Rg. Goldie et al., PREDOMINANCE OF ENDOTHELIN(A) (ET(A)) RECEPTORS IN OVINE AIRWAY SMOOTH-MUSCLE AND THEIR MEDIATION OF ET-1-INDUCED CONTRACTION, British Journal of Pharmacology, 112(3), 1994, pp. 749-756
1 Autoradiographic studies were conducted to investigate the receptor
subtypes for endothelin-1 (ET-1) that were present in the ovine respir
atory tract. In addition, the receptor subtypes mediating contraction
of airway smooth muscle and the possible involvement of extracellular
Ca2+ and inositol phosphate generation in intracellular signal transdu
ction were assessed. 2 Specific [I-125]-ET-1 binding in ovine trachea
increased in a time- and concentration-dependent manner. Autoradiograp
hic studies demonstrated that significant binding was associated with
airway smooth muscle, although higher densities of specific binding we
re associated with submucosal glands and with cells immediately below
the epithelial basement membrane (lamina propria). The ET(A) receptor-
selective antagonist, BQ 123 (1 mu M), virtually abolished specific bi
nding to airway smooth muscle. Quantitative analyses of autoradiograph
ic data describing the time-dependence of specific [I-125]-ET-1 bindin
g in ovine airway smooth muscle in the presence and absence of BQ 123
or sarafotoxin S6c, revealed a homogeneous population of ET(A) recepto
rs. BQ 123 (1 mu M) also abolished specific binding to structures asso
ciated with submucosal glands, whereas the ET(B) receptor selective ag
onist, sarafotoxin S6c (100 nM) had little effect on this binding, ind
icating the predominance of ET(A) receptors at these sites. In contras
t, ET(B) receptors predominated in the lamina propria, since sarafotox
in S6c abolished specific binding in this tissue. 3 High levels of spe
cific [I-125]-ET-1 binding were also detected in the alveoli and in th
e walls of blood vessels and small airways in ovine peripheral lung. S
pecific binding associated with alveoli was reduced to similar extents
by BQ 123 (1 mu M; 54%) and sarafotoxin S6c (100 nM; 40%), suggesting
the coexistence of both ET(A) and ET(B) receptors in approximately eq
ual proportions in this tissue. In contrast, specific binding to blood
vessels and to peripheral bronchial smooth muscle was abolished in th
e presence of BQ 123 (1 mu M), but was unaffected by sarafotoxin S6c,
indicating the presence of only ET(A) receptors at these sites. 4 ET-1
caused concentration-dependent contractions of ovine tracheal smooth
muscle which were inhibited in the presence of BQ 123 (1 mu M). ET-1 a
lso caused concentration-dependent contraction of ovine lung parenchym
a strips. In contrast, the ET(B) receptor-selective agonists, sarafoto
xin S6c and BQ 3020, were virtually inactive as spasmogens in both tra
cheal smooth muscle and lung strip preparations. Thus contraction was
mediated by ET(A) receptors in ovine tracheal smooth muscle and this i
s consistent with binding and autoradiographic data demonstrating a ho
mogeneous population of these binding sites in this tissue. Contractio
n of parenchymal lung strip preparations to ET-1 was mediated via non-
ET(B) receptors, presumably ET(A) receptors, with contributions to thi
s response perhaps coming from airway and vascular smooth muscle and f
rom alveolar wall contractile cells. 5 ET-1-induced contraction of tra
cheal smooth muscle was not Significantly altered in the presence of i
ndomethacin (5 mu M), indicating that cyclo-oxygenase metabolites of a
rachidonic acid were not involved in this response. Contraction induce
d by ET-1 was virtually abolished in Ca2+-free medium containing 0.1 m
M EGTA, indicating that this response was dependent upon the influx of
extracellular Ca2+. Contraction was inhibited by about 50% in the pre
sence of nicardipine (1 mu M), indicating that a significant component
of this response was mediated via the activation of L-type Ca2+ chann
els.