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 (HctCAP) are key determinants of microvascular O2 exchange. However, the mechanisms leading to the changes in F RBC, V RBC 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 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 (Δ )i nV RBC and HctCAP. The values at rest (R) and end-recovery (ER) were significantly lower (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 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 HctCAP within individual capillaries following muscle contractions.