Bronchoconstrictor effect of thrombin and thrombin receptor activating peptide in guinea‐pigs in vivo

Abstract

1 Several thrombin cellular e ects are dependent upon stimulation of proteinase activated receptor-1 (PAR-1) localized over the cellular surface. Following activation by thrombin, a new Nterminus peptide is unmasked on PAR-1 receptor, which functions as a tethered ligand for the receptor itself. Synthetic peptides called thrombin receptor activating peptides (TRAPs), corresponding to the N-terminus residue unmasked, reproduce several thrombin cellular e ects, but are devoid of catalytic activity. We have evaluated the bronchial response to intravenous administration of human a-thrombin or a thrombin receptor activating peptide (TRAP-9) in anaesthetized, arti®cially ventilated guinea-pigs. 2 Intravenous injection of thrombin (100 u kg 71 ) caused bronchoconstriction that was recapitulated by injection of TRAP-9 (1 mg kg 71 ). Animal pretreatment with the thrombin inhibitor Hirulog TM (10 mg kg 71 i.v.) prevented thrombin-induced bronchoconstriction, but did not a ect bronchoconstriction induced by TRAP-9. Both agents did not induce bronchoconstriction when injected intravenously to rats. 3 The bronchoconstrictor e ect of thrombin and TRAP-9 was subjected to tolerance; however, in animals desensitized to thrombin e ect, TRAP-9 was still capable of inducing bronchoconstriction, but not vice versa. 4 Depleting animals of circulating platelets prevented bronchoconstriction induced by both thrombin and TRAP-9. 5 Bronchoconstriction was paralleled by a biphasic change in arterial blood pressure, characterized by a hypotensive phase followed by a hypertensive phase. Thrombin-induced hypotension was not subject to tolerance and was inhibited by Hirulog TM ; conversely, hypertension was subject to tolerance and was not inhibited by Hirulog TM . Hypotension and hypertension induced by TRAP-9 were neither subject to tolerance nor inhibited by Hirulog TM . 6 Our results indicate that thrombin causes bronchoconstriction in guinea-pigs through a mechanism that requires proteolytic activation of its receptor and the exposure of the tethered ligand peptide. Platelet activation might be triggered by the thrombin e ect.

Asthma and coagulation

“…72 These reports suggest a functional relationship of PAF in the pathophysiology of asthma. The potential role of platelets in lung function disorders accompanying asthma is further supported by findings that bronchoconstriction induced by either intratracheal instillation of lipopolysaccharide 73 or intravenous injection of thrombin 74 required the presence of functionally intact platelets in the circulation. In accordance, platelet depletion resulted in a significant inhibition of allergen-induced airway hyper-responsiveness to inhaled histamine in rabbits, 75 and P-selectin-deficient mice demonstrated lower levels of airway hyper-responsiveness than wild-type mice 61 with cockroach allergen-induced lung inflammation.…”

The role of platelets in the pathophysiology of asthma

“…In platelet-depleted animals, pulmonary pressure increased by only 17% of the value recorded for animals with normal platelet numbers. Cicala et al [39] showed that substantial bronchoconstriction in the guinea-pig airways can also be achieved by intravenous thrombin administration. However, they observed that reduction in lung function in this model is averted in animals previously treated with anti platelet serum (APS), suggesting that platelets are essential for a bronchoconstrictive reaction.…”

Direct Thrombin Inhibition Reduces Lung Collagen Accumulation and Connective Tissue Growth Factor Mrna Levels in Bleomycin-Induced Pulmonary Fibrosis

“…Although PAR-1 expression had never been described in the lung, PAR-1 mRNA had previously been detected in cultured alveolar macrophages. 48 Furthermore, specific agonists of this receptor have been shown to cause bronchoconstrictor responses in experimental animals, 49 suggesting that the lung does indeed express functional PAR-1 receptors. Our studies showed that PAR-1 was expressed on the bronchial epithelium in the lungs of all experimental animals, with no evidence of altered expression after bleomycin instillation.…”