Bradykinin modulates arginine vasopressin-induced calcium influx in vascular myocytes

We investigated direct, endothelium-independent effects of bradykinin on arginine vasopressin-induced calcium influx in vascular smooth muscle cells. We studied cultured rat vascular smooth muscle cells by using the whole-cell voltage-clamp and calcium fluorescence imaging methods. Exposing cultured vascular smooth muscle cells (A7r5 cell line) to arginine vasopressin (100 nM) produced a transient increase in [Ca²⁺](i), followed by a sustained increase in [Ca²⁺](i). This was readily reversible (n=28). At a holding potential of -40 to -60 mV, arginine vasopressin induced a sustained inward current correlated with a sustained increase in [Ca²⁺](i). Bradykinin (30 nM to 30 μM) had no effect on arginine vasopressin-induced [Ca²⁺](i) transients. However, during the sustained phase of increased [Ca²⁺](i), bradykinin reversibly attenuated relative fluorescence and inward current in the presence of arginine vasopressin (n=14). This was concentration dependent and inhibited by [D-Phe⁷]-bradykinin (30 μM), a kinin receptor antagonist. Also, sustained arginine vasopressin-mediated increases in [Ca²⁺](i) and inward current were attenuated by Ca²⁺-free or La³⁺-supplemented perfusate but not by nifedipine (n=5). Conclusions: (1) Bradykinin can attenuate arginine vasopressin-induced and sustained Ca²⁺ influx and sustained inward current through a novel endothelium-independent process. (2) The direct effect of bradykinin on arginine vasopressin-induced increases in [Ca²⁺](i) sustained Ca²⁺ influx in vascular smooth muscle cells is concentration dependent and kinin-receptor mediated. (3) Arginine vasopressin-induced sustained [Ca²⁺](i) elevation correlates with the activation of a dihydropyridine-insensitive, Ca²⁺-conducting inward current.