Is cytotoxic cellular edema real? The effect of calcium ion on water homeostasis in the rat heart

The objective of this study was to determine the effect of calcium on water homeostasis in cardiac muscle and whether cytotoxic edema is involved in ischemic-reperfusion injury. Cellular water was quantified by multinuclear nuclear magnetic resonance (NMR) spectroscopy in isolated rat hearts during 30 min of ischemia and reperfusion. An increase in extracellular calcium from 0.75 to 2.25 mM increased ischemic swelling twofold, with a marginal effect on functional recovery at reperfusion. Inhibition of Ca2+ channels with verapamil and bepridil reduced ischemic swelling by 58-66% and improved cardiac viability and functional recovery. Inhibition of the Na/Ca exchange with amiloride resulted in reperfusion swelling resulting from the inhibition of calcium efflux via the Na/Ca exchange. This was confirmed by inhibition with amiloride of 63% of cellular swelling induced by calcium overload via the Na/Ca exchange after calcium-free perfusion (calcium paradox). The volume-related activity of amiloride was attributed to the inhibition of Na/Ca exchange because the inhibition of the Na/H antiport resulted in low cellular volumes. The consistent changes in cellular volumes induced by the various agents used to alter calcium fluxes provide evidence of a pro-edematous effect of intracellular calcium accumulation during myocardial ischemia. The administration of toxic doses of bepridil to the perfused and ischemic heart resulted in the shrinkage of cellular volumes and in functional failure. The toxicity of bepridil was mediated by unrestricted release of calcium from the sarcoplasmic reticulum because magnesium cardioplegia, which limits calcium release, lowered ischemic swelling without cellular shrinkage and improved functional recovery. In all experimental groups, cellular edema subsided during the early stages of reperfusion, indicating efficient dissipation of intracellular hyperosmolarity. There was no evidence that cytotoxic edema develops during ischemia or reperfusion, or both. On the contrary, calcium-mediated cardiac toxicity was associated with cellular volume shrinkage as a result of the efflux of osmolytes through permeabilized sarcolemmal membranes. It is concluded that calcium ion is involved in the regulation of cellular volumes, and its intracellular accumulation promotes swelling in ischemic myocytes with intact membranes.