NITRIC-OXIDE CAUSES APOPTOSIS IN PULMONARY VASCULAR SMOOTH-MUSCLE CELLS

Nitric oxide (NO), a product of certain cytokine-activated cells, affe cts rates of apoptosis, a mechanism of programmed cell death. We asked whether NO affected rates of apoptosis in pulmonary vascular cells. U sing rat pulmonary artery smooth muscle cells, we studied direct effec ts of the NO donor S-G-nitrosoacetyl-D,L-penicillamine (SNAP) and the effects of NO endogenously synthesized in response to bacterial lipopo lysaccharide (LPS) and inflammatory cytokines interleukin-1 beta, inte rferon-gamma, and tumor necrosis factor-alpha (a combination called cy tomix for convenience). We determined apoptosis on the basis of light microscopy and the bromodeoxyuridine terminal deoxynucleotidyl transfe rase reaction (BrdUTdT). Both SNAP- and cytomix-induced synthesis of N O resulted in histologic evidence of apoptosis based upon fluorescence microscopy using propidium iodide. SNAP (10(-5) M) increased BrdUTdT- positive cells from 17.5 to 78.4% compared with basal medium alone, wi th the maximal response occurring at 15 h or exposure. Exposing cells to LPS and cytokines induced NO production (from 0.1 +/- 0.1 to 24.6 /- 0.5 mu M, P < 0.05) caused cytological changes consistent with apop tosis and led to an increase of increased BrdUTdT-positive cells from 11 to 41% at 12 h compared with basal medium alone. The competitive NO synthase inhibitor N-G-monomethyl-L-arginine inhibited both NO synthe sis and NO apoptosis, returning the proportion of BrdUTdT-positive cel ls (6%) to levels below control. L-Arginine (0.5 mM) restored percenta ges to those increase in response to endogenously synthesized NO, and NO is a potential mechanism of acute lung injury in response to inflam matory cytokines. (C) 1998 Academic Press.