Effects of calcium modulators on vagally-mediated constriction in the guinea-pig isolated trachea

1. The effects of calcium modulators on tracheal constriction evoked by vagal stimulation were examined in the isolated, innervated trachea of the guinea-pig. Responses were assessed in the Krebs-filled trachea as changes in intraluminal pressure (ILP), increases and decreases reflecting constriction and dilatation, respectively. 2. Preparations had a positive resting ILP, indicating significant spontaneous tone. Verapamil and nifedipine reduced baseline ILP, whilst Bay K 8644 had mixed effects. 3. Verapamil and nifedipine attenuated vagal responses in a concentration-dependent manner. At lower concentrations attenuation was due entirely to postjunctional effects but at higher concentrations prejunctional effects may have contributed to attenuation. 4. Verapamil and nifedipine attenuated vagal responses in the absence or presence of flurbiprofen, indicating that their effects are largely independent of the generation of cyclo-oxygenase products. Nifedipine, however, was less effective in reducing responses to low frequency vagal stimulation (up to 5 Hz) when flurbiprofen was present. 5. Bay K 8644 augmented vagal responses, the degree varying widely between preparations. 6. It was concluded that influx of Ca2+ through voltage-operated Ca2+ channels contributes significantly to vagally-mediated tracheal constriction in normal trachea and in trachea where endogenous release of cyclo-oxygenase products is inhibited.