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

Vasodilator responses to anandamide (arachi-donylethanolamide) and potassiu m ions were compared with those mediated by endothelium-derived hyperpolari zing factor (EDHF) in guinea-pig isolated basilar artery contracted with pr ostaglandin F-2 alpha. In this artery, EDHF-mediated responses can be evoke d by acetylcholine in the presence of both indomethacin (10 mu M) and N-G-n itro-L-arginine (0.3 mM). In endothelium-denuded arterial segments, which f ailed to respond to acetylcholine, anandamide was still able to evoke a com plete relaxation. Anandamide (10 mu M) did not affect the resting membrane potential, whereas acetylcholine (10 mu M) hyperpolarized the smooth muscle cells by 23 mV in the presence of indomethacin and NG-nitro-L-arginine. Pr e-treatment with capsaicin (10 mu M) or resiniferatoxin (0.1 mu M) abolishe d the anandamide-induced relaxation, but had no effect on the EDHF-mediated relaxation induced by acetylcholine. Treatment with a mixture of the calci um-sensitive potassium channel inhibitors, apamin and charybdotoxin, which abolishes EDHF-mediated relaxation in this artery, did not affect the relax ation evoked by anandamide. The additional presence of glibenclamide or cic lazindol, inhibitors of ATP-sensitive and voltage-dependent potassium chann els, also had no effect on the anandamide-induced relaxation. Increasing th e potassium ion concentration by 2-10 mM induced inconsistent vasodilator r esponses. However, re-admission of potassium ions to preparations incubated in potassium-free solution elicited almost complete and sustained relaxati ons. A short incubation period with ouabain (10 mu M for 10 min) or cooling (18-22 degrees C) abolished these responses, whereas the acetylcholine-ind uced relaxation in the presence of indomethacin and N-G-nitro-L-arginine wa s unaffected (ouabain) or partially reduced (cooling). The anandamide-induc ed relaxation was also abolished by ouabain and cooling. Furthermore, ouaba in inhibited the vasodilator response to capsaicin, bur not that to calcito nin gene-related peptide (CGRP), and per se evoked a release of CGRP from t he artery. The gap junction uncoupler, 18 alpha-glycyrrhetinic acid (100 mu M), 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 indic ate that EDHF is neither anandamide nor potassium ions in this artery.