Asialoglycoprotein receptor mRNAs are expressed in most extrahepatic rat tissues during development

The expression of an asialoglycoprotein receptor, comprised of a major (R- 1) and minor (R-2/3) component, was characterized in various rat tissues during postnatal development. Two R-1 mRNA subspecies were identified in the liver by Northern blots. The 1.4-kb form, which was the predominant subspecies in the liver, was detected in small amounts only in the stomach and kidney. The 1.7-kb mRNA, however, was present in all extrahepatic tissues examined. R-1 mRNA content in the liver increased by only 38% during the suckling period, but the extrahepatic R-1 mRNA had a dramatic biphasic developmental pattern with the highest levels at birth and after weaning. The R-2/3 mRNA was present as a single species on Northern blot in the liver and in a restricted number of extrahepatic tissues (salivary glands, ileum, and kidney) but was detected in most tissues by polymerase chain reaction. The R- 2/3 mRNA content in the liver increased fourfold after birth but was consistently low in the other tissues. [⁵⁷Co]cobalamin-haptocorrin binding activity was present in all extrahepatic tissues examined, although only at a level 2-5% of hepatic binding. Binding affinities of the adult kidney brush- border membranes (1.9 x 10⁹ M^-1) and the adult liver plasma membranes (2.4 x 10⁹ M^-1) were comparable. On Western blot, <4% of adult liver R-1 content and a relatively larger amount of R-2/3 were detected in all four extrahepatic tissues examined. Thus the low binding activity in extrahepatic tissues correlates with the presence of both receptor subunits, which are closely related to or identical with the hepatic receptor subunits.