Thrombin inhibits active sodium-potassium transport in porcine lens

PURPOSE. Although thrombin is best known for its role in blood coagulation, it has been reported to change the activity of ion motive ATPases in some tissues. In the present study, experiments were conducted to determine the influence of thrombin enactive sodium-potassium transport in porcine lenses . METHODS. Ouabain-sensitive potassium (Rb-86) uptake by intact porcine lense s was used as an index of Na,K-ATPase-mediated active sodium-potassium tran sport. Na,K-ATPase activity was measured by determining ouabain-sensitive A TP hydrolysis in isolated membrane material. RESULTS. In the presence of thrombin (1 unit/ml) the rate of ouabain-sensit ive potassium (Rb-86) uptake was reduced by 40% to 60%, but ouabain-insensi tive potassium (Rb-86) uptake was unchanged. The inhibitory effect of throm bin on ouabain-sensitive potassium (Rb-86) uptake was suppressed in the pre sence of hirudin (an antagonist for thrombin receptors) but persisted in th e presence of amphotericin B (a pseudo ionophore that effectively clamps pl asma membrane sodium permeability at a high value). Enzyme measurements sho wed ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) was significant ly inhibited in membrane material isolated from the capsule epithelium of l enses, which had been pretreated with thrombin for 30 minutes. However, thr ombin failed to exert a direct inhibitory effect on Na,K-ATPase activity wh en added directly to membrane fragments isolated from the epithelium of con trol (nonincubated) lenses. Both genistein and herbimycin (tyrosine kinase inhibitors) suppressed the effect of thrombin on the Rb-86 uptake response. Results from Western blot studies suggested that tyrosine kinases are acti vated in the epithelium of lenses exposed to thrombin, CONCLUSIONS. The results suggest the inhibitory effect of thrombin on lens active sodium-potassium transport could involve the activation of a recepto r-second-messenger mechanism in intact lens cells. The response appears to involve a tyrosine kinase-mediated step. The functional significance of the thrombin-mediated change of lens active sodium-potassium transport is uncl ear since appreciable amounts of thrombin may only be presented to the lens during instances of blood-aqueous-barrier breakdown. It is possible that l ens receptors are functionally activated by other proteases, possibly cathe psins, which may enter aqueous humor from the ciliary body.