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Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226
Address all correspondence and requests for reprints to: William B. Campbell, Ph.D., Department of Pharmacology and Toxicology, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, Wisconsin 53226. E-mail: wbcamp{at}mcw.edu.
The present study characterizes the vascular responses of isolated small bovine adrenal cortical arteries to acetylcholine, an endogenous neurotransmitter in the adrenal gland. Acetylcholine (10–10 to 10–6 M) elicited a concentration-dependent relaxation, with a maximal relaxation of 96 ± 1% and EC50 of 4.2 nM. The relaxation was abolished by endothelial removal and attenuated by the nitric oxide synthase inhibitor N-nitro-L-arginine (L-NA, 30 µM) but not by the cyclooxygenase inhibitor indomethacin (10 µM). The maximal relaxation and EC50 of acetylcholine in the presence of L-NA were 87 ± 4% and 22 nM, respectively. The acetylcholine-induced, indomethacin- and L-NA-resistant relaxation was eliminated by high K+ and markedly inhibited by the cytochrome P450 inhibitors SKF 525A (10 µM) and miconazole (10 µM). The maximal relaxations and EC50s with SKF 525A and miconazole were 56 ± 8 and 72 ± 2% and 0.8 and 0.5 µM, respectively. In indomethacin- and L-NA-treated arteries, acetylcholine induced a smooth muscle hyperpolarization, which was blocked by SKF 525A (3 ± 1 mV vs. 15 ± 2 mV of control). Arachidonic acid (10–9 to 10–5 M) and 14,15-epoxyeicosatrienoic acid (14,15-EET, 10–9 to 10–5 M), a cytochrome P450 metabolite of arachidonic acid, also evoked relaxations in small adrenal arteries, with maximal relaxations of 56 ± 4 and 90 ± 5%, respectively. The arachidonic acid-induced relaxation was blocked by SKF 525A. Using high-pressure liquid chromatography and gas chromatography/mass spectrometry analysis, EETs were identified in small adrenal arteries. These results demonstrate that acetylcholine is a potent vasodilator of small adrenal cortical arteries. The acetylcholine-induced relaxation is largely mediated by an endothelium-dependent hyperpolarization mechanism, presumably through cytochrome P450 metabolites of arachidonic acid.
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