METABOLIC ADAPTATION OF THE HORSESHOE CRAB,LIMULUS POLYPHEMUS, DURING EXERCISE AND ENVIRONMENTAL HYPOXIA AND SUBSEQUENT RECOVERY

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Abstract
Metabolic responses to exercise, to exposure to environmental anaerobiosis, and to subsequent recovery were investigated in muscle, hepatopancreas, and hemolymph of the horseshoe crab, Limulus polyphemus. Exercise caused a considerable decline in arginine phosphate and the formation of D-lactate in muscle tissue, whereas the adenylate energy charge was maintained. Some of the D-lactate appears to have been transported from muscle tissue into the hemolymph. This occurred, apparently, during exercise as well as during recovery. Hemolymph postbranchial PO2, which decreased during exercise and tissue phosphagen stores were rapidly restored to aerobic control values upon recovery, while D-lactate oxidation was protracted, especially in muscle. Environmental anaerobiosis for 48 h was fueled by the breakdown of arginine phosphate (considerable only in muscle tissue) and glycogen, resulting in the accumulation of arginine, D-lactate, and alanine in both muscle and hepatopancreas. Alanine production may occur via glutamate-pyruvate transaminase and glutamate dehydrogenase, which take over the role of D-lactate dehydrogenase to maintain redox balance during the later phases of anaerobiosis. Recovery from anaerobiosis was characterized by a rapid replenishment of the phosphagen, a rapid drop in alanine concentration, and a protracted time-course for the decline in D-lactate levels, which was somewhat faster in the hepatopancreas than in muscle tissue.