Protamine, a mixture of polypeptides that is rich in arginine, has been used clinically as an antidote to heparin overdoses and a complexing agent in a long-acting insulin preparation. When protamine is administered intravenously, its abundant accumulation in the kidneys has been reported. However, the renal uptake mechanism for protamine is not clear. In this study, we examined the transport mechanism for protamine in opossum kidney (OK) cells, a suitable in vitro model for renal proximal tubular epithelial cells. Flow cytometric analysis revealed that the association of fluorescein isothiocyanate (FITC)-labeled protamine from salmon (FITC-protamine) by OK cells was inhibited by unlabeled protamine in a concentration-dependent manner. The association of FITC-protamine was temperature-and energy-dependent. Confocal microscopy analysis showed that the fluorescence was localized in the cytoplasm and nucleus of OK cells. In addition, FITC-protamine association was inhibited by cationic drugs such as polycationic gentamicin and polymixin B, but it was increased by a basic amino acid, arginine. Inhibitors for clathrin-and caveolin-dependent endocytosis showed inhibitory effects on FITC-protamine association. Pretreatment with heparinase III partially but significantly decreased the association of FITC-protamine. These results suggest that protamine may be taken up by OK cells via receptor-mediated endocytosis, which may result in its localization in the cytoplasm and nucleus of the cells.

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