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

Abstract

Increases in Picombined 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 Pion 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 Piat both low (15°C) and high (30°C) temperature. In slow fibers, 30 mM Pisignificantly 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 Piat 15°C but was significantly increased at 30°C. At both low and high temperatures, 30 mM Piincreased the Ca2+required to elicit half-maximal force (pCa50) in both slow and fast fibers, with the effect of Pitwofold greater at the higher temperature. These data suggest that during fatigue, reductions in the myoplasmic Ca2+and increases in Piact 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.