LOCALIZATION, EXPRESSION, AND CHARACTERIZATION OF GUANYLIN IN THE RATADRENAL-MEDULLA

The peptide guanylin, recently isolated from the intestine, and locali zed to cells of the gut mucosa, is involved in electrolyte/water trans port in the intestinal epithelium by means of a paracrine mode of regu lation. Since high amounts of this peptide are present also in the sys temic circulation, we investigated the adrenal gland as a potential en docrine source of guanylin. Using a reverse transcriptase-polymerase c hain reaction and hybridization with an internal oligonucleotide desig ned for rat guanylin, 514-bp signals were obtained in intestinal tissu e and adrenal gland. Successive analyses of extracts from intestine an d adrenal gland by HPLC, western blotting, and radioimmunoassay reveal ed the presence of the same high-molecular mass (about 12.4 kDa) guany lin that corresponds to the mass of the guanylin prohormone. About 60 fmol/ml of circulating immunoreactive guanylin was determined in plasm a. Localization studies with antisera directed against different epito pes of guanylin revealed that, in the adrenal gland, guanylin immunore activity is restricted to the medulla, where it is mainly confined to norepinephrine chromogranin A-containing cells. On the ultrastructural level, guanylin immunoreactivity was exclusively located to secretory granules of chromaffin cells. The present data indicate that, in addi tion to entero-endocrine cells, the adrenal medulla represents a furth er source of guanylin. Thus, an endocrine mode of function of guanylin may accrue to its hitherto evidenced paracrine action in fluid transp ort in the intestinal epithelium. Furthermore guanylin may be consider ed as a neurohormonal peptide.