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Received January 4, 2006
Revised February 7, 2006
Accepted after revision March 26, 2006
1 Kansas State University
* To whom correspondence should be addressed. E-mail: poole{at}vet.k-state.edu.
In skeletal muscle capillaries, red blood cell flux
(FRBC), velocity (VRBC) and
hematocrit (HctCAP) are key determinants of
microvascular O2 exchange. However, the
mechanisms entailing the changes in
FRBC, VRBC and
HctCAP during muscle contractions and
recovery thereafter are not fully understood. To
address this issue we used intravital microscopy to
investigate the temporal profile of the rat
spinotrapezius muscle (n = 5) capillary hemodynamics
during recovery from 3 min of twitch muscle contractions
(1 Hz, 4-6 V). Specifically, we hypothesized that 1)
during early recovery FRBC and
VRBC would decrease rapidly and
FRBC would display a biphasic response
(consistent with a muscle-pump effect on capillary
hemodynamics); and 2) there would be a dynamic
relationship between changes (
) in
VRBC and HctCAP. The values at
rest (R) and end-recovery (ER) were significantly lower
(P < 0.05) than at end-contraction (EC) for
FRBC (in cell/s, R = 30.1 ±
7.8, EC = 46.2 ± 7.3 and ER = 26.0 ±
6.1), VRBC (in µm/s, R = 368 ±
83, EC = 497 ± 62 and ER = 334 ± 59) and
HctCAP (R = 0.193 ± 0.016, EC = 0.214
± 0.023 and ER = 0.185 ± 0.019). The
first data point where a significant decrease in
FRBC, HctCAP and
VRBC occurred was at 5, 5 and 20 s post-
contraction, respectively. The decrease in
FRBC approximated a monoexponential
response (half-time of ~ 26 s). The relationship
between
VRBC and
HctCAP was not significant (P > 0.05).
Based on the early decrease in FRBC
(within 5 s), overall dynamic profile of
FRBC and the ~ 20 s delay to the
decrease in VRBC we conclude that the muscle-
pump does not appear to contribute substantially to the steady-state capillary
hemodynamics in the contracting rat spinotrapezius
muscle. Moreover, our findings suggest that alterations
in VRBC do not obligate proportional changes
in HctCAP within individual capillaries
following muscle contractions.
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