CAPILLARY-FIBER GEOMETRY IN PECTORALIS-MUSCLES OF ONE OF THE SMALLESTBATS

We previously reported striking similarities in the structural capacit y for O, flux in the highly aerobic flight muscles of a hummingbird an d bat despite their significant differences in capillary-fiber geometr y and number, and fiber size. However, the bats of that study (Eptesic us fuscus, BW 15-16 g) were about 5 times larger than the hummingbirds (Selasphorus rufus; BW 3-4 g) In this study, we examined the flight m uscle in a bat of approximately the same size as the hummingbird to de termine whether features found in the big brown bat would be accentuat ed or if there would be additional similarities with the hummingbird. The pectoralis muscle of pipistrelle bats Pipistrellus hesperus (BW 3- 5 g) was perfusion-fixed in situ, processed for electron microscopy an d analyzed by morphometry. Fiber size (group mean +/- SE, 314 +/-22 mu m(2) at 2.1 mu m sarcomere length) and capillary geometry (high degre e of tortuosity and branching) were remarkably similar to those in pec toralis muscle of the big brown bat. Thus distances from capillaries t o the center of the fibers were not reduced in pipistrelle flight musc le (as in hummingbird) nor was capillary tortuosity and branching furt her increased (compared with big brown bat). Capillary-fiber surface r atio at a given mitochondrial volume/mu m length of fiber was high and similar to that in big brown bat and hummingbird, consistent with the idea that the size of the capillary-fiber interface plays an importan t role in providing the great O-2 flux potential in these muscles. In addition, capillary-fiber number at a given fiber mitochondrial volume per mu m length of fiber was similar to that in other muscles includi ng big brown bat and hummingbird flight muscle, bat hindlimb and rat M . soleus. This supports the notion of a close relationship between cap illary number and mitochondrial volume on an individual fiber basis in aerobic muscles.