Thrombin-elicited contractile responses of aortic smooth muscle.
Human alpha-thrombin at physiologically relevant concentrations of 0.75 to 225 nM (0.01 to 22 clotting units/ml) caused rabbit thoracic aorta to slowly contract in isolated organ baths. Near maximum contractile tension (10% below the irreversible contraction caused by 22 units/ml) was slowly generated by 75 nM alpha-thrombin (8 units/ml) over 18 min and was 50% that of the norepinephrine (NE) control. The initial tonus could only be regained by repeated washing and 60 min equilibration. Tissues were refractory to a second 75 nM alpha-thrombin challenge but responded fully to 1.0 microM NE. Conversion of human alpha-thrombin to nonclotting but estero/amidolytically active gamma-thrombin (less than 0.1% clotting activity) or nitration of the enzyme (1% clotting activity) did not interfere with the contractile activity, whereas chemical conjugates of the parent enzyme at the catalytic site were inactive. Neither atropine, phentolamine, nor indomethacin blocked the thrombin-induced contractions, whereas D-600 was markedly inhibitory. Preincubation of the tissue with inactive forms of thrombin did not prevent the alpha-thrombin-induced response. Removal of the vascular endothelium did not prevent contraction. Aorta preparations with intact endothelium relaxed in the presence of very low concentrations (0.75 to 750 pM) of alpha-thrombin prior to contracting in response to higher concentrations during cumulative dose-response experiments. Our data suggest that catalytically active thrombin forms (alpha- or nonclotting beta- and gamma-thrombins) may have hemostatic functions at the vascular levels in hemostasis.
D000818:Animals; D001013:Aorta, Thoracic / Q000502:physiology; D001285:Atropine / Q000494:pharmacology; D055598:Chemical Phenomena; D002621:Chemistry; D004305:Dose-Response Relationship, Drug; D004727:Endothelium / Q000502:physiology; D006801:Humans; D007213:Indomethacin / Q000494:pharmacology; D007231:Infant, Newborn; D007700:Kinetics; D009119:Muscle Contraction / Q000187:drug effects*; D009131:Muscle, Smooth, Vascular / Q000502:physiology*; D009638:Norepinephrine / Q000494:pharmacology; D010646:Phentolamine / Q000494:pharmacology; D011817:Rabbits; D013329:Structure-Activity Relationship; D013917:Thrombin / Q000494:pharmacology*
D001285:Atropine; D013917:Thrombin; D009638:Norepinephrine; D007213:Indomethacin; D010646:Phentolamine