The role of ATP in energy-deprivation contractures in unloaded rat ventricular myocytes

Energy-deprivation contractures were investigated in unloaded rat ventricular myocytes. Application of 2 mM cyanide in the presence of 10 mM 2-deoxyglucose (metabolic blockade) led to a rapid shortening 'contracture' (maximum speed 1.5 ± 0.2% control cell length/s). Cells shortened to a constant length of 69 ± 1.6% of the control length. Removal of cyanide caused cells to shorten further ('recontracture'), before relaxing towards to the control length. Cells shortened to 57 ± 2.0% during the recontracture. Similar behaviour was observed in zero extracellular [Ca²⁺]. Cells permeabilized with saponin (0.1% w/v) responded to the removal of ATP from the bathing solution, and to readdition of ATP, as intact cells did to complete metabolic blockade and its removal. In these permeabilized cells, the extent and speed of contracture shortening were similar at pCa = 7 and pCa > 9. When the bath concentration of ATP ([ATP](b)) was lowered to zero, shortening stopped at about 70% of the control length. However, when [ATP](b) was lowered to an intermediate level (4-20 μM), cells contracted to lengths as short as 30% of the control length. Similarly, when [ATP](b) was restored from zero to an intermediate concentration (4-20 μM), recontracture shortening continued without relaxation. The peak speed of this Ca²⁺-independent shortening showed a sigmoidal dependence on pMgATP (pMgATP_0.5 = 4.0). Phosphocreatine (10 mM) shifted the ATP dependence of Ca²⁺-independent shortening to lower [ATP](b) (pMgATP_0.5 = 5.0), suggesting that gradients of [ATP] could exist between the bath and the myofilaments. Ca²⁺-independent shortening was inhibited by the chemical phosphatase 2,3-butanedione monoxime (BDM), although BDM did not relax cells from the shortened state during energy deprivation. Using a simple model, we show that the results can be explained by cross-bridge cycling occurring independently of Ca²⁺ over a 'window' range of [MgATP] (0.1-100 μM), shortening ceases as the rate of cross-bridge cycling declines. Recontracture occurs on restoring ATP production, because stiffness falls and Ca²⁺-independent cross-bridge cycling initially increases. As [MgATP] rises above 100 μM, Ca²⁺-independent cross-bridge cycling ceases and the cell relaxes towards the control length. We conclude that energy-deprivation contractures, and recontractures, can result from changes in [MgATP] and do not necessarily require changes in [Ca²⁺](i).