EFFECT OF LOW PERFUSATE [CA2-RETICULUM IN MYOCARDIAL STUNNING(] AND DILTIAZEM ON CARDIAC SARCOPLASMIC)

The ability of low perfusate Ca2+ concentration ([Ca2+]) or diltiazem to improve sarcoplasmic reticulum (SR) function and mechanical perform ance after ischemia-reperfusion was investigated using an isovolumic L angendorff preparation. SR function was evaluated by the oxalate-suppo rted Ca2+ uptake rates of ventricular homogenates. Influx of Ca2+ was estimated from the rate of Ca2+ uptake in the presence of high concent rations of ryanodine (500 mu M) to close the Ca2+ efflux channel. Ca2 efflux under the assay conditions was estimated as the difference in Ca2+ uptake rate in the presence and absence of ryanodine. Ten and fif teen min of global, normothermic ischemia decreased the Ca2+ uptake ra te in the presence of ryanodine, suggesting that Ca2+ influx was decre ased. The effect of ischemia on Ca2+ influx was not altered by preperf usion with low [Ca2+] (0.2 mM) or with 1.0 mu M diltiazem. Ischemia de creased SR Ca2+ uptake rate twice as much in the absence of ryanodine as in its presence, indicating an increased efflux of Ca2+. This incre ased efflux was reduced by preperfusion with either low [Ca2+] or dilt iazem. The decreased Ca2+ influx was completely reversed by 15 min of reperfusion, whereas the increased Ca2+ efflux was only partially reve rsed. These results indicate that ischemia exerts independent-effects on the SR Ca2+ influx and efflux pathways. The results also suggest th at one site of cardiac protection by low [Ca2+] or diltiazem is the ry anodine-sensitive Ca2+ efflux pathway of the SR, rather than the Ca2influx pathway.