Necessity of dual blockade of endothelin ET(A) and ET(B) receptor subtypes for antagonism of endothelin-1-induced contraction in human bronchi

1 Endothelin (ET)-1 has been postulated to be involved in the development of obstructive airway diseases in man. In the present study, we attempted to characterize ET receptor subtypes mediating ET-1-induced contraction in human isolated bronchi. The ET receptor antagonists used in the present study were BQ-123 (ET(A) receptor-selective), BQ-788 (ET(B) receptor-selective) and BQ-928 (ET(A)/ET(B) dual). Sarafotoxin S6c (S6c) was also used as an ET(B) receptor-selective agonist. 2 In human bronchi, ET-1 and S6(c) (10-(12) M to 10(-7) M) produced concentration-dependent contraction with almost equal potency (pD(2): 8.88 +/- 0.16 for ET-1 and 9.42 +/- 0.15 for S6c). The contraction induced by S6c was competitively antagonized by BQ-788 alone (1 and 10 mu M) with a pK(B) value of 7.49 +/- 0.21, suggesting that the stimulation of ET(B) receptors causes contraction of human bronchi. However, contrary to expectation, the concentration-response curves for ET-1 were not affected by BQ-788. The ET-1- and S6c-induced contractions were not affected by BQ-123 (10 mu M). Thus, ET-1-induced contraction of human bronchi is not antagonized by BQ-123 alone or by BQ-788 alone. 3 Combined treatment with 10 mu M BQ-123 and 10 mu M BQ-788 significantly antagonized the contraction induced by ET-1 with a dose-ratio of 11. BQ-928 also significantly antagonized ET-1-induced contraction with a pK(B) value of 6.32 +/- 0.24. 4 The specific binding of [I-125]-ET-1 to human bronchial membrane preparations was inhibited by BQ-123 (100 pM to 1 mu M) by approximately 40%. Combination treatment with BQ-788 (100 pM to 1 mu M) completely inhibited the BQ-123-resistant component of [I-125]-ET-1 specific binding. 5 In conclusion, the present study demonstrates that BQ-788 alone cannot inhibit ET-1-induced contractions in human bronchi, although human bronchial ET(B) receptors are BQ-788-sensitive. Furthermore, it was shown that blockade of both receptor subtypes antagonizes ET-1-induced contraction, and that both receptor subtypes co-exist in human bronchial smooth muscles. These findings suggest that ET(A) receptors as well as ET(B) receptors are involved in ET-1-induced contraction in human bronchi. If ET-1 is involved in human airway diseases, dual blockade of ET(A) and ET(B) receptors may be necessary to treat the diseases.