Pm. Zygmunt et al., INVOLVEMENT OF VOLTAGE-DEPENDENT POTASSIUM CHANNELS IN THE EDHF-MEDIATED RELAXATION OF RAT HEPATIC-ARTERY, British Journal of Pharmacology, 121(1), 1997, pp. 141-149
1 In the rat hepatic artery, the acetylcholine-induced relaxation medi
ated by endothelium-derived hyperpolarizing factor (EDHF) is abolished
by a combination of apamin and charybdotoxin, inhibitors of small (SK
Ca) and large (BKCa) conductance calcium-sensitive potassium (K)-chann
els, respectively, but not by each toxin alone. The selective BKCa inh
ibitor iberiotoxin cannot replace charybdotoxin in this combination. S
ince delayed rectifier K-channels (K-V) represent another target for c
harybdotoxin, we explored the possible involvement of K-V in EDHF-medi
ated relaxation in this artery. 2 The K-V inhibitors, agitoxin-2 (0.3
mu M), kaliotoxin (0.3 mu M), beta-dendrotoxin (0.3 mu M), dofetilde (
1 mu M) and terikalant (10 mu M), each in combination with apamin (0.3
mu M) had no effect on the EDHF-mediated relaxation induced by acetyl
choline in the presence of N-omega-nitro-L-arginine (0.3 mM) and indom
ethacin (10 mu M), inhibitors of nitric oxide (NO) synthase and cyclo-
oxygenase, respectively (n=2-3). Although the K-V inhibitor margatoxin
(0.3 mu M) was also without effect (n=5), the combination of margatox
in and apamin produced a small inhibition of the response (pEC(50) ant
i E-max values were 7.5+/-0.0 and 95+/-1% in the absence and 7.0+/-0.1
and 81+/-6% in the presence of margatoxin plus apamin, respectively;
n=6; P<0.05). 3 Ciclazindol (10 mu M) partially inhibited the EDHF-med
iated relaxation by shifting the acetylcholine-concentration-response
curve 12 fold to the right (n=6; P<0.05) and abolished the response wh
en combined with apamin (0.3 mu M; n=6). This combination did not inhi
bit acetylcholine-induced relaxations mediated by endothelium-derived
NO (n=5). 4 A 4-aminopyridine-sensitive delayed rectifier current (I-K
(V)) was identified in freshly-isolated single smooth muscle cells fro
m rat hepatic artery. None of the cells displayed a rapidly-activating
and -inactivating A-type current. Neither charybdotoxin (0.3 mu M; n=
3) nor ciclazindol (10 mu M; n=5), alone or in combination with apamin
(0.3 mu M; n=4-5), had an effect on I-K(V). A tenfold higher concentr
ation of ciclazindol (0.1 mM, n=4) markedly inhibited I-K(V), but this
effect was not increased in the additional presence of apamin (0.3 mu
M; n=2). 5 By use of membranes prepared from rat brain cortex, [I-125
]-charybdotoxin binding was consistent with an interaction at a single
site with a K-D of approximately 25 pM. [I-125]-charybdotoxin binding
was unaffected by iberiotoxin (0.1 mu M, n=6), but was increased by a
pamin in a concentration-dependent manner (E-max 43+/-10%, P<0.05 and
pEC(50) 7.1+/-0.2; n=7-8). Agitoxin-2 (10 nM) displaced [I-125]-charyb
dotoxin binding by 91+/-3% (n=6) and prevented the effect of apamin (1
mu M; n=6). 6 It is concluded that the EDHF-mediated relaxation in th
e rat hepatic artery is not mediated by the opening of either K-V or B
KCa. Instead, the target K-channels for EDHF seem to be structurally r
elated to both K-V and BKCa. The possibility that a subtype of SKCa ma
y be the target for EDHF is discussed.