Changes in fluid-phase endocytosis in the rat with age and their relation to total albumin elimination.

Rates of fluid-phase endocytosis were determined in several organs and tissues of female WAG/Rij rats of several ages by using 125I-labelled polyvinylpyrrolidone ([125I]-PVP) as a marker. Liver, muscle and skin accounted for a high level of [125I]PVP uptake 28 h after injection. When PVP uptake was expressed per gram of organ/tissue, the liver and spleen were found to be the most active. An age-related increase in [125I]PVP uptake was seen at between 12 and 36 months of age in liver, kidneys and heart. Except for the kidneys this increase is caused by an increase in wet weight of these organs and not by an increase in the specific endocytic rate. These data, together with reported findings on the major sites of albumin catabolism, in liver, kidney, spleen and intestine, indicate that fluid-phase endocytosis is a main mechanism for the observed age-related increase in albumin elimination in these rats.