Myoglobin Adaptation in Terrestrial and Diving Birds and Mammals

Abstract

Myoglobin (Mb) is an oxygen binding hemoprotein in vertebrate skeletal muscle that functions in intracellular oxygen storage and transport. Due to the unique oxygen storage demands of diving birds and mammals, these vertebrates can have Mb concentrations ten-fold those found in their terrestrial counterparts making them ideal animal models for studying Mb function. Increased Mb bound muscle oxygen stores are advantageous for diving vertebrates, but Mb concentration optimized to maintain aerobic metabolism while diving or limiting to aerobic dive duration? A numeric model simulating a diving Weddell seal was created to examine physiological factors that influence dive duration and optimal Mb concentration. Mb concentration was limiting to dive duration in postabsorptive dives. However, Mb concentration was optimized for postprandial dives which were limited by blood-bound oxygen stores due to the additional metabolic costs of digestion.

While Mb concentration is adaptive in diving vertebrates, less is known about molecular adaptation of Mb functional properties. Novel methods were developed to extract Mb from frozen muscle and determine Mb oxygen affinity (P_(50)) by generating a high resolution oxygen dissociation curve at 37°C. For comparison, Mb P_(50) was determined for 25 species of diving and terrestrial birds and mammals. Myoglobin P_(50) was conserved among terrestrial vertebrates and most cetaceans at approximately 3.7 mmHg with the exception of the melon-headed whale that had a significantly higher P_(50) (lower oxygen affinity) of 4.85 mmHg. Among pinnipeds (seals and sea lions) the P_(50) ranged from 3.23-3.81 mmHg and showed a trend for higher oxygen affinity in species with longer dive durations. Among diving birds the P_(50) ranged from 2.40-3.36 mmHg and also showed a trend of higher affinities in species with longer dive durations. Both myoglobin concentration and oxygen affinity appear adaptive in diving vertebrates to maintain aerobic metabolism and minimize hypoxic cellular damage in ischemic muscle.