Goldfish acclimated to 20 and 5°C were subjected to anoxia (3 h at 20°C and 18 h at 5°C). The intracellular pH (pHi) and the levels of high-energy phosphate compounds in the epaxial muscle were continuously monitored by in vivo31P-n. m.r. spectroscopy. Free [ADP] was calculated from n.m.r.-measured reactants of the creatine kinase reaction and the creatine kinase equilibrium constant. A rapid initial drop in pHi was followed by a sudden decrease in the rate of development of acidosis. Similar biphasic time courses were observed for the increases in the levels of free ADP and inorganic phosphate and the decrease in phosphocreatine concentration.

In a parallel series of experiments, fish were placed in the flow cell of the n.m.r. probe (outside the magnet) and anesthetized after different anoxic intervals. Metabolites in red muscle, white muscle and blood were then assayed, using classical biochemical techniques. The transition to a less rapid decline in pH appears to be primarily due to a suppression of energy demand and activation of H+ extrusion; ethanol production becomes significant only after longer periods of anoxia. Anoxic exposure has no influence on the kinetic properties of muscle alcohol dehydrogenase, but acclimation of goldfish to low temperature increases alcohol dehydrogenase activity in white muscle.

Note: Present address: PET Center, Academic Hospital, Groningen University, PO Box 30.001, 9700RB Groningen, The Netherlands.

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