The relationship between ATP hydrolysis and active force in compressed and swollen skinned muscle fibers of the rabbit

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Abstract
We investigated whether the inhibition of force generation observed in compressed muscle fibers is accompanied by a coupled reduction in hydrolytic activity. Isometric force and rates of ATP hydrolysis (ATPase) were measured as functions of the relative width of chemically skinned skeletal muscle fiber segments immersed in relaxing (pCa>8) and activating (pCa 4.9) salt solutions. Osmotic radial compression of the fiber segment was produced (with little or no affect on striation spacing) by adding Dextran T500 to the bathing media. ADP as a product of ATP hydrolysis in fibers undergoing 10–15 min contractions was measured using high pressure liquid chromatography. Compression of the (initially swollen) fiber segment with dextran produced a slight (4%) increase in average active force and then, with further compression, a sharp decrease (with maximum around in situ width). With compression, the average ATPase of the fiber decreased monotonically, and with extreme compression (with 0.22 g dextran per ml), ATPase fell to a fifth of its level determined in dextran-free solution while force was abolished. The time course of active force development was described by the sum of two exponential functions, the faster of which characterized the rate of rise. Fiber compression (0.14 g dextran per ml) reduced the rate of rise of force ten-fold compared to that in dextran-free solution. Hindrance of cross movement is proposed to account for the inhibition of active force generation and (coupled) ATPase in compressed fibers.