In skeletal muscle capillaries, red blood cell (RBC) flux (F_(rbc))> velocity ( F_(rbc)) and haematocrit (HctcAp) are key determinants of microvascular O_2 exchange. However, the mechanisms leading to the changes in F_(rbc), V_(rbc) and Hct_(CAP) 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 haemodynamics during recovery from 3 min of twitch muscle contractions (1 Hz, 4-6 V). Specifically, we hypothesized that (1) during early recovery F_(rbc) and F_(rbc) would decrease rapidly and F_(rbc) would display a biphasic response (consistent with a muscle pump effect on capillary haemodynamics), and (2) there would be a dynamic relationship between changes (A) in F_(rbc) and Hct_(CAP)- The values at rest (R) and end-recovery (ER) were significantly lower (P < 0.05) than at end-contraction (EC) for F_(rbc) (in cells s~(-1), R= 30.1 (+-)7.8, EC = 46.2 (+-)7.3 and ER = 26.0 (+-)6.1), F_(rbc) (in mum s~(-1), R = 368 (+-) 83, EC = 497 (+-) 62 and ER = 334 (+-) 59) and Hct_(CAP) (R = 0.193 (+-) 0.016, EC = 0.214 (+-) 0.023 andER = 0.185 (+-) 0.019). Thefirst data point where a significant decrease in F_(rbc)> Hct_(CAP) and F_(rbc) occurred was at 5,5 and 20 s post-contraction, respectively. The decrease in F_(rbc) approximated a monoexponential response (half-time of ~26 s). The relationship between A F_(rbc) and AHct_(CAP) was not significant (P > 0.05). Based on the early decrease in F_(rbc) (within 5 s), overall dynamic profile of F_(rbc) and the ~20 s 'delay' to the decrease in F_(rbc) we conclude that the muscle pump does not appear to contribute substantially to the steady-state capillary haemodynamics in the contracting rat spinotrapezius muscle. Moreover, our findings suggest that alterations in F_(rbc) do not obligate proportional changes in Hct_(CAP) within individual capillaries following muscle contractions.
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