STRUCTURAL BASIS FOR OXYGEN DELIVERY - MUSCLE CAPILLARIES AND MANIFOLDS IN TUNA RED MUSCLE

We summarize our morphometric data on fiber vascularization and aerobi c capacity in red muscle of tuna (Katsuwonus pelamis), compared to int ensely aerobic flight muscles of hummingbird (Selasphorus rufus, BW 3- 4 g) and bat (Eptesius fuscus, BW 15-16 g, Pipistrellus hesperus, BW 3 -5 g). Three characteristic features of high nux paths for oxygen: (a) small fiber size, (b) dense capillary network and (c) high mitochondr ial volume density were found in tuna, but they were not as pronounced as in hummingbird and bat flight muscles. A particular arrangement of capillary manifolds, also seen in flight muscle of birds but not in b ats, was found in tuna, forming dense envelopes of capillary branches around portions of muscle fibers. However, all indexes of fiber capill arization were relatively low in tuna red muscle for its mitochondrial volume, compared with other intensely aerobic muscles. Capillary leng th per unit volume of mitochondria, and capillary surface per mitochon drial inner (and outer) membrane surface area, were about one half of those in hummingbird or bat flight muscles. Consistent differences exi st in the size of the capillary network for the size of the mitochondr ial compartment in highly aerobic red muscle of tuna compared with bir d and mammal.