Temporal profile of rat skeletal muscle capillary haemodynamics during recovery from contractions

in skeletal muscle capillaries, red blood cell (RBC) flux (F-RBC), velocity (V-RBC) and haematocrit (Hct(CAP)) 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 V-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 (delta) in V-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), V-RBC (in mu m 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 and ER = 0.185 +/- 0.019). The first data point where a significant decrease in F-RBC, Hct(CAP) and V-RBC occurred was at 5,5 and 20 s post-contraction, respectively. The decrease in F-RBC approximated a monoexponential response (half-time of similar to 26 s). The relationship between Delta V-RBC and Delta Hct(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 similar to 20 s 'delay' to the decrease in V-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 V-RBC do not obligate proportional changes in Hct(CAP) Within individual capillaries following muscle contractions.