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

Abstract

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 μm. At 2.7 μm the fiber width was reduced by 17% relative to that at 2.0 μm and the pCa₅₀ for force development was increased by ∼0.3 pCa units. In the presence of 5% Dextran T-500 the fiber width at sarcomere length 2.0 μm was also decreased by 17% and the Ca²⁺ sensitivity was increased to the same value as at 2.7 μm. In contrast, at sarcomere length 2.7 μm the addition of as much as 10% Dextran T-500 had no effect on Ca²⁺ sensitivity. At sarcomere length 3.4 μm there was an additional 7% compression and the Ca²⁺ sensitivity was increased slightly (∼0.1 pCa units) relative to that at 2.7 μm. However at 3.4 μm the addition of 5% Dextran T-500 caused the Ca²⁺ sensitivity to decrease to the level seen at 2.0 μm. 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.