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¹ Clarenburg Research Laboratory, Departments of Anatomy & Physiology and Kinesiology, Kansas State University, Manhattan, KS 66506-5802, USA

In skeletal muscle capillaries, red blood cell (RBC) flux (F_RBC), velocity (V_RBC) and haematocrit (Hct_CAP) are key determinants of microvascular O₂ 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 () 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 µm 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 26 s). The relationship between V_RBC and 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 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.

(Received 4 January 2006; accepted after revision 26 March 2006; first published online 31 March 2006)
Corresponding author D. C. Poole: Department of Anatomy and Physiology, College of Veterinary Medicine, 228 Coles Hall, 1600 Denison Avenue, Manhattan, KS 66506-5602, USA. Email: poole{at}vet.ksu.edu

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