Ca2+‐dependent and ‐independent mechanism of cyclic‐AMP reduction: mediation by bradykinin B2 receptors

1 Bradykinin caused a transient reduction of about 25% in the cyclic AMP level in forskolin prestimulated DDT1 MF-2 smooth muscle cells (IC50: 36.4 +/- 4.9 nM) and a pronounced, sustained inhibition (40%) of the isoprenaline-stimulated cyclic AMP level (IC50: 37.5 +/- 1.1 nM).2 The Ca2+ ionophore, ionomycin, mimicked both the bradykinin-induced transient reduction in the forskolin-stimulated cyclic AMP level and the sustained reduction in the isoprenaline-stimulated cyclic AMP level.3 The Ca2+-dependent effect on cyclic AMP induced by bradykinin was mediated solely by Ca2+ release from internal stores, since inhibition of Ca2+ entry with LaCl3 did not reduce the response to bradykinin.4 The involvement of calmodulin-dependent enzyme activities, protein kinase C or an inhibitory GTP binding protein in the bradykinin-induced responses was excluded since a calmodulin inhibitor, calmidazolium, a PKC inhibitor, staurosporine and pertussis toxin, respectively did not affect the decline in the cyclic AMP level.5 Bradykinin enhanced the rate of cyclic AMP breakdown in intact cells, which effect was not mimicked by ionomycin. This suggested a Ca2+-independent activation of phosphodiesterase activity by bradykinin in DDT1 MF-2 cells.6 The bradykinin B-1 receptor agonist, desArg(9)-bradykinin, did not affect cyclic AMP formation in isoprenaline prestimulated cells, while the bradykinin Bz receptor antagonists, Hoe 140 (D-Arg[Hyp(3), Thi(5), D-Tic(7), Oic(8)]-BK) and D-Arg[Hyp(3), Thi(5,8), D-Phe(7)]-BK completely abolished the bradykinin response in both forskolin and isoprenaline prestimulated cells.7 Bradykinin caused an increase in intracellular Ca2+, which was antagonized by the bradykinin B-2 receptor antagonists, Hoe 140 and D-Arg[Hyp(3), Thi(5,8), D-Phe(7)]-BK. The bradykinin B-2 receptor agonist, desArg(9)-bradykinin, did not evoke a rise in cytoplasmic Ca2+.8 It is concluded, that stimulation of bradykinin B-2 receptors causes a reduction in cellular cyclic AMP in DDT1 MF-2 cells. This decline in cyclic AMP is partly mediated by a Ca2+/calmodulin independent activation of phosphodiesterase activity. The increase in [Ca2+](i) mediated by bradykinin B-2 receptors inhibited forskolin- and isoprenaline-activated adenylyl cyclase differently, most likely by interfering with different components of the adenylyl cyclase signalling pathway.