INNER SARCOLEMMAL LEAFLET CA2- ITS ROLE IN CARDIAC NA( BINDING )CA EXCHANGE/

A recently completed model of Ca concentration and movements in the ca rdiac cell diadic cleft space predicts that removal or neutralization of inner sarcolemmal (St) leaflet anionic Ca-binding sites at the sarc olemmal border of this space will greatly diminish Na/Ca exchange-medi ated Ca efflux. The present study tests this prediction using the loca l anesthetic dibucaine as a probe. It is shown, in isolated SL, that d ibucaine competitively displaces Ca specifically from anionic phosphol ipid headgroups. Dibucaine also displaces Ca from the SL when applied to intact cells. It does not affect the content or release of Ca from sarcoplasmic reticulum (SR) in these cells. This eliminates a primary effect on SR Ca as a contributing factor to dibucaine's effect on Na/C a exchange-mediated Ca efflux. Measurement of this efflux from whole c ells shows a highly significant reduction of 58% (p < 0.001) by 0.5 mM dibucaine. The inhibiting effect of dibucaine on Na/Ca exchange-media ted Ca efflux can be significantly reversed by augmentation of Ca rele ase from SR by caffeine at the time of activation of Na/Ca exchange, T his supports the contention that the dibucaine-SL interaction is a com petitive one vis-a-vis Ca. The results are supportive of the model in which inner SL leaflet Ca-binding sites account for the delay of Ca di ffusion from the diadic cleft, thereby prolonging the time for which [ Ca] remains elevated in the cleft. The prolonged increased [Ca] signif icantly enhances the ability of Na/Ca exchange to remove Ca from the c ell during the excitation-contraction cycle.