Involvement of voltage-dependent potassium channels in the EDHF-mediated relaxation of rat hepatic artery

Abstract

1. In the rat hepatic artery, the acetylcholine-induced relaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) is abolished by a combination of apamin and charybdotoxin, inhibitors of small (SK(Ca)) and large (BK(Ca)) conductance calcium-sensitive potassium (K)-channels, respectively, but not by each toxin alone. The selective BK(Ca) inhibitor iberiotoxin cannot replace charybdotoxin in this combination. Since delayed rectifier K-channels (K(V)) represent another target for charybdotoxin, we explored the possible involvement of K(V) in EDHF-mediated relaxation in this artery. 2. The K(V) inhibitors, agitoxin-2 (0.3 μM), kaliotoxin (0.3 μM), β-dendrotoxin (0.3 μM), dofetilide (1 μM) and terikalant (10 μM), each in combination with apamin (0.3 μM) had no effect on the EDHF-mediated relaxation induced by acetylcholine in the presence of N(ω)-nitro-L-arginine (0.3 mM) and indomethacin (10 μM), inhibitors of nitric oxide (NO) synthase and cyclo-oxygenase, respectively (n = 2-3). Although the K(V) inhibitor margatoxin (0.3 μM) was also without effect (n = 5), the combination of margatoxin and apamin produced a small inhibition of the response (pEC50 and 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 μM) partially inhibited the EDHF-mediated relaxation by shifting the acetylcholine-concentration-response curve 12 fold to the right (n =.6; P < 0.05) and abolished the response when combined with apamin (0.3 μM; n = 6). This combination did not inhibit 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 from rat hepatic artery. None of the cells displayed a rapidly-activating and -inactivating A-type current. Neither charybdotoxin (0.3 μM; n = 3) nor ciclazindol (10 μM; n = 5), alone or in combination with apamin (0.3 μM; n = 4-5), had an effect on I(K(V)). A tenfold higher concentration 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 μM; n = 2). 5. By use of membranes prepared from rat brain cortex, [125I]-charybdotoxin binding was consistent with an interaction at a single site with a K(D) of approximately 25 pM. [125I]-charybdotoxin binding was unaffected by iberiotoxin (0.1 μM, n = 6), but was increased by apamin in a concentration-dependent manner (E(max) 43 ± 10%, P < 0.05 and pEC50 7.1 ± 0.2; n = 7-8). Agitoxin-2 (10 nM) displaced [125I]-charybdotoxin binding by 91 ± 3% (n = 6) and prevented the effect of apamin (1 μM; n = 6). 6. It is concluded that the EDHF-mediated relaxation in the rat hepatic artery is not mediated by the opening of either K(V) or BK(Ca). Instead, the target K-channels for EDHF seem to be structurally related to both K(V) and BK(Ca). The possibility that a subtype of SK(Ca) may be the target for EDHF is discussed.