MECHANISM OF CONTRACTION INDUCED BY BRADYKININ IN THE RABBIT SAPHENOUS-VEIN

1 By using fura-PE3 fluorometry and receptor-coupled permeabilization by alpha-toxin, the mechanism of the bradykinin (BK)-induced contracti on was determined in the rabbit saphenous vein (RSV). The receptor sub type responsible for the BK-induced contraction of RSV was determined by means of a pharmacological blocker study and reverse transcription polymerase chain reaction (RT-PCR). 2 In the presence of extracellular Ca2+ (1.25 mM), BK (10(-11)-3 x 10(-7) M) induced increases in both t he cytosolic Ca2+ concentration ([Ca2+](i)) and force, in a concentrat ion-dependent manner. Both the release of Ca2+ from the store site and the influx of extracellular Ca2+ contribute to an increase in [Ca2+]( i) induced by BK. 3 In the absence of extracellular Ca2+, the applicat ion of 10(-7) M BK induced transient elevations of [Ca2+](i) and force , both of which thereafter declined to the levels observed before the application of BK. When extracellular Ca2+ was replenished (1.25 mM), [Ca2+](i) and force increased to form a peak, followed by a sustained elevation in the presence of BK. When an RSV strip was pretreated with 10(-5) M thapsigargin for 20 min, the BK-induced transient increases in both [Ca2+](i) and force were markedly inhibited. 4 These responses induced by BK were inhibited by Hoe 140 (D-Arg-[Hyp(3), Thi(5), D-Tic (7), Oic(8)] bradykinin), a highly specific bradykinin B-2 receptor an tagonist, in a concentration-dependent manner. In RT-PCR, B-2-receptor mRNA was expressed in the smooth muscle of RSV. 5 The [Ca2+](i)-force relationships, which were determined by cumulative applications of ex tracellular Ca2+ (0-5 mM) during 118 mM K+-depolarization, shifted to the upper left in the presence of BK, thus indicating that BK induced a greater force than 118 mM K+-polarization for a given level of [Ca2](i). 6 In alpha-toxin-permeabilized preparations of RSV, application of 10(-7) M BK after a steady state contraction had been induced by a mixture of 5 x 10(-7) M Ca2+, 10(-6) M GTP and 10(-6) M captopril caus ed an additional force development at a constant [Ca2+](i). However, t reatment with 1 mM guanosine-5'-O-(beta-thiodiphosphate) (GDP beta S) for 5 min before and during the application of BK (10(-7) M), abolishe d this BK-induced additional contraction. 7 These results indicated th at in RSV: (1) BK elicits vasoconstriction by increasing the Ca2+ infl ux from the extracellular space, Ca2+ release from intracellular thaps igargin-sensitive storage sites and increasing the Ca2+ sensitivity of the contractile apparatus, (2) the BK-induced increase in Ca2+ sensit ivity is mediated by G-protein, (3) the BK-induced contractions are me diated via B-2-receptors and (4) the smooth muscle cells express B-2-r eceptor mRNA.