Ca2+- and myosin phosphorylation-independent relaxation by halothane in K+-depolarized rat mesenteric arteries.
BACKGROUND
Volatile anesthetics inhibit vascular smooth muscle contraction, but the mechanisms responsible are uncertain. In this study, the effects of halothane on Ca2+ signaling and Ca2+ activation of contractile proteins were examined in high K+-depolarized smooth muscle from rat mesenteric resistance arteries.


METHODS
Vessels were cannulated and held at a constant transmural pressure (40 mmHg). Image analysis and microfluorimetry were used to simultaneously measure vessel diameter and smooth muscle intracellular [Ca2+] concentration ([Ca2+]i). Myosin light chain (MLC) phosphorylation was measured using the Western blotting technique.


RESULTS
Step increases in extracellular [Ca2+] concentration (0-10 mM) in high K+ (40 mM)-depolarized smooth muscle produced incremental increases in [Ca2+]i, MLC phosphorylation, and contraction. Halothane (0.5-4.5%) inhibited contraction in a concentration-dependent manner, but the decrease in [Ca2+]i was small, and there was a marked shift in the [Ca2+]i-contraction relationship to the right, indicating an important Ca2+ desensitizing effect. Halothane (0.5-4.5%) did not affect MLC phosphorylation or the [Ca2+]-MLC phosphorylation relationship, but the MLC phosphorylation-contraction relationship was also shifted rightward, indicating an "MLC phosphorylation" desensitizing effect. In contrast, control relaxations produced by the Ca2+ channel blocker nifedipine were accompanied by decreases in both [Ca2+]i and MLC phosphorylation, and nifedipine had no affect on the [Ca2+]i-contraction, [Ca2+]i-MLC phosphorylation, and MLC phosphorylation-contraction relationships.


CONCLUSIONS
In high K+-depolarized vascular smooth muscle, halothane relaxation is largely mediated by a Ca2+ and MLC phosphorylation desensitizing effect. These results suggest that the relaxing action of halothane is independent of the classic Ca2+-induced myosin phosphorylation contraction mechanism.
Tsuneyoshi
Isao
I
Department of Anesthesiology, Washington University School of Medicine, St. Louis, MO 63110, USA.
Zhang
Dongya
D
Boyle
Walter A
WA
D000465:Algorithms; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D020013:Calcium Signaling / Q000187:drug effects*; D004730:Endothelium, Vascular / Q000166:cytology / Q000502:physiology; D006221:Halothane / Q000494:pharmacology*; D066298:In Vitro Techniques; D008297:Male; D008638:Mesenteric Arteries / Q000187:drug effects; D009126:Muscle Relaxation / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D018994:Myosin Light Chains / Q000502:physiology*; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology*; D051381:Rats; D020318:Rats, Long-Evans
D018685:Anesthetics, Inhalation; D018994:Myosin Light Chains; D011188:Potassium; D006221:Halothane