SARCOMERE-LENGTH AND CAPILLARY CURVATURE OF RAT HINDLIMB MUSCLES IN-VIVO

Mammalian skeletal muscle fibers have been reported to develop maximum force at a sarcomere length (L(s)) of similar to 2.5 mu m. However, the functional range of muscle length (L(m)) and L(s) encountered by skeletal muscle in vivo is not well defined. Changes in L(s) markedly influence capillary geometry, but this effect has been shown only in fixed preparations. The purpose of this study was to evaluate the influence of limb position on L(m), L(s), and capillary geometry in living undisturbed hindlimb muscles; We tested the hypothesis that maximal excursion of the foot would have similar effects on L(s) and capillary geometry of antagonistic soleus (Sol) and extensor digitorum longus (EDL) muscles in vivo. Female Sprague-Dawley rats (n = 9; 243 +/- 3 g) were anesthetized (pentobarbital sodium; 35 mg/kg). The right Sol and EDL muscles were exposed and irrigated with physiological saline solution (34 degrees C; pH 7.4). Sarcomeres and capillaries were observed with video microscopy (total magnification x 1,900; spatial resolution <1 mu m); sarcomeres were labeled with a fluorescent dye [4-(4-diethylaminostyryl)-N-methylpyridinium iodide]. As foot angle increased from 30 degrees (maximal dorsiflexion) to 170 degrees (maximal plantarflexion), L(m) and L(s) increased for EDL muscles (27.51 +/- 0.42 to 30.97 +/- 0.25 mm and 2.33 +/- 0.01 to 3.09 +/- 0.05 mu m, respectively; P < 0.05) and decreased for Sol muscles (26.09 +/- 0.38 to 20.27 +/- 0.34 mm and 3.17 +/- 0.03 to 2.22 +/- 0.04 mu m, respectively; P < 0.05). Muscle fiber lengths changed in parallel with L(m) and L(s) for each muscle, and the ranges of L(m) and L(s) were greater (P < 0.05) for Sol muscles compared with EDL muscles. Capillary curvature index (CCI; %capillary segments with axial deviation greater than or equal to 5 mu m along a 50-mu m distance) increased with shortening in both muscles and was greater (P < 0.05) at each corresponding length in Sol muscle compared with EDL muscle. This in vivo difference in CCI between muscles is consistent with higher capillary volume density in the Sol muscle compared with the EDL muscle.