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MUSCLE CELL BIOLOGY AND CELL MOTILITY

The depressive effect of P_i on the force-pCa relationship in skinned single muscle fibers is temperature dependent

Department of Biological Sciences, Marquette University, Milwaukee, Wisconsin

Submitted 10 July 2005 ; accepted in final form 3 November 2005

Increases in P_i combined with decreases in myoplasmic Ca²⁺ are believed to cause a significant portion of the decrease in muscular force during fatigue. To investigate this further, we determined the effect of 30 mM P_i on the force-Ca²⁺ relationship of chemically skinned single muscle fibers at near-physiological temperature (30°C). Fibers isolated from rat soleus (slow) and gastrocnemius (fast) muscle were subjected to a series of solutions with an increasing free Ca²⁺ concentration in the presence and absence of 30 mM P_i at both low (15°C) and high (30°C) temperature. In slow fibers, 30 mM P_i significantly increased the Ca²⁺ required to elicit measurable force, referred to as the activation threshold at both low and high temperatures; however, the effect was twofold greater at the higher temperature. In fast fibers, the activation threshold was unaffected by elevating P_i at 15°C but was significantly increased at 30°C. At both low and high temperatures, 30 mM P_i increased the Ca²⁺ required to elicit half-maximal force (pCa₅₀) in both slow and fast fibers, with the effect of P_i twofold greater at the higher temperature. These data suggest that during fatigue, reductions in the myoplasmic Ca²⁺ and increases in P_i act synergistically to reduce muscular force. Consequently, the combined changes in these ions likely account for a greater portion of fatigue than previously predicted based on studies at lower temperatures or high temperatures at saturating Ca²⁺ levels.

force-pCa relationship; phosphate; fatigue

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