the sarcoplasmic reticulum or the sarcolemmal membrane.

SUMMARY Oxygen consumption, as an indicator of routine metabolic rate (RoMR), and tissue-specific changes in protein synthesis, as measured by 3 H-labelled phenylalanine incorporation rates, were determined in Astronotus ocellatus to investigate the cellular mechanisms behind hypoxia-induced metabolic depression and recovery. RoMR was significantly depressed, by approximately 50%, when dissolved oxygen levels reached 10%saturation (0.67±0.01 mg l –1 at 28±1°C). This depression in RoMR was accompanied by a 50–60% decrease in liver,heart and gill protein synthesis, but only a 30% decrease in brain protein synthesis. During recovery from hypoxia, an overshoot in RoMR to 270% of the normoxic rate was observed, indicating the accumulation of an oxygen debt during hypoxia. This conclusion was consistent with significant increase in plasma lactate levels during the hypoxic exposure, and the fact that lactate levels rapidly returned to pre-hypoxic levels. In contrast, a hyperactivation of protein synthesis did not occur, suggesting the overshoot in oxygen consumption during recovery is attributed to an increase in cellular processes other than protein synthesis.

SUMMARY The effects of hypoxia on energy economy of cardiac muscle were compared between the hypoxia-tolerant freshwater turtle at 20°C and the hypoxia-sensitive rainbow trout at 15°C. Isolated ventricular preparations were left either at rest or stimulated at 30 min -1 to develop isometric twitch force. Under oxygenated conditions, twitch force and oxygen consumption were similar for the two species. Overall metabolism was reduced during severe hypoxia in both resting and stimulated preparations and under these conditions most of the ATP production was derived from anaerobic metabolism. During hypoxia, a metabolic depression of approximately 2/3 occurred for non-contractile processes in both turtle and trout preparations. During hypoxia, lactate production and residual oxygen consumption were similar in turtle and trout. Cellular energy state and phosphorylation potential decreased during severe hypoxia in both species and this reduction was more severe in preparations stimulated to contraction. However, in turtle ventricular preparations the energy state and phosphorylation potential stabilised at higher levels than in trout, and turtle preparations also maintained a higher twitch force throughout the hypoxic period. Moreover,twitch force relative to total ATP hydrolysis was markedly increased during hypoxia in turtle while this ratio was unchanged for trout. The main findings of this study are: (1) cellular energy liberation and the energy demand of non-contractile processes decreased to similar levels in hypoxic turtle and trout myocardium; (2) turtle myocardium maintained a substantially higher cellular energy state and twitch force development than trout myocardium during hypoxia and (3) the ratio of twitch force to ATP hydrolysis increased during hypoxia in turtle but was unchanged in trout. It is possible that this superior economy of the contracting turtle myocardium contributes to the remarkable hypoxia tolerance of freshwater turtles.

SUMMARY Mechanisms of intracellular pH (pHi) regulation were investigated in anoxia-tolerant hepatocytes from goldfish Carassius auratus , and compared to the situation in the anoxia-intolerant hepatocytes from trout Oncorhynchus mykiss . Under normoxic conditions, the pHi of goldfish hepatocytes was regulated by a Na + /H + exchanger and a Na + -independent Cl – /HCO 3 – exchanger, the latter being activated only after acidification of the cells. Mechanisms of acid secretion appear to be fuelled, at least in part, by lactate formation under fully aerobic conditions, as inhibition of glycolysis caused a drastic reduction of steady state proton release. In trout hepatocytes both a Na + /H + exchanger and a Cl – /HCO 3 – exchanger were found to be tonically active, as described previously. During chemical anoxia a constant pHi was maintained in goldfish hepatocytes, whereas it was reversibly reduced by 0.3 units in the trout cells. Under these conditions a reversible increase in the rate of acid secretion was induced in the cells from both species. In the goldfish cells this was based on a SITS-sensitive transporter, possibly involving export of lactate, with no contribution from Na + /H + exchange. By contrast, in hepatocytes from trout, CN-induced acid secretion was dominated by the activity of the Na + /H + exchanger. Brief exposure to extracellular acidosis had no dramatic effects on the energetics of hepatocytes from either species.