MECHANISMS OF NATRIURETIC-PEPTIDE-INDUCED GROWTH-INHIBITION OF VASCULAR SMOOTH-MUSCLE CELLS

Objective: While natriuretic peptides can inhibit growth of vascular s mooth muscle cells (VSMC), controversy exists as to whether this effec t is mediated via the guanylate cyclase-coupled receptors, NPR-A and N PR-B, or the clearance receptor, NPR-C. The original aim of this study was to examine the mechanism by which the NPR-C receptor regulates gr owth. Methods: Rat VSMC were characterized with regard to natriuretic peptide receptor expression by RT/PCR and radioligand binding studies. The effect on growth following addition of the peptides and the Ligan ds for NPR-C was measured by [H-3]thymidine incorporation. Cyclic guan osine monophosphate (cGMP) levels were determined by radioimmunoassay and mitogen activating protein kinase activity was based on the phosph orylation of myelin basic protein. Results: In rat VSMC, passages 4-12 , both atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) dose-dependently inhibited serum and PDGF-induced VSMC growth. In contrast, NPR-C specific ligands alone had no effect on cell growth but enhanced growth inhibition when co-administered with ANP and CNP. ANP and CNP also decreased PDGF-BB-stimulated MAP kinase activity. On ce again, NPR-C specific ligands alone had no effect but enhanced the effects of ANP. Furthermore, a cGMP specific phosphodiesterase inhibit or dose-dependently inhibited VSMC growth and markedly enhanced natriu retic-peptide-induced inhibition at low peptide concentrations. To exa mine a potential mechanism for the controversy concerning the NPR-C, w e investigated the autocrine expression of ANP and CNP by VSMC and fou nd that mRNA encoding both peptides could be detected by RT/PCR. Concl usion: Our findings indicate that the guanylyl-cyclase-linked receptor s mediate the antiproliferative actions of the natriuretic peptides on vascular smooth muscle cell growth. Moreover, we hypothesize that the apparent inhibition of growth by NPR-C specific ligands reported by o thers may be due to stabilization of natriuretic peptides produced by the cultured VSMC and subsequent action of these peptides at guanylyl- cyclase-linked receptors.