Role of Changes in [Ca]; in Energy Deprivation Contracture

Mechanisms of energy deprivation contracture were investigated in cultured chick embryo ventricular cells. In the presence of zero-extracellular-Na* (choline chloride substitution)-nominal-zero-Ca ([Ca] ~ 5 fiM), exposure of ventricular cells to 1 mM cyanide (CN) and 20 mM 2-deoxyglucose (2-DG)-zero-glucose solution resulted in the development of a contracture (video motion detector) in 5.9 ±0.5 minutes. Early after contracture development, the resupply of extracellular Na, in the continued presence of CN + 2-DG, resulted in a rapid partial relaxation (t,,2= 1.9 ±0.3 seconds), associated with an increase in °Ca efflux, presumably due to transsarcolemmal Ca extrusion due to Na-Ca exchange. Ressupply of glucose and removal of CN + 2-DG, in the continued absence of Na, resulted in an initially slower ( t u = 11.6 ±2.5 seconds), but more complete relaxation of contracture, which was not associated with increased Ca efflux. Pretreatment with 20 mM caffeine delayed the onset of contracture (9.2 ±1.1 minutes) and resulted in a contracture that could not be relaxed by ressupply of external Na only. Studies using the fluorescent Ca probe indol demonstrated that in zero-Na-zero-Ca solutions, contracture due to CN + 2-DG was associated with an initial rise in [Ca], but that this did not account for all of contracture force development. In cells exposed to CN + 2-DG in the presence of normal extracellular Na and Ca concentrations, a small rise in [Ca], was associated with initial contracture development, consistently preceding the development of a larger accelerated contracture presumably due to ATP depletion. We conclude that an early component of ATP depletion contracture is due to an increase in [Ca],. The rate of this increase in [Ca], depends to some extent on the loading of internal stores of Ca, particularly sarcoplasmic reticulum. Elevation of [Ca], may promote subsequent rigor by hastening ATP depletion by activation of Ca-ATPases. (Circulation Research 1987;61:726-734)