INHIBITORY EFFECTS OF NORDIHYDROGUAIARETIC ACID ON ET(A)-RECEPTOR-MEDIATED CONTRACTIONS TO ENDOTHELIN-1 IN RAT TRACHEA

1 It has been shown previously that nordihydroguaiaretic acid (NDGA) inhibits endothelin-1 (ET-1)-induced contractions in rat isolated tracheal smooth muscle. To investigate the underlying mechanisms, this study examined the effects of NDGA on various aspects of the ET(A) and ET(B) receptor-effector systems which mediate ET-1-induced contractions in this preparation. 2 NDGA inhibited contractions induced by each of the isoforms of ET (ET-1, ET-2 and ET-3) but not those induced by the ET(B) receptor-selective agonist, sarafotoxin S6c, the cholinoceptor agonist, carbachol or the depolarizing spasmogen, KCl. 3 Quantitative autoradiographic studies of [I-125]-ET-1 binding to rat tracheal smooth muscle indicated that NDGA was not an ET receptor antagonist. 4 NDGA inhibited the ET(A) receptor-mediated, intracellular Ca2+-dependent contractions induced by 100 nM ET-1 in Ca2+-free solution (by 75%, P<0.01). Furthermore, NDGA markedly inhibited the contractions induced by ryanodine and cyclopiazonic acid; contractions purportedly due to Ca2+ release from intracellular stores. 5 Like NDGA, the sarcoplasmic reticulum Ca2+-ATPase inhibitors cyclopiazonic acid and thapsigargin inhibited contractions to ET-1, but not carbachol or KCl. However, cyclopiazonic acid, but not NDGA, also (a) induced transient contractions in rat trachea, (b) potentiated contractions induced by KCl, and (c) potentiated the extracellular Ca2+-dependent phase of ET-1-induced contractions, indicating that NDGA did not inhibit ET-1-induced contractions through Ca2+-ATPase inhibition and depletion of sarcoplasmic reticular Ca2+. 6 In control preparations, ET-1 induced a slowly developing, sustained contraction. However, in the presence of NDGA or the ET(A) receptor antagonist, BQ123, ET-1-induced contractions resembled the transient contractions induced by sarafotoxin S6c. In nominally Ca2+-free solution, ET(A) receptor-mediated contractions induced by ET-1 developed very slowly and were inhibited by NDGA. 7 Additional studies indicated that the inhibitory effects of NDGA on endothelin-1-induced contractions were not the result of any significant actions of NDGA on lipoxygenase, cytochrome P-450, L- or T-type Ca2+-channels, Na+-channels or protein kinase C. 8 In summary, NDGA selectively inhibited ET-1-induced contractions in rat tracheal smooth muscle via a lipoxygenase-independent mechanism involving inhibition of the ET(A) but not the ET(B), receptor-effector system. NDGA did not appear to inhibit the initial events in the ET(A) signal transduction pathway, such as receptor binding and protein kinase C activation. However, NDGA inhibited the intracellular Ca2+-dependent component of ET-1-induced contraction, possibly by inhibiting mobilisation of intracellular Ca2+. As an apparent direct consequence of inhibiting the ET(A) receptor-effector system, NDGA markedly changed the time course of ET-1-induced contractions; from a slowly developing and sustained contraction into a transient contraction resembling that induced by sarafotoxin S6c.