We modeled changes in contractile element kinetics derived from the cyclic relationship between myoplasmic [Ca2+], measured by indo 1 fluorescence, and left ventricular pressure (LVP). We estimated model rate constants of the Ca2+ affinity for troponin C (TnC) on actin (A) filament (TnCA) and actin and myosin (M) cross-bridge (A · M) cycling in intact guinea pig hearts during baseline 37°C perfusion and evaluated changes at 1) 20 min 17°C pressure, 2) 30-min reperfusion (RP) after 30-min 37°C global ischemia during 37°C RP, and 3) 30-min RP after 240-min 17°C global ischemia during 37°C RP. At 17°C perfusion versus 37°C perfusion, the model predicted: A · M binding was less sensitive; A · M dissociation was slower; Ca2+ was less likely to bind to TnCA with A · M present; and Ca2+ and TnCA binding was less sensitive in the absence of A · M. Model results were consistent with a cold-induced fall in heart rate from 260 beats/min (37°C) to 33 beats/min (17°C), increased diastolic LVP, and increased phasic Ca2+. On RP after 37°C ischemia vs. 37°C perfusion, the model predicted the following: A · M binding was less sensitive; A · M dissociation was slower; and Ca2+ was less likely to bind to TnCA in the absence of A · M. Model results were consistent with reduced myofilament responsiveness to [Ca2+] and diastolic contracture on 37°C RP. In contrast, after cold ischemia versus 37°C perfusion, A · M association and dissociation rates, and Ca2+ and TnCA association rates, returned to preischemic values, whereas the dissociation rate of Ca2+ from A · M was ninefold faster. This cardiac muscle kinetic model predicted a better-restored relationship between Ca2+ and cross-bridge function on RP after an eightfold longer period of 17°C than 37°C ischemia.
The effect of partial extraction of troponin C on the elementary steps of the cross-bridge cycle in rabbit psoas muscle fibers