Fig. 6. Relaxation of a substance P–induced contraction is enhanced by the selective positive allosteric effect of propofol on the γ-aminobutyric acid subtype A channel (GABAA). ( A ) Cumulatively increasing concentration response curves expressed as a percent of initial force after a substance P contraction for propofol ( squares ), dimethyl sulfoxide (DMSO) vehicle ( down triangles ), or time control ( up triangles ). Relaxation of guinea pig tracheal rings contracted with 1 μm substance P was significantly enhanced by propofol (≥ 20 μm). * P < 0.05 compared to time control; *** P < 0.001 compared to time control. ( B ) Cumulatively increasing concentration response curves expressed as a percent of initial force after a substance P contraction for propofol ( squares ) or with 5 μm gabazine pretreatment ( up triangles ) before propofol. Partial but significant reversal of propofol-induced relaxation occurred with low-dose gabazine (5 μm) pretreatment (at 50 μm and 100 μm propofol; ** P < 0.01 and *** P < 0.001, respectively). Gabazine alone ( diamonds ) was not different from untreated time controls ( down triangles ).