Abstract

1Single bovine tracheal smooth muscle (BTSM) cells were cultured and used to measure bradykinin-induced changes in [Ca2+]i by dynamic video imaging.
2Bradykinin (10 pm-10 μm)-induced an increase in [Ca2+]i over basal levels (69 ± 2 nm; n = 353) which was concentration-dependent (log EC50 = −8.7 m) in the presence of extracellular calcium ions (2 mm). The bradykinin B2 receptor antagonist, d-Arg[Hyp3,Thi5,8,d-Phe7]-bradykinin, produced a parallel shift to the right of the bradykinin concentration-response curve (log EC50 = −7.1 m and −5.8 m in the presence of 1 μm and 10 μm antagonist respectively) yielding an apparent KD of 26 nm.
3In the absence of extracellular calcium ions (with 0.1 mm EGTA), bradykinin (10 pm– 10 μm) produced a uniform increase in [Ca2+]i from a basal level of 33 ± 2 nm (n = 140) to approximately 180 nm in BTSM cells indicating an ‘all-or-nothing’ release of intracellular calcium ions. In the presence of 10 μm d-Arg[Hyp3,Thi5,8,d-Phe7]-bradykinin no responses could be induced by bradykinin at concentrations below 100 nm. However, at 100 nm and 1 μm bradykinin there was no change in the uniform increase in [Ca2+]i in these cells previously observed.
4In both the absence or presence of d-Arg[Hyp3,Thi5,8,d-Phe7]-bradykinin, there was a concentration-dependent increase in the percentage of cells responding to bradykinin (frequency) under calcium-rich or calcium-free conditions. Individual cells also demonstrated a difference in the sensitivity to any particular concentration of bradykinin.
5A latency in the response of cells to bradykinin was observed both in calcium-containing and calcium-free conditions.
6We conclude that bradykinin B2 receptors are expressed by BTSM cells and are involved in the bradykinin-induced increase in [Ca2+]i. It appears that the increase in [Ca2+]i can be mediated via a graded influx of calcium ions from the extracellular space or an ‘all-or-nothing’ release from intracellular stores.