Thrombin contracts isolated bovine retinal small arteries in vitro.

The effect of thrombin was tested in vitro on rings of bovine retinal small arteries (internal diameter approximately 200 microns). Cumulative addition of thrombin (0.001-10 units/ml) induced a variable concentration-dependent contraction of the retinal arteries. The contractions were slow in onset and reached a plateau after 5-8 minutes when thrombin was added cumulatively to the organ bath. Two vessels remained contracted greater than 1 hour after wash-out of thrombin. Maximum vessel contraction induced by thrombin was equal to 43% of the vessel Emax (1.36 N/m), with an effective concentration at the 50% level of 0.04 units/ml. Vessel contraction induced by 1 unit/ml of thrombin was, in contrast, transient, reaching a maximum within 2-4 minutes. Thereafter the vessel tension declined again almost back to baseline within the next 10-20 minutes. Contractions to thrombin could not be repeated nor could it be elicited with thrombin inactivated with heat or antithrombin-III. Treatment of vessels with 10(-6) M phenoxybenzamine had no effect on the thrombin-induced vessel response. These findings indicate that the contractile effect of thrombin depends on its catalytic activity. Indomethacin at a concentration of 10(-5) M did not affect the thrombin-induced vessel response. Methylene blue at a concentration of 3 X 10(-6) M potentiated the thrombin-induced response in the larger, greater than 200-microns diameter retinal arteries. The ensuing relaxation of the arteries, after maximal tone was reached, was slower than in the control.(ABSTRACT TRUNCATED AT 250 WORDS)