Length-dependent effects of osmotic compression on skinned rabbit psoas muscle fibers

The goal of this study was to characterize the interrelationship between sarcomere length and interfilament spacing in the control of Ca2+ sensitivity in skinned rabbit psoas muscle fibers. Measurements were made at sarcomere lengths 2.0, 2.7 and 3.4 mu m. At 2.7 mu m the fiber width was reduced by 17% relative to that at 2.0 mu m and the pCa(50) for force development was increased by similar to 0.3 pCa units. In the presence of 5% Dextran T-500 the fiber width at sarcomere length 2.0 mu m was also decreased by 17% and the Ca2+ sensitivity was increased to the same value as at 2.7 mu m. In contrast, at sarcomere length 2.7 mu m the addition of as much as 10% Dextran T-500 had no effect on Ca2+ sensitivity. At sarcomere length 3.4 mu m there was an additional 7% compression and the Ca2+ sensitivity was increased slightly (similar to 0.1 pCa units) relative to that at 2.7 mu m. However at 3.4 mu m the addition of 5% Dextran T-500 caused the Ca2+ sensitivity to decrease to the level seen at 2.0 mu m. Given that the skinning process causes a swelling of the filament lattice it is evident that the relationship between sarcomere length and Ca2+ sensitivity observed in skinned fibers may not always be applicable to intact fibers. These data are consistent with measurements of Ca2+ in intact fibers which indicate that there might be a decline in Ca2+ sensitivity at long sarcomere lengths.