We measured intracellular acid-base balance and indicators of carbohydrate and high-energy phosphate metabolism as lampreys recovered from exhaustive exercise. A combined respiratory and metabolic acidosis was observed in the locomotory muscle following ‘burst’ exercise. Muscle pH decreased from approximately 7.2 to 6.7, whereas intracellular PCO2 rose from approximately 0.6 to 1.6 kPa. Unlike the situation in similarly stressed teleost fish such as rainbow trout, the respiratory acidosis in muscle persisted for several hours. This apparent CO2 retention in lamprey muscle may be the result of a restricted ability of the circulatory system to transport CO2 due to reduced erythrocyte anion exchange, or it could represent a restricted ability of the muscle itself to clear the intracellular pool of CO2 due to reduced carbonic anhydrase activity. Maximal lactate dehydrogenase activity of lamprey muscle exhibited a marked dependence on pH, increasing in activity by 30 % as pH decreased from 7.2 to 6.7 (reflecting the ‘resting’ to ‘post- exercise’ pH change observed in vivo). Following exhaustive exercise, the acid-base balance of the muscle is influenced by both proton- consuming (e.g. AMP deamination, glycogen replenishment) and proton-producing (e.g. rephosphorylation of creatine) metabolic processes. The net effect is that, although intracellular pH is maximally depressed, energy stores such as phosphocreatine and glycogen are partially restored within 1 h of exhaustive exercise, placing the animal in good stead for further locomotory work.

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