Effects of skeletal muscle sarcomere length on in vivo capillary distensibility

The structural integrity of the capillary wall is such that capillary lumin al distensibility is largely determined by support provided by the tissue i n which it is located. Given that O-2 flux density is greatest across the s keletal muscle capillary endothelium, any changes in capillary diameter (d( c)) would be expected to affect O-2 diffusing capacity as well as hemodynam ic resistance. We used intravital microscopy techniques to study the maxima lly vasodilated rat (n = 5) spinotrapezius muscle microcirculation in vivo within the physiological sarcomere length range, at high and low mean arter ial pressures (MAP) systematically altered by blood withdrawal and infusion . We tested the hypothesis that in vivo capillary diameter alterations in r esponse to changes of MAP would be reduced at extended sarcomere lengths. A t 2.4-mu m sarcomere length, mean d(c) ((d) over bar(c)) within the spinotr apezius increased from 5.6 +/- 0.1 to 5.9 +/- 0.1 mu m (P < 0.01) as MAP in creased from 33 to 94 mm Hg. However, there was absolutely no change (i.e., 5.2 +/- 0.1 vs 5.2 +/- 0.1 mu m) in (d) over bar(c) in response to changes in MAP at 3.2-mu m sarcomere length. Furthermore, at sarcomere lengths <2. 8 mu m there was a significant increase (P < 0.01) in (d) over bar(c) (n = 40) as MAP increased, whereas (d) over bar(c) (n = 49) remained unchanged w ith alterations of MAP when sarcomere length was greater than or equal to 2 .8 mu m (P > 0.05). These data suggest that pressure-induced alterations in capillary luminal diameter and thus "in vivo capillary distensibility" are dependent upon the presiding sarcomere length. Furthermore, we conclude th at the MAP-induced increases in capillary luminal diameter at the shorter s arcomere lengths are modest (similar to 5%) and unlikely to affect O-2 diff using capacity and vascular resistance appreciably. (C) 1999 Academic Press .