Fluid phase endocytosis by cultured rat hepatocytes and perfused rat liver: implications for plasma membrane turnover and vesicular trafficking of fluid phase markers.

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Abstract
Hepatocytes take up a variety of ligands via receptor-mediated endocytosis, yet little is known regarding either the volume of fluid or the amount of membrane internalized via endocytosis in liver cells. In these studies, we have utilized radiolabeled insulin to characterize fluid phase endocytosis by rat hepatocytes in primary culture and perfused rat liver. Uptake of inulin by cultured hepatocytes was nonlinear with time, occurring most rapidly during the first 2 min. Inulin uptake and efflux in cultured hepatocytes and inulin uptake by perfused rat liver were kinetically compatible with the entry of inulin into a rapidly (t1/2, 1-2 min) turning-over (presumably endosomal) compartment that exchanged contents with the extracellular space and comprised .apprxeq. 3% of hepatocyte volume, as well as entry into and concentration of inulin within slowly (t1/2, > 1 hr) turning-over storage compartments. Based on inulin uptake, it is estimated that cultured hepatocytes endocytosed the equivalent of 20% or more of their volume and 5 or more times their plasma membrane surfae area each hour. Neither chloroquine (1 mM) nor taurocholate (200 .mu.M) affected inulin handling by cultured cells, whereas colchicine (10 .mu.M) inhibited transfer to storage compartments by > 50%. In conjunction with our previous observations, the present findings suggest that inulin endocytosed across the basolateral membrane is largely (.simeq. 80%) regurgitated back into plasma, with smaller amounts transported to intracellular storage compartments (.simeq. 18%) or to bile (.simeq. 2%). Transport of inulin via these pathways is unaffected by taurocholate and does not require vesicle acidification, whereas intact microtubular function is required for transfer to storage compartments or biliary secretion.