In vitro relaxation of vascular smooth muscle by atropine: involvement of K+ channels and endothelium.
Cumulative addition of atropine to the organ bath containing endothelium-intact (+E) rat aorta, which was precontracted with phenylephrine (PE, 1 microM) and subsequently relaxed with carbachol (1 microM), caused biphasic changes in the vascular contractility of +E rat aortic rings. Low concentrations of atropine (10 nM-1.0 microM) caused progressive restoration of contraction to PE; whereas at higher concentrations (1-100 microM), atropine caused progressive relaxation. Atropine-induced aortic relaxation was significantly inhibited upon endothelium removal by either rubbing or saponin treatment, but considerable relaxation still persisted in the range of 30-100 microM atropine. Similar findings were also obtained when the nitric oxide (NO) generation was inhibited with 300 microM NO synthase inhibitor, L-NAME. Atropine-induced relaxation was also observed when 5-hydroxytryptamine (5-HT) was used as the agonist and the atropine-relaxation was more potent at lower concentrations of PE and 5-HT. However, atropine had no effect on the contraction elicited by KCl or prostaglandin F(2 alpha). Also, atropine-induced relaxation was not affected by indomethacin (1-10 microM), nicotine (10-100 microM) or hexamethonium (30 microM). Pretreatment of +E aorta with tetraethylammonia (TEA, 3-10 mM) or 4-aminopyridine (4-AP, 1-3 mM) showed prominent inhibitory effect on atropine-induced relaxation; on the other hand, preincubation with glibenclamide (1-10 microM), BaCl(2) (1-30 microM) or 2 microM charybdotoxin and apamin, had little effect on the relaxation induced by atropine. When added to tissues after relaxation to atropine, TEA and 4-AP concentration-dependently reversed the relaxation in -E aorta, whereas in +E aorta, TEA up to 30 mM and 4-AP up to 10 mM only partially affected atropine-induced relaxation. Although TEA and 4-AP potentiated the PE-contraction, such potentiation is unlikely to contribute to the change in sensitivity to atropine-induced relaxation, since in the presence of 15 mM KCl, which also potentiated PE-contraction to a comparable extent, the atropine-relaxation remains unchanged. Scopolamine also acts like atropine, except that the effect of scopolamine was smaller than that of atropine and is primarily endothelium-dependent. Atropine-induced relaxation also occurs in medium artery (renal artery) and small muscular artery (mesenteric artery). In conclusion, atropine-relaxation is mediated in part via voltage-dependent K(+) channels in both smooth muscle and endothelium and forms the mechanistic basis for the observed vasodilation, reduced blood pressure and facial flushing following atropine overdose.
Kwan
Chiu-Yin
CY
Department of Medicine, Faculty of Health Sciences, McMaster University, Hamilton, Ontario, Canada. kwancy@mcmaster.ca
Zhang
Wen-Bo
WB
Kwan
Tony K
TK
Sakai
Yasushi
Y
D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects / Q000502:physiology; D001285:Atropine / Q000494:pharmacology*; D004730:Endothelium, Vascular / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D008638:Mesenteric Arteries / Q000187:drug effects / Q000502:physiology; D018727:Muscarinic Antagonists / Q000494:pharmacology*; D009119:Muscle Contraction / Q000187:drug effects; D009126:Muscle Relaxation / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000502:physiology; D009569:Nitric Oxide / Q000502:physiology; D026902:Potassium Channel Blockers / Q000494:pharmacology; D015221:Potassium Channels / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012077:Renal Artery / Q000187:drug effects / Q000502:physiology
D018727:Muscarinic Antagonists; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D009569:Nitric Oxide; D001285:Atropine