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Articles in PresS, published online ahead of print February 15, 2002
J Appl Physiol, 10.1152/jap.01222.2001
Submitted on December 12, 2001
Accepted on February 12, 2002
1 Department of Medicine, University of California-San Diego, La Jolla, CA, USA
2 Department of Anatomy, Physiology and Kinesiology, Kansas State University, Manhattan, KS, USA
* To whom correspondence should be addressed. E-mail: ckindig{at}ucsd.edu.
Muscle contractions evoke an immediate rise in blood flow. The distribution of this increased blood flow within the capillary bed and its implications for muscle O2 diffusing capacity (DO2m) and O2 exchange remain undetermined. We developed the exteriorized rat (n=4) spinotrapezius muscle preparation for evaluation of capillary hemodynamics prior to (rest), during, and immediately following (post) a bout of twitch contractions (indirect, bipolar stimulation for 3 min; 1 Hz frequency, ~5 volts, 2 ms pulse duration) to resolve (second-by-second) alterations in red blood cell velocity (VRBC) and flux (FRBC). Contractions increased both intracapillary VRBC (rest, 270±62; post, 428±47 µm/s; p<0.005) and FRBC (rest, 22.4±5.5; post, 44.3±5.5 cells/s; p<0.005) but not the % of capillaries supporting continuous RBC flow (rest, 84.0±0.7; post, 89.5±1.4%, p>0.05). Furthermore, contractions elevated capillary hematocrit (Hctcap) and reduced RBC spacing (rest vs. post, both p<0.05). Within the first 1 or 2 contractions, intracapillary VRBC rose to its peak value for the 3 min contraction bout. At contraction onset, FRBC increased biphasically i.e., immediate increase to a plateau (first 12-20 s) followed by a second rise to steady-state. The temporal profiles of both VRBC and FRBC were such that Hctcap was not elevated immediately and actually tended to decrease over the first 10-15 s following onset of contractions. These data support the notion that contraction-induced alterations in capillary RBC flux and distribution augment both convective and diffusive mechanisms for blood-myocyte O2 transfer from rest to steady-state conditions. However, over the first 10-15 s following contraction onset, the immediate and precipitous rise in VRBC compared to the biphasic and prolonged increase of FRBC may act to lower DO2m by not only reducing capillary transit time but by delaying the increase in the instantaneous RBC-to-capillary surface contact thought crucial for blood-myocyte O2 flux.
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