Bradykinin binding to B2 kinin receptors and stimulation of phosphoinositide turnover and arachidonic acid release in primary cultures of cells from late pregnant rat myometrium

Abstract

Primary cultures of cells from late pregnant rat myometrium contain B2 kinin receptors through which bradykinin (BK) stimulates inositol phosphate (InsP) formation and arachidonic acid (20:4) release. Equilibrium binding at 4 °C revealed that [³H]BK identified a maximal number of cell surface B2 kinin receptor binding sites on rat myometrial cells of 308 ± 78 fmol/10⁶ cells with apparently a single equilibrium dissociation constant of 1.8 ± 0.2 nM. At 37 °C, [³H]BK binding was associated with a time-dependent decrease in the reversibility of the binding. This decrease was due in part to formation of slowly dissociating cell surface receptor [³H]BK binding and in part to internalization of the receptor-bound [³H]BK. Exposure of labeled cells to BK resulted in dose-dependent increases in [³H]InsP₃, [³H]InsP₂ ([³H]Ins(1,4)P₂), and [³H]InsP₁ [([³H]Ins(1)P₁) formation and [³H]20:4 release. Pretreatment with 100 ng/mL pertussis toxin did not perturb BK stimulation of [³H]InsP formation but partially (~30%) inhibited BK stimulation of [³H]20:4 release. BK stimulation of [³H]20:4 release was directly proportional to the number of receptor sites occupied by BK. In contrast, stimulation of [³H]InsP formation required a threshold level of receptor occupancy, which decreased as a function of time of BK exposure. These results show that BK interacts with B2 kinin receptors on rat myometrial cells with apparently a single affinity through which BK stimulates [³H]InsP formation and [³H]20:4 release. BK stimulation of [³H]InsP formation requires a threshold BK concentration, which decreases with time, and we suggest that the decrease is due to a time-dependent formation of a BK receptor binding state from which BK slowly dissociates.Key words: bradykinin, receptors, myometrium, inositol phosphates, arachidonic acid.