A direct [3H]-bradykinin ([3H]-BK) binding assay has been used to characterize the BK receptors in canine cultured tracheal epithelial cells (TECs). Based on receptor binding assay, TECs have specific, saturable, high-affinity binding sites for [3H]-BK. 2. The specific [3H]-BK binding was time- and temperature-dependent. Equilibrium of association of [3H]-BK with the BK receptors was attained within 30 min at room temperature and 1 h at 4 degrees C, respectively. 3. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 1.5 +/- 0.2 nM and a maximum receptor density (Bmax) of 53.2 +/- 5.2 fmol mg-1 protein. The Hill coefficient for [3H]-BK binding was 1.00 +/- 0.02. The association (K1) and dissociation (K-1) rate constants were (7.6 +/- 1.1) x 10(6) M-1 min-1 and (9.2 +/- 1.5) x 10 M-3 min-1, respectively. KD, calculated from the ratio of K-1 and K1, was 1.2 +/- 0.3 nM, a value close to that calculated from Scatchard plots of binding isotherms. 4. Neither a B1 receptor selective agonist (des-Arg9-BK, 0.1 nM - 10 microM) nor antagonist ([Leu8, des-Arg9]-BK, 0.1 nM - 10 microM) significantly inhibited [3H]-BK binding to TECs, which excludes the presence of B1 receptors in canine TECs. 5. The specific binding of [3H]-BK to canine TECs was inhibited by the B2 receptor selective antagonists ([D-Arg0, Hyp3, Thi5, D-Tic7, Oic8]-BK (Hoe 140, 0.1 nM-10 microM) and [D-Arg0, Hyp3, Thi5.8, D-Phe7]-BK, 0.1 nM - 10 microM) and agonists (BK and kallidin, 0.1 nM-10 microM) with a best fit by a one-binding site model. The order of potency for the inhibition of [3H]-BK binding was kallidin = BK = Hoe 140 > [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK. 6. BK and kallidin significantly induced concentration-dependent accumulation of IPs with a half-maximal response (EC50) at 17.6 +/- 3.5 and 26.6 +/- 5.3 nM, respectively, while the B1-selective agonist, des-Arg9-BK did not stimulate IPs accumulation and the B1-selective antagonist [Leu8, des-Arg9]-BK did not inhibit BK-induced IPs accumulation. Two B2-selective antagonists, Hoe 140 and [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK, inhibited BK-stimulated IPs accumulation with apparent pKB values of 8.8 +/- 0.3 and 7.0 +/- 0.3, respectively. 7. It is concluded that the pharmacological characteristics of the BK receptors in canine cultured TECs are primarily of the B2 receptor subtype which might regulate the function of tracheal epithelium through the activation of this receptor subtype coupling to PI hydrolysis.
