PROTEOLYTIC ACTIVATION OF A PUTATIVE RECEPTOR LEADING TO VASOCONSTRICTION AND PLATELET-AGGREGATION

Submandibular enzymatic vasoconstrictor (SEV), a member of the kallikr ein family of enzymes, elicits biological effects by a proteolytically mediated mechanism. We studied 1) whether SEV is able to aggregate pl atelets and 2) whether SEV may activate a receptor other than the clon ed thrombin receptor. SEV (10(-8) M) aggregated platelets, released AT P and increased intracellular Ca2+. Elastase treatment rendered human platelets unresponsive to SEV and thrombin (TH), but not to cathepsin G. In desensitization experiments performed with gamma-TH, after two s uccessive additions of approximately 50 nM gamma-TH, a third dose elic ited 15.8+/-3.4% of the initial response (n=4), but platelets responde d to approximately 20 nM SEV by 33.8+/-7.2% of control (p <0.03 vs las t response to gamma-TH). After desensitization to SEV (n=4), the respo nse to a third dose was 4+/-1.3% of control, but gamma-TH still induce d 37.7+/-12.4% aggregation (p <0.02 vs last response to SEV). Incubati on of washed rabbit platelets with alpha-TH digested with elastase (10 (-10) M TH added to 7 mu g/ml elastase for 1 min) rendered them unresp onsive to additional challenges with TH, but they still responded to a n equipotent dose of SEV (2.7x10(-9) M) by 86+/-48% of control. In iso lated rabbit aortic rings contracted with 10(-6) M norepinephrine (NE) to 42+/-3% of maximum, SEV (2.8x10(-8) M) caused further contraction to 87+/-4%. In contrast, alpha-TH (1.6x10(-7) M) fended to relax both NE- and SEV-contracted rings by 14+/-2 and 16.2+/-2%, respectively (n= 3 each). We concluded that part of the platelet-aggregating effect of SEV may be mediated by activation of a receptor(s) different from that of TH. Copyright (C) 1997 Elsevier Science Ltd.