Effects of MgATP on ATP utilization and force under normal and simulated ischaemic conditions in rat cardiac trabeculae

The dependency of ATP utilization and isometric force on [MgATP] was studie d in skinned rat trabeculae under normal (pH 7.0) and simulated ischaemic ( pH 6.2, 30 mM added P-i) conditions at 20 +/-1 degreesC. At saturating [Ca2 +], mean (+/- SEM) ATP utilization at 5 mM MgATP (A(0)) was 0.48 +/-0.03 mM /s and force (F-0) was 37 +/-2 kN/m(2). At 10 muM MgATP under normal condit ions ATP utilization decreased gradually to 66 +/-3% of A(0), and force inc reased to 169 +/-7% of F-0, Under ischaemic conditions at, 10 muM MgATP, AT P utilization decreased from 30 +/-5% to 11 +/-2% of A(0) whereas force inc reased eight-fold from 12 +/-4% to 97 +/-7% of F-0. The [MgATP] at half-max imal ATP utilization (K-m) under ischaemic conditions was 21 +/-3 muM. At p H 7.0, K-m was estimated to be less than 10 muM. These results show that te nsion cost decreases markedly with decreasing MgATP. Under ischaemic condit ions parallel changes in Ca2+ sensitivity of force and ATP utilization were observed, corresponding to 1.3 pCa units. Reducing [MgATP] from 0.5 to 0.0 5 mM caused a modest reversal of this change in Ca2+ sensitivity. These cha nges in Ca2+ sensitivity are consistent with a marked reduction in active f orce and force-related ATP utilization during ischaemia but are insufficien t to explain the ischaemic contracture, on the basis of active force develo pment.