CNP IS PRESENT IN CANINE RENAL TUBULAR CELLS AND SECRETED BY CULTUREDOPOSSUM KIDNEY-CELLS

C-type natriuretic peptide (CNP) is a vasoactive and antimitogenic pep tide that is structurally similar but genetically distinct from atrial natriuretic peptide. While first discovered in the brain, CNP has bee n shown to be produced by endothelial cells and may function in a para crine and autocrine fashion in the control of vascular tone. Recently, CNP immunoreactivity and B-type natriuretic peptide receptors (NPR-B) , for which CNP is a specific ligand, have been identified in the kidn ey. The present study was designed to determine whether renal epitheli al cells produce and secrete CNP and whether CNP immunoreactivity is p resent in canine kidney. Opossum kidney (OK) cells that express proxim al tubular cell characteristics were incubated for 6 h in fetal calf s erum-free Dulbecco's modified Eagle's medium (DMEM). CNP immunoreactiv ity was measured in the preincubation and 6-h conditioned media by rad ioimmunoassay (RIA) using a specific antibody to CNP-22. Furthermore t he molecular form of this CNP-like protein was determined by reverse-p hase high-performance liquid chromatography (HPLC), and intracellular localization of the CNP immunoreactivity was determined by immunohisto chemical staining. CNP immunoreactivity was also determined in renal t issue from dogs subjected to saline or endothelin infusion. Six-hour i ncubation in DMEM resulted in accumulation of CNP immunoreactivity (ba seline below detection level vs. 6 h = 117.3 +/- 8.3 pg/ml, P < 0.001) . Intracellular CNP concentration determined after sonication was 1.9 +/- 0.2 mu g/g protein, and immunohistochemical staining for CNP was m arkedly positive in the cytoplasm. HPLC demonstrated that the CNP immu noreactivity in the conditioned media corresponded to the CNP-53 molec ular form. CNP immunohistochemical staining was mild in renal tubular cells of saline-infused dogs and intense in renal tubular cells of end othelin-infused dogs. This study demonstrates that CNP-like peptide is present in renal tubular cells in vivo and is produced and secreted b y cultured renal epithelial cells in vitro. These findings suggest tha t CNP may play a role in the paracrine and/or autocrine regulation of renal function.