Differential actions of anandamide, potassium ions and endothelium-derived hyperpolarizing factor in guinea-pig basilar artery

Abstract.

Vasodilator responses to anandamide (arachidonylethanolamide) and potassium ions were compared with those mediated by endothelium-derived hyperpolarizing factor (EDHF) in guinea-pig isolated basilar artery contracted with prostaglandin F_{2 α}. In this artery, EDHF-mediated responses can be evoked by acetylcholine in the presence of both indomethacin (10 µM) and N ^G-nitro-l-arginine (0.3 mM). In endothelium-denuded arterial segments, which failed to respond to acetylcholine, anandamide was still able to evoke a complete relaxation. Anandamide (10 µM) did not affect the resting membrane potential, whereas acetylcholine (10 µM) hyperpolarized the smooth muscle cells by 23 mV in the presence of indomethacin and N ^G-nitro-l-arginine. Pre-treatment with capsaicin (10 µM) or resiniferatoxin (0.1 µM) abolished the anandamide-induced relaxation, but had no effect on the EDHF-mediated relaxation induced by acetylcholine. Treatment with a mixture of the calcium-sensitive potassium channel inhibitors, apamin and charybdotoxin, which abolishes EDHF-mediated relaxation in this artery, did not affect the relaxation evoked by anandamide. The additional presence of glibenclamide or ciclazindol, inhibitors of ATP-sensitive and voltage-dependent potassium channels, also had no effect on the anandamide-induced relaxation. Increasing the potassium ion concentration by 2–10 mM induced inconsistent vasodilator responses. However, re-admission of potassium ions to preparations incubated in potassium-free solution elicited almost complete and sustained relaxations. A short incubation period with ouabain (10 µM for 10 min) or cooling (18–22°C) abolished these responses, whereas the acetylcholine-induced relaxation in the presence of indomethacin and N ^G-nitro-l-arginine was unaffected (ouabain) or partially reduced (cooling). The anandamide-induced relaxation was also abolished by ouabain and cooling. Furthermore, ouabain inhibited the vasodilator response to capsaicin, but not that to calcitonin gene-related peptide (CGRP), and per se evoked a release of CGRP from the artery. The gap junction uncoupler, 18α-glycyrrhetinic acid (100 µM), affected neither the EDHF-mediated relaxation induced by acetylcholine nor the vasodilator responses to anandamide and potassium ions. Thus, EDHF-mediated vasorelaxation in the guinea-pig basilar artery does not seem to involve Na⁺/K⁺-ATPase, sensory nerves or gap junctions. These results indicate that EDHF is neither anandamide nor potassium ions in this artery.