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Metabolic responses to experimental diving in adult and fetal weddell seals Murphy, Brian Joseph

Abstract

Metabolic potentials and activities of the heart, lung and brain were examined in the Antarctic Weddell seal, a species displaying outstanding diving abilities. The activities of representative enzymes in oxidative and fermentative metabolism in both the adult and near-term fetus are similar to those in homologous organs of the adult ox, but the brain and heart in both groups of seals contained elevated levels of lactate dehydrogenase. The isozyme data indicate that all three organs in the adult and fetus have the potential for either lactate production or utilization depending upon metabolic conditions. During experimental diving (up to 30 min) glucose appears to be a critical carbon and energy source for the adult. Glucose is utilized in a mixed aerobic and anaerobic metabolism. The consequent fall in blood glucose levels and rise in lactate levels are due predominantly to peripheral, hypoperfused tissues, but the central organs influence these metabolite pools as well. The brain of the adult seal utilizes glucose at a rate of approximately 0.3 μmol/gm/hr., releasing 20-25% as lactate; this proportion does not change throughout diving-recovery cycles. The lung consumes lactate and thus diminishes its accumulation in the blood during the dive. One, and possibly the main, fate of lactate absorbed by the lung is oxidation since ¹⁴CO₂ is the only measurable derivative found in aortic blood following ¹⁴C-lactate infusion into the right ventricle. During recovery, when blood lactate levels rise above 6 μmol/ml, the brain switches from lactate release to lactate uptake at a rate high enough to readily support the normal metabolic rate of this organ (about 8 μmol ATP/gm/hr). Enzyme and metabolite measurements suggest that the lung and heart also contribute to lactate clearance and re-establishment of metabolic homeostasis following diving in the adult. Blood metabolite analyses suggest little involvement of the majority of free amino acids during dive-recovery cycles. Only alanine and glutamine increase throughout both phases and both probably function as waste nitrogen carriers. Alanine accumulation could also be caused by anerobic catabolism of protein and/or anaerobic-aerobic transitions in tissues such as skeletal muscles. Both the physiological data and metabolite recovery profiles indicate fetal peripheral vasoconstriction and a fully developed diving response as a consequence of maternal diving. The fetus has apparently adapted to long-duration diving by significantly increasing its blood storage capacity of glucose to beyond that of the mother. Furthermore, these elevated stores appear to be under tight regulation during the latter stages of experimental diving.

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