The involvement of a novel mechanism distinct from the thrombin receptor in the vasocontraction induced by trypsin

Abstract

1. The vasocontracting effect of a serine protease trypsin and its mechanisms were investigated by monitoring the isometric tension in endothelium-denuded rings of rabbit thoracic aortae and its effects on intracellular free Ca²⁺ concentrations ([Ca²⁺]_i) in dispersed rabbit vascular smooth muscle cells with a Ca²⁺ indicator fura-2. The actions of trypsin were compared with those of thrombin.

2. Both thrombin and trypsin reversibly contracted aortic rings without endothelium in a concentration-dependent manner. The vasocontraction induced by trypsin was well correlated with the protease activity of trypsin actually added to the tissue baths containing the aortic rings and was completely blocked by soybean trypsin inhibitor and phenylmethylsulphonyl fluoride (PMSF), a serine protease inhibitor.

3. The trypsin-induced contractions of the aortic rings were not the result of irreversible damage to vascular smooth muscle cells, since the contractile responses induced by noradrenaline or 30 mM KCl were unaffected by pretreatment with trypsin.

4. The contractions induced by either thrombin or trypsin were reduced to about 30% of control responses after removal of extracellular Ca²⁺, indicating that most of the contraction is dependent on extracellular Ca²⁺. By contrast, the contractions induced by either of the proteases were reduced by an antagonist of L-type voltage-operated Ca²⁺ channels, nifedipine, to about 70% of control responses, indicating that both nifedipine-sensitive and -resistant Ca²⁺ channels are involved in these contractions.

5. In the aortic rings precontracted by a maximally effective concentration of thrombin, the second application of thrombin virtually failed to induce contractions but trypsin could still induce contractions amounting to 10% of control values by it's protease activity.

6. After the first application of a maximal concentration of thrombin, the second application of thrombin could not induce an increase in [Ca²⁺]_i, but an application of trypsin could still induce an increase in [Ca²⁺]_i in dispersed rabbit vascular smooth muscle cells.

7. These data suggest that in addition to activation of a thrombin receptor, trypsin can contract rabbit aortae by a proteinase-activated receptor 2 or a novel mechanism.

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