A COMPARISON OF THE MICROCIRCULATION IN THE RAT SPINOTRAPEZIUS AND DIAPHRAGM MUSCLES

Of all skeletal muscles examined in the rat, the spinotrapezius (S) an d diaphragm (D) have the closest fiber-type composition. However, thei r oxidative capacities differ by two- to threefold. We have developed an intravital microscopy preparation to study diaphragm microcirculati on in vivo. Using this preparation and the standard spinotrapezius mod el first described by S. D. Gray (1973, Microvasc. Res. 5, 395-400), w e tested the hypothesis that pronounced microcirculatory differences w ould exist between these two muscles as a function of their disparate oxidative capacities. The lineal density of all capillaries in the spi notrapezius was 33.6 +/- 1.5 compared to 65.1 +/- 3.3 capillaries/mm i n the diaphragm (P < 0.001). In the diaphragm compared with the spinot rapezius muscle, a significantly (P < 0.05) greater proportion of capi llary countercurrent now (D, 29 +/- 6% vs 8 +/- 6%) existed. Within bo th muscles, there was a similar proportion of capillaries supporting r ed blood cell (RBC) now (S, 89 +/- 7% vs D, 92 +/- 2%). However, the d iaphragm supported significantly (P < 0.001) greater intracapillary RB C velocities (D, 302 +/- 11 vs S, 226 +/- 9 mu m/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 diaphrag m (0.32 +/- 0.02) than in the spinotrapezius (0.22 +/- 0.03) muscle. T hese data demonstrate that microcirculatory now characteristics in res ting muscle can be regulated independent of muscle fiber-type composit ion and may be related to muscle oxidative capacity. (C) 1998 Academic Press.