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 transmurat 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 [Ca ²⁺]_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.