ARE CAPILLARY HEMODYNAMICS RELATED TO FIBER TYPE OR OXIDATIVE CAPACITY IN PASSIVE SKELETAL MUSCLE?

Abstract

1298 Fiber type composition, oxidative enzyme activity, and capillarity determine, in part, performance and fatiguability characteristics of skeletal muscle. In this regard, the rat spinotrapezius and diaphragm muscles offer an interesting comparison: their fiber type composition is almost exactly the same, yet there is a 2-3 fold disparity in oxidative capacity. We have developed an intravital microscopy preparation to study the passive in vivo diaphragm (D) microcirculation. Using this preparation and the established spinotrapezius (S) preparation (Poole et al. Am. J. Physiol. H2107-2114, 1997), we tested the hypothesis that, at optimum muscle sarcomere length in passive muscles, red blood cell velocity and flux would be greater in the highly oxidative diaphragm compared to the low oxidative spinotrapezius muscle. Both muscles exhibited a similar proportion of flowing capillaries (D, 92 ± 2%; S, 89 ± 7%), however, due to a smaller diaphragm fiber cross-sectional area, the lineal density of flowing capillaries was substantially greater in this muscle (D, 59.6 ± 2.7; S, 29.8 ± 1.7 cap/mm). Within the capillaries supporting RBC flow, RBC velocity and flux were 34% and 74% greater (both p<0.001) respectively, in the diaphragm compared with the spinotrapezius. Furthermore, capillary“tube” hematocrit was greater (p=0.01) in the diaphragm (0.32± 0.02) compared to the spinotrapezius (0.22 ± 0.02) muscle. This increased capillary hematocrit elevates the effective capillary surface area available for O2 exchange in the diaphragm compared to the spinotrapezius at rest. Skeletal muscle blood flow is regulated by a complex array of neural, humoral, metabolic, mechanical and myogenic mechanisms. However, these data suggest that resting microcirculatory flow characteristics may be regulated, in part, as a function of muscle oxidative capacity rather than fiber type perse.