Ultrastructural immunocytochemical localization of the asialoglycoprotein receptor in rat hepatocytes

The asialoglycoprotein receptor is present on the plasma membrane of rat hepatocytes and mediates the endocytosis of exogenous asialoglycoproteins. The intracellular locations of the receptor are incompletely defined, and whether the receptor is present on the canalicular membrane is controversial. To investigate these unresolved issues, we localized the asialoglycoprotein receptor on the surfaces of and within rat hepatocytes in the absence of exogenous asialoglycoprotein. Cryostat sections of perfusion-fixed livers of fasted rats were incubated with peroxidase-labeled antibodies to the rat asialoglycoprotein receptor and were examined by light and electron microscopy. The asialoglycoprotein receptor on the surface of hepatocytes was present on the sinusoidal plasma membrane and was concentrated in coated pits; it was present also on the lateral plasma membrane but was absent from the bile canalicular membrane. Within hepatocytes, the receptor was present in saccules of the Golgi complexes and occasionally in segments of the endoplasmic reticulum. Definite endocytic vesicles containing receptor were absent, and lysosomes were devoid of the receptor. These observations indicate that in the absence of exogenous asialoglycoprotein (a) the asialoglycoprotein receptor on the plasma membrane of hepatocytes is present only on the sinusoidal and lateral surfaces, (b) the receptor is being synthesized and assembled, (c) endocytic internalization of the receptor is minimal or absent, and (d) lysosomes are devoid of the receptor.