Fig. 1. Effect of halothane on inositol triphosphate (IP³) concentrations in porcine airway smooth muscle cells. IP³concentrations were measured using radioimmunoreceptor assay. Compared with unstimulated control, IP³concentrations were significantly (P < 0.05) elevated by exposure to acetylcholine, 2 minimum alveolar concentration (MAC) halothane, and a combination thereof. The IP³response to simultaneous application of acetylcholine and halothane was significantly greater than that induced by either agent alone. Inhibition of phospholipase C by U73122 significantly blunted the IP³response to both acetylcholine and halothane, but the effect on halothane was considerably smaller. *Significant (P < 0.05) difference from unstimulated control. †Significant difference from acetylcholine exposure alone. §Significant difference from halothane exposure alone.