High aerobic capacities in the skeletal muscles of pinnipeds: adaptations to diving hypoxia

The objective was to assess the aerobic capacity of skeletal muscles in pin nipeds. Samples of swimming and nonswimming muscles were collected from Ste ller sea lions (Eumetopias jubatus, n = 27), Northern fur seals (Callorhinu s ursinus, n = 5), and harbor seals (Phoca vitulina, n = 37) by using a nee dle biopsy technique. Samples were either immediately fixed in 2% glutarald ehyde or frozen in Liquid nitrogen. The volume density of mitochondria, myo globin concentration, citrate synthase activity, and beta-hydroxyacyl-CoA d ehydrogenase was determined for all samples. The swimming muscles of seals had an average total mitochondrial volume density per volume of fiber of 9. 7%. The swimming muscles of sea lions and fur seals had average mitochondri al volume densities of 6.2 and 8.8%, respectively. These values were 1.7- t o 2.0-fold greater than in the nonswimming muscles. Myoglobin concentration , citrate synthase activity, and beta-hydroxyacyl-CoA dehydrogenase were 1. 1- to 2.3-fold greater in the swimming vs. nonswimming muscles. The swimmin g muscles of pinnipeds appear to be adapted for aerobic lipid metabolism un der the hypoxic conditions that occur during diving.