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

Abstract

Submandibular enzymatic vasoconstrictor (SEV), a member of the kallikrein family of enzymes, elicits biological effects by a proteolytically mediated mechanism. We studied 1) whether SEV is able to aggregate platelets and 2) whether SEV may activate a receptor other than the cloned thrombin receptor. SEV (10 −8 M) aggregated platelets, released ATP and increased intracellular Ca 2+. Elastase treatment rendered human platelets unresponsive to SEV and thrombin (TH), but not to cathepsin G. In desensitization experiments performed with γ-TH, after two successive additions of approximately 50 nM γ-TH, a third dose elicited 15.8±3.4% of the initial response ( n = 4), but platelets responded to approximately 20 nM SEV by 33.8 ± 7.2% of control ( p < 0.03 vs last response to γ-TH). After desensitization to SEV ( n=4), the response to a third dose was 4 ± 1.3% of control, but γ-TH still induced 37.7 ± 12.4% aggregation ( p < 0.02 vs last response to SEV). Incubation of washed rabbit platelets with α-TH digested with elastase (10 −10 M TH added to 7 μg/ml elastase for 1 min) rendered them unresponsive to additional challenges with TH, but they still responded to an equipotent dose of SEV (2.7 × 10 −9 M) by 86 ± 48% of control. In isolated rabbit aortic rings contracted with 10 −6 M norepinephrine (NE) to 42 ± 3% of maximum, SEV (2.8 × 10 −8 M) caused further contraction to 87 ± 4%. In contrast, α-TH (1.6 × 10 −7 M) tended 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 © 1997 Elsevier Science Ltd