Role of TRPV1 and intracellular Ca²⁺ in excitation of cardiac sensory neurons by bradykinin

Bradykinin is an important mediator produced during myocardial ischemia and infarction that can activate and/or sensitize cardiac spinal (sympathetic) sensory neurons to trigger chest pain. Because a long-onset latency is associated with the bradykinin effect on cardiac spinal afferents, a cascade of intracellular signaling events is likely involved in the action of bradykinin on cardiac nociceptors. In this study, we determined the signal transduction mechanisms involved in bradykinin stimulation of cardiac nociceptors. Cardiac dorsal root ganglion (DRG) neurons in rats were labeled by intracardiac injection of a fluorescent tracer, 1,1′-dioctadecyl-3,3,3′,3′- tetramethylindocarbocyanine percholate (DiI). Whole cell current-clamp recordings were performed in acutely isolated DRG neurons. In DiI-labeled DRG neurons, 1 μM bradykinin significantly increased the firing frequency and lowered the membrane potential. Iodoresiniferatoxin, a highly specific transient receptor potential vanilloid type 1 (TRPV1) antagonist, significantly reduced the excitatory effect of bradykinin. Furthermore, the stimulating effect of bradykinin on DiI-labeled DRG neurons was significantly attenuated by baicalein (a selective inhibitor of 12-lipoxygenase) or 2-aminoethyl diphenylborinate [an inositol 1,4,5-trisphosphate (IP₃) antagonist]. In addition, the effect of bradykinin on cardiac DRG neurons was abolished after the neurons were treated with BAPTA-AM or thapsigargin (to deplete intracellular Ca²⁺ stores) but not in the Ca²⁺-free extracellular solution. Collectively, these findings provide new evidence that 12-lipoxygenase products, IP₃, and TRPV1 channels contribute importantly to excitation of cardiac nociceptors by bradykinin. Activation of TRPV1 and the increase in the intracellular Ca²⁺ are critically involved in activation/ sensitization of cardiac nociceptors by bradykinin.