MEMBRANE ASYMMETRY IN ISOLATED CANINE CARDIAC SARCOPLASMIC-RETICULUM - COMPARISON WITH SKELETAL-MUSCLE SARCOPLASMIC-RETICULUM

Cardiac sarcoplasmic reticulum (CSR), isolated from dog hearts, was sh own to be asymmetric rn the distribution of phospholipids across the C SR bilayer. Phosphatidylethanolamine was mostly resident in the outer leaflet, phosphatidylcholine was equally distributed across both monol ayers and phosphatidylserine was found primarily in the inner monolaye r, This distribution of headgroups is similar to that found in fast sk eletal muscle sarcoplasmic reticulum (SSR); however, the asymmetry in CSR is not as striking as that in SSR. Phospholipids retained by the C SR calcium pump protein (CaATPase) after detergent ''stripping'' were similar to those intimate to the SSR CaATPase, although the percentage s of unsaturated phospholipids and plasmalogenic phospholipids are not as great as in the skeletal system. Lipids associated with the CSR Ca ATPase following DFDNB cross-linking showed a preference for retention of the aminophospholipids, again similar to the SSR CaATPase, Because the nonrandom distribution of membrane lipids modifies SSR function, it is likely these membrane lipids impact in situ the function of the CSR.