IN-VIVO MICROVASCULAR STRUCTURAL AND FUNCTIONAL CONSEQUENCES OF MUSCLE LENGTH CHANGES

As muscles are stretched, blood flow and oxygen delivery are compromis ed, and consequently muscle function is impaired. We tested the hypoth esis that the structural microvascular sequellae associated with muscl e extension in vivo would impair capillary red blood cell hemodynamics . We developed an intravital spinotrapezius preparation that facilitat ed direct on-line measurement and alteration of sarcomere length simul taneously with determination of capillary geometry and red blood cell flow dynamics. The range of spinotrapezius sarcomere lengths achievabl e in vivo was 2.17 +/- 0.05 to 3.13 +/- 0.11 mu m. Capillary tortuosit y decreased systematically with increases of sarcomere length up to 2. 6 mu m, at which point most capillaries appeared to be highly oriented along the fiber longitudinal axis. Further increases in sarcomere len gth above this value reduced mean capillary diameter from 5.61 +/- 0.0 3 mu m at 2.4-2.6 mu m sarcomere length to 4.12 +/- 0.05 mu m at 3.2-3 .4 mu m sarcomere length. Over the range of physiological sarcomere le ngths, bulk blood flow (radioactive microspheres) decreased similar to 40% from 24.3 +/- 7.5 to 14.5 +/- 4.6 ml.100 g(-1).min(-1). The propo rtion of continuously perfused capillaries, i.e., those with continuou s flow throughout the 60-s observation period, decreased from 95.9 +/- 0.6% at the shortest sarcomere lengths to 56.5 +/- 0.7% at the longes t sarcomere lengths and was correlated significantly with the reduced capillary diameter (r = 0.711, P < 0.01; n = 18). We conclude that alt erations in capillary geometry and luminal diameter consequent to incr eased muscle sarcomere length are associated with a reduction in mean capillary red blood cell velocity and a greater proportion of capillar ies in which red blood cell flow is stopped or intermittent. Thus not only does muscle stretching reduce bulk blood (and oxygen) delivery, i t also alters capillary red blood cell flow dynamics, which may furthe r impair blood-tissue oxygen exchange.