Relationship between pinocytic rate and uptake of transferrin by suspended rat hepatocytes

Abstract

The aim of this study was to compare quantitatively the capacity to transcytose (i.e. to uptake and release) transferrin (Tf) with the pinocytic activity of suspended adult rat hepatocytes. An oligodisperse preparation of 131I-polyvinylpyrrolidone (PVP; Mr 36,000) was used to measure the inward and outward aspects of the pinocytic process in separate experiments. Cell association of rat 125I-Tf was measured at Tf concentrations approaching physiological, where 59Fe uptake obeyed first-order kinetics. Release studies with both PVP and Tf were carried out under conditions which minimized the probability of de novo endocytosis of a molecule already released. Sets of experimental points representing cell-associated radioactivities were converted into continuous algebraic functions by fitting with two-term (release studies) or three-term (uptake studies) exponential equations. Transport of PVP and Tf through the cells was computed from these equations by deconvolution. This analysis showed that, under the present experimental conditions, the fractional transcytosis rates of Tf and PVP by hepatocytes were in the ratio of 1:0.77. These values imply that, in the physiological range of Tf concentrations, about 75% of the Fe taken up by hepatocytes may be due to a pinocytic mechanism (fluid-phase or mixed). Inclusion of chloroquine (1 mM) in the suspending medium, both in uptake and release experiments, resulted in more PVP and Tf passing through the cells, while Fe uptake was reduced. It is suggested that the base probably exerted its enhancing effect on transcytosis by shunting the subcellular transport of PVP and Tf to the outward leg through a shorter circuit.

A Nanoparticle-Based Model Delivery System To Guide the Rational Design of Gene Delivery to the Liver. 2. In Vitro and In Vivo Uptake Results

“…Based on these uptake results, it is estimated that hepatocytes pinocytose the equivalent of 20% or more of their volume and at least 5 times their plasma membrane surface area each hour (19). This high recycle rate of pinocytosed materials has also been confirmed with suspended rate hepatocytes (20), and the shape of the MeO-50 uptake curve in Figure 1a can be explained by this mechanism (21). For the first 30-60 s of hepatocyte exposure to MeO-50 nanoparticles, uptake is proceeding quickly without significant nanoparticle release back into the extracellular fluid.…”

“…This can be understood if the primary mechanism of uptake of Gal-50 in vitro is also pinocytosis, but with additional ASGPr-mediated uptake as well. Suspended rat hepatocytes have been shown (20) to uptake transferrin primarily by pinocytosis, despite the presence of a transferrin receptor on their cell surface. Nearly 80% of transferrin uptake was attributed to pinocytosis, with the remainder coming through receptormediated endocytosis.…”