Age-related changes in the galactose recognition system in rat liver cells

In the present report, the galactose recognition system in 3- and 24-month-old rat livers has been studied with in vitro and with in situ binding experiments and in vivo ligand uptake. The galactose-specific receptors were visualized by using colloidal gold particles of different sizes (5 nm, 17 nm and 50 nm mean diameter), coated with lactosylated bovine serum albumin (LacBSA) as electron dense ligands. The data show that aging affects the expression of galactose-specific receptors and the rate of endocytosis. In the in vitro and in situ experiments hepatocytes and liver macrophages from old rats on the plasma membrane express a decreased number of binding sites with respect to those present on the adult rat liver cells. Approximately 80% of the total number of liver macrophages from aged rats show a binding distribution which is very different from the typical clustered receptor arrangement: the binding sites appear as single or small clusters of gold granules. As a direct consequence of the altered pattern of the receptor distribution, the capacity of liver macrophages from 24-month-old rats to internalize the larger ligands (17 nm and 50 nm) is decreased, as compared with adult rats. Aging, therefore, influences the galactose recognition system in two ways: (i) by decreasing the number of binding sites expressed on the liver cell surfaces; and (ii) by modifying the receptor distribution on liver macrophages and consequently affecting the internalization of galactose exposing particles.