Low levels of extracellular glucose limit cardiac anaerobic metabolism in some species of fish

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 it is 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 media 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 treatment. 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.