A comparison of the microcirculation in the rat spinotrapezius and diaphragm muscles.

Of all skeletal muscles examined in the rat, the spinotrapezius (S) and diaphragm (D) have the closest fiber-type composition. However, their oxidative capacities differ by two- to threefold. We have developed an intravital microscopy preparation to study diaphragm microcirculation in vivo. Using this preparation and the standard spinotrapezius model first described by S. D. Gray (1973, Microvasc. Res. 5, 395-400), we tested the hypothesis that pronounced microcirculatory differences would exist between these two muscles as a function of their disparate oxidative capacities. The lineal density of all capillaries in the spinotrapezius was 33.6 +/- 1.5 compared to 65.1 +/- 3.3 capillaries/mm in the diaphragm (P < 0.001). In the diaphragm compared with the spinotrapezius muscle, a significantly (P < 0.05) greater proportion of capillary countercurrent flow (D, 29 +/- 6% vs 8 +/- 6%) existed. Within both muscles, there was a similar proportion of capillaries supporting red blood cell (RBC) flow (S, 89 +/- 7% vs D, 92 +/- 2%). However, the diaphragm supported significantly (P < 0.001) greater intracapillary RBC velocities (D, 302 +/- 11 vs S, 226 +/- 9 micron/s) and fluxes (D, 33.4 +/- 1.1 vs S, 19.2 +/- 2.1 cells/s) compared with the spinotrapezius. Capillary "tube" hematocrit was greater (P = 0.01) in the diaphragm (0.32 +/- 0.02) than in the spinotrapezius (0.22 +/- 0.03) muscle. These data demonstrate that microcirculatory flow characteristics in resting muscle can be regulated independent of muscle fiber-type composition and may be related to muscle oxidative capacity.