No abstract

The submandibular rat 1 protein (SMR1) is selectively processed at pairs of basic amino acid residues in a tissue- and sex-specific manner. We have mapped peripheral targets for the final secretory maturation product of SMR1, the pentapeptide QHNPR, by examining in vivo the tissue distribution of the radiolabeled peptide using β-radio imager whole body autoradiography. The characteristics of tissue uptake allowed specific binding sites at physiological peptide concentrations to be identified within the renal outer medulla, bone and dental tissue, glandular gastric mucosa, and pancreatic lobules. Direct evidence that pentapeptide binding sites are localized in selective portions of the male rat nephron, within the S3, S2, and S1 segments of the proximal tubules, was obtained. In bone tissue the pentapeptide exclusively accumulates within the trabecular bone remodeling unit, and in dental tissue it concentrates within the tubules of the dentinal rat incisor. In relation to male rat-specific behavioral characteristics, our data suggest that the circulating androgen-regulated SMR1-derived pentapeptide is primarily involved in the modulation of mineral balance between at least four systems: kidney, bone, tooth, and circulation.

The in vivo bioavailability of exogenous fibroblast growth factor 2 (FGF2) was studied after i.v. injection of uniformly 14 C-labeled FGF2 into young rats.14 C-FGF2 was rapidly accumulated in almost all solid organs within 5 min. After 30 min, more than 65% of FGF2 was retained in liver, 4.5% in kidneys, 1.2% in spleen, 0.15% in adrenal glands, and trace amounts in bone marrow, eyes, lungs, and heart. Suborgan distribution of 14 C-FGF2 showed that for kidneys and adrenal glands, the labeling was mainly concentrated in the cortical zone. Incubation of organ sections with 2 M NaCl or heparin eluted all the radioactivity, indicating that labeling was due to FGF2-heparan sulfate proteoglycan (HSPG) interactions. Electrophoretic analysis show only native 14 C-FGF2 in the blood and extracellular matrix; however, FGF2 is continuously catabolized in solid organs, indicating that all participate in the clearance of FGF2 by cellular internalization and subsequent catabolism. All FGF2 catabolic fragments bound heparin, demonstrating the preservation of their HSPG-binding site during the in vivo intracellular catabolism of FGF2. Analysis of the high-affinity receptors of FGF2 (FGFR-1 and FGFR-3) and the mitogen-activated protein kinase did not show any increase in either FGFR tyrosine phosphorylation or in mitogen-activated protein kinase activation. This study shows for the first time that exogenous FGF2 is cleared by HSPG cellular internalization and catabolism without inducing the activation of FGFRs within at least five organs in vivo, which strongly suggests that the HSPG-dependent internalization and catabolism pathway may control the in vivo bioavailability of FGF2.