Sarcomere length-induced alterations of capillary hemodynamics in rat spinotrapezius muscle: Vasoactive vs passive control

Skeletal muscle blood flow is reduced as fibers are stretched longitudinall y. Neither the underlying cause(s) of this decrement in blood flow nor the consequences in terms of capillary red blood cell (rbc) hemodynamics has be en established clearly within the physiological range of muscle sarcomere l ength. Using intravital microscopy, this investigation determined arteriola r diameter and capillary rbc velocity (V-rbc), flux (F-rbc), and hematocrit (Hct(t)) in the rat spinotrapezius muscle at shortened/resting (2.6 mum) a nd physiological extended (3.2 mum) sarcomere lengths under control (c) and local maximally vasodilated (v, phentolamine, 1 mu mol/L; prazosin, 0.1 mu mol/L; nitroprusside, 10 mu mol/L) conditions. The hypothesis tested was t hat muscle stretch would reduce V-rbc and F-rbc proportionally such that Hc t(t) would remain unchanged and that these reductions in V-rbc and F-rbc wo uld be attenuated following maximal vasodilation. V-rbc and F-rbc were incr eased significantly following maximal vasodilation at 2.6-mum (S9 and 84%) and 3.2-mum (64 and 104%) sarcomere lengths, respectively. Irrespective of sarcomere length, Hct(t) was elevated significantly following vasodilation (c, 0.20 +/- 0.01; v, 0.27 +/- 0.01). At 3.2 mum compared with the 2.6-mum sarcomere length, V-rbc and F-rbc were both reduced significantly under con trol and vasodilated conditions as expected. However, the percent reduction in either V-rbc (c, 27%, and v, 29%) or F-rbc (c, 26%, and v, 33%) was not significantly different between the 2.6- and 3.2-mum sarcomere lengths. In addition, arteriolar diameter was not altered discernably as sarcomere len gth was increased from 2.7 mum (c, 29.0 +/- 4.5; v, 37.9 +/- 6.7 mum) to 3. 2 mum (c, 29.4 +/- 4.5; v, 37.3 +/- 6.2 mum). These data suggest that incre asing sarcomere length from resting to the upper extreme of the physiologic al range in the rat spinotrapezius muscle reduces V-rbc and F-rbc (at const ant hematocrit) by a mechanism that is independent of stretch-activated art eriolar vasoconstriction. (C) 2001 Academic Press.