High levels of docosahexaenoic acid (22:6n-3)-containing phospholipids in high-frequency contraction muscles of hummingbirds and rattlesnakes.

Phospholipids containing docosahexaenoic acid (22:6n-3) have been proposed to be required as conformational cofactors for the functional assembly of membrane proteins such as rhodopsin, ion pumps and the various complexes of the mitochondrial electron transport chain (Infante, 1987, Mol. Cell. Biochem. 74, 111-116; Infante and Huszagh, 2000, FEBS Lett. 468, 1-5). This hypothesis predicts that high-frequency contraction muscles, which are endowed with a high content of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) and mitochondrial respiration enzymes, would have higher concentrations of 22:6n-3-containing phospholipids when compared with other muscles in the same species known to have a much lower contraction frequency. We have analyzed the fatty acid composition of ruby-throated hummingbird (Archilochus colubris) pectoral and leg muscles and of rattlesnake (Crotalus atrox) shaker and ventral muscles. We have found that hummingbird pectoral muscles, which are high contraction frequency muscles with the highest known respiratory rate among vertebrates, have a 22:6n-3 concentration of 20.8% vs. 4.9% for the low frequency leg muscles. Similarly, rattler muscles in rattlesnakes, also high contraction frequency muscles, have a higher 22:6n-3 concentration than that of their ventral muscles (15.1% vs. 10.6%, respectively). These results are consistent with a specific molecular role for 22:6n-3-containing phospholipids, as proposed.