Metabolic activities of heart, lung, and brain during diving and recovery in the Weddell seal

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Abstract
The metabolic potentials and activities of the heart, lung and brain were studied in the Weddell seal, a species displaying outstanding diving abilities. The activities of representative enzymes in oxidative and fermentative metabolism were similar to those in homologous bovine organs, but the brain and heart contained elevated levels of lactate dehydrogenase. Isozyme analyses indicated the potential in all 3 organs for either lactate production or lactate utilization, both conditions being found during diving-recovery cycles. During awake simulated diving, overall energy needs were supplied by glucose in a mixed aerobic and anaerobic metabolism; the consequent fall in blood glucose levels and rise in lactate levels were due predominantly to peripheral hypoperfused tissues, but the central organs influenced these metabolite pools as well. The brain utilized blood glucose at a rate of 0.3 .mu.mol .cntdot. g-1 .cntdot. min-1, releasing 20-25% as lactate; this proportion did not change through diving-recovery cycles. The lung utilized blood lactate preferentially at a rate of about 0.5 mmol/min and thus diminished its accumulation in the blood during diving. The main fate of lactate taken up by the lung was oxidation inasmuch as 14CO2 was the only measurable derivative found in aortic blood following [14C]lactate infusion into the right ventricle. During recovery, when blood lactate levels rose about 6 .mu.mol/ml, the brain switched from lactate release to lactate uptake at a rate high enough to readily support the normal metabolic rate of this organ (about 8 .mu.mol ATP .cntdot. g-1 .cntdot. min-1). Enzyme and metabolite measurements suggested that the lung and heart also contributed to lactate clearance and reestablishment of metabolic homeostasis following diving.

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