EFFECT OF HINDLIMB UNLOADING ON RAT SOLEUS FIBER FORCE, STIFFNESS, AND CALCIUM SENSITIVITY

The purpose of this study was to examine the time course of change in soleus muscle fiber peak force (N), tension (P-o, KN/m(2)), elastic mo dulus (E(o)), and force-pCa and stiffness-pCa relationships. After 1, 2, or 3 wk of hindlimb unloading (HU), single fibers were isolated and placed between a motor arm and a transducer, and fiber diameter, peak absolute force, Po-,Po- E(o), and force-pCa and stiffness-pCa relatio nships were characterized. One week of HU resulted in a significant re duction in fiber diameter (68 +/- 2 vs. 57 +/- 1 mu m), force (3.59 +/ - 0.15 vs. 2.19 +/- 0.12 x 10(-4) N), P-o (102 +/- 4 vs. 85 +/- 2 kN/ m(2)), and E(o) (1.96 +/- 0.12 vs. 1.37 +/- 0.13 x 10(7) N/m(2)), and 2 wk of HU caused a further decline in fiber diameter (45 +/- 1 mu m), force (1.31 +/- 0.06 x 10(-4) N), and E(o) (0.96 +/- 0.09 x 10(7) N/m (2)). Although the mean fiber diameter and absolute force continued to decline through 3 wk of HU, P-o recovered to values not significantly different from control. The P-o/E(o) ratio was significantly increase d after 1 (5.5 +/- 0.3 to 7.1 +/- 0.6), 2, and 3 wk of HU, and the 2-w k (9.5 +/- 0.4) and 3-wk (9.4 +/- 0.8) values were significantly great er than the 1-wk values. The force-pCa and stiffness-pCa curves were s hifted rightward after 1, 2, and 3 wk of HU. At 1 wk of HU, the Ca2+ s ensitivity of isometric force, assessed by Ca2+ concentration required for half-maximal force, was increased from the control value of 1.83 +/- 0.12 to 2.30 +/- 0.10 mu M. In conclusion, after HU, the decrease in soleus fiber P-o can be explained by a reduction in the number of m yofibrillar cross bridges per cross-sectional area. Our working hypoth esis is that the loss of contractile protein reduces the number of cro ss bridges per cross-sectional area and increases the filament lattice spacing. The increased spacing reduces cross-bridge force and stiffne ss, but P-o/E(o) increases because of a quantitatively greater effect on stiffness.