THROMBIN PREVENTS THE EXPRESSION OF INDUCIBLE NITRIC-OXIDE SYNTHASE IN VASCULAR SMOOTH-MUSCLE CELLS BY A PROTEOLYTICALLY-ACTIVATED THROMBINRECEPTOR

Proteolytically active forms of thrombin (alpha- and gamma-thrombin) a nd thrombin receptor peptides inhibited the release of nitrite, a stab le endproduct of nitric oxide, evoked by interleukin-1 beta (IL-1 beta ) in cultured vascular smooth muscle cells while proteolytically inact ive forms [D-Phe-Pro-Arg chloromethyl ketone-alpha-thrombin (PPACK-alp ha-thrombin) and diisopropylphosphoryl-alpha-thrombin (DIP-alpha-throm bin)] had either no or only minimal inhibitory effects. Under bioassay conditions, perfusates from columns containing IL-1 beta-activated va scular smooth muscle cells or cells treated with IL-1 beta plus PPACK- alpha-thrombin relaxed detector blood vessels. These relaxations were abolished by the inhibitor of nitric oxide synthesis, N-G-nitro-L-argi nine. No relaxations were obtained with untreated cells or IL-1 beta-t reated cells in the presence of alpha-thrombin. The expression of indu cible nitric oxide synthase mRNA and protein in vascular smooth muscle cells by IL-1 beta was impaired by alpha-thrombin. These results demo nstrate that thrombin regulates the expression of the inducible nitric oxide synthase at a transcriptional level via the proteolytic activat ion of the thrombin receptor in vascular smooth muscle cells.