Direct evidence of changes in myofilament responsiveness to Ca²⁺ during hypoxia and reoxygenation in myocardium

In the presence of 1 μM ryanodine, muscles loaded with the calcium indicator aequorin were stimulated at 15-20 Hz to produce steady levels of force and intracellular Ca²⁺ concentrations ([Ca²⁺](i)) at various extracellular Ca²⁺ concentration ([Ca²⁺](o)). After 5, 10, and 15 min of hypoxia and 3 min of reoxygenation, tetani were initiated. Force vs. [Ca²⁺](i) relation was shifted to the right 0.11, 0.18 and 0.24 pCa units, and maximal force was down 66, 48, and 37% after start of hypoxia. During reoxygenation, the relationship was shifted up by 26%. In skinned fiber preparations, an increase in inorganic phosphate ion concentration from 0 to 10 mM and 15 mM decreased maximal force development by 32 and 53%, respectively, and shifted the pCa-force curve to the right by 0.08 and 0.14 pCa. A decrease in pH from 7.1 to 6.8 shifted the pCa-force curve to the right by 0.20 pCa units without affecting maximal force. These changes indicate that during hypoxia, a decrease in the sensitivity of the myofilaments to Ca²⁺ and a depression of maximal Ca²⁺-activated force occur, whereas during reoxygenation, there is an increase in maximal Ca²⁺-activated force.