Contractile effects of thrombin in porcine pulmonary arteries and the influence of thrombin inhibitors.

Thrombin catalyzes not only the conversion of fibrinogen to fibrin but also activates several receptor-mediated cell responses. In ring segments of porcine pulmonary arteries the contractile effect of thrombin was studied in the presence and absence of endothelium. The integrity of endothelium was assessed by the bradykinin-induced relaxation of PGF2 alpha (3 mumol/l)-precontracted vessels which was absent after mechanical removal of endothelium. Thrombin at 0.1 to 10 U/ml (i.e. about 1-100 nmol/l) caused a sustained contraction in endothelium-denuded arteries with a maximum at 20-30 min. In vessels with intact endothelium a significant increase in tension up to 1 U/ml was observed preceded by a transient relaxant response. The contractile effect in vessels with intact endothelium was comparatively weaker. This is probably due to the release of EDRF from endothelial cells since blockade of EDRF synthesis by NG-nitro-L-arginine augmented the thrombin-induced contractions in arteries with intact endothelium. Indomethacin did not alter the contractile effect. However, in vessels with endothelium and in endothelium-denuded vessels the contractions were reduced when extracellular calcium was omitted. Verapamil (10 mumol/l) significantly diminished the contractile effect only in endothelium-denuded vessels. On preincubation of endothelium-denuded arterial ring segments with myo-[2-3H]inositol the addition of thrombin (10 U/ml) caused an accumulation of [3H]inositol-1,4,5-triphosphate (IP3). A maximum was observed after 2 min preceding the maximum increase in contraction. Measurement of thrombin-induced endogenous IP3 generation by radioreceptor assay yielded the same results. The thrombin-induced contractile effect requires the proteolytic activity of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)