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1 Dept. of Biological Sciences, Marquette University, Milwaukee, WI, USA
* To whom correspondence should be addressed. E-mail: ndebold{at}physiology.med.uvm.edu.
Increases in inorganic phosphate (Pi) combined with decreases in myoplasmic calcium (Ca2+) are believed to cause a significant portion of the decrease in muscular force during fatigue. To investigate this further the effect of 30 mM Pi on the force-Ca2+ relationship of chemically skinned single muscle fibers was determined 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+ in the presence and absence of 30 mM Pi at both low (15°C) and high (30°C) temperature. In slow fibers, 30 mM Pi significantly increased the Ca2+ required to elicit measurable force, referred to as the activation threshold (Act. T.) at both low and high temperatures, however the effect was 2-fold greater at the higher temperature. In fast fibers, Act.T. was unaffected by elevating Pi at 15°C, but significantly increased at 30°C. At both low and high temperatures, 30 mM Pi increased the Ca2+ required to elicit half-maximal force (pCa50) in both slow and fast fibers, with the effect of Pi 2-fold greater at the higher temperature. These data suggest that during fatigue, reductions in the myoplasmic Ca2+ and increases in Pi 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.
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