The nature of anionic sites and the endocytic pathway in heart muscle cells

The distribution and internalization of anionic sites in heart muscle cells (HMC) were studied by direct measurements of their zeta potential (ZP) and by ultrastructural cytochemistry. Our data showed that HMC are negatively charged and that their anionic sites are distributed over the entire sarcolemma. Treatments with neuraminidase and trypsin altered the ZP value and also reduced binding of cationized ferritin (CF) to the sarcolemma. Sialic acid was shown to be an important component on the surface of HMC, since its removal reduced the cell surface negative charge by 25%. Phospholipase C did not significantly change the surface charge, nor did it alter HMC reactivity to CF particles when compared with control cells. Endocytosis of anionic sites was investigated using two different protocols that allow follow-up of this dynamic process. Incubation of HMC with cationized ferritin particles at 37 degrees C induced a redistribution of ligand-bound anionic sites, followed by their internalization or detachment. The clustering of anionic sites on the surface of HMC indicates that these cells are characterized by a high level of membrane fluidity. CF particles were localized inside early and late endosomes, lysosomes, and also in ferritin-enriched vesicles near the sarcolemma. An endocytic pathway for anionic sites in HMC is discussed.