A PHOSPHOLIPASE-A(2) INHIBITOR (RO-31-4493) PREVENTS PROTEIN LOSS ASSOCIATED WITH THE CALCIUM PARADOX IN ISOLATED GUINEA-PIG HEARTS WITHOUTEFFECT ON CONTRACTURE, CALCIUM OVERLOAD, OR THE CURRENTS THROUGH L-TYPE CALCIUM CHANNELS

Objective: Although it is widely accepted that the hypercontraction pr ovoked by the influx of Ca2+ on return to normal Tyrode solution after a period of Ca2+ depletion (the calcium paradox) contributes to the d amage to the cell membrane, it remains possible that the Ca2+ overload activates intracellular enzymes that are important in the initial deg radation of the cell membrane. This possibility was examined in this w ork. Experimental design: The effect of Ro 31-4493, a phospholipase A( 2) inhibitor, upon protein loss, hypercontracture, and ultrastructural changes in Langendorff perfused guinea pig hearts during the calcium paradox was studied. Because the degree of Ca2+ overload might also be affected, the action of the drug on the resting and action potentials and the whole cell currents through the L-type Ca-2 channels and the changes in [Ca2+](i) in isolated guinea pig ventricular myocytes were also studied. Results: Ro 31-4493, at 10-50 mu LM, inhibited the relea se of proteins from guinea pig Langendorff perfused hearts during the calcium paradox in a dose dependent way. This protective effect requir ed preincubation with the agent before the Ca2+ depletion. No signific ant effect upon the contractile behaviour, as recognised from the sust ained increase in intraventricular pressure on Ca2+ repletion, was pro duced. However, structural changes suggest that the extent of hypercon traction, the disruption of cell membrane and the release of mitochond ria was less in treated hearts. Ro 31-4493 produced no change in the r esting and action potentials or the behaviour of the L-type Ca2+ chann els with either Ca2+ or Na+ as the charge carrier, while measurements of [Ca2+], indicated a similar Ca2+ overload in both treated and untre ated hearts. Conclusion: The effects of Ro 31-4493 are inconsistent wi th mechanical consequences of hypercontraction upon the cell membranes , being solely responsible for the protein loss during the calcium par adox and suggest that activation of a Ca2+ dependent phospholipases ma y be an important step.