Bradykinin activates a cross-signaling pathway between sensory and adrenergic nerve endings in the heart: A novel mechanism of ischemic norepinephrine release?
We had shown that bradykinin (BK) generated by cardiac sympathetic nerve endings (i.e., synaptosomes) promotes exocytotic norepinephrine (NE) release in an autocrine mode. Because the synaptosomal preparation may include sensory C-fiber endings, which BK is known to stimulate, sensory nerves could contribute to the proadrenergic effects of BK in the heart. We report that BK is a potent releaser of NE from guinea pig heart synaptosomes (EC(50) approximately 20 nM), an effect mediated by B(2) receptors, and almost completely abolished by prior C-fiber destruction or blockade of calcitonin gene-related peptide and neurokinin-1 receptors. C-fiber destruction also greatly decreased BK-induced NE release from the intact heart, whereas tyramine-induced NE release was unaffected. Furthermore, C-fiber stimulation with capsaicin and activation of calcitonin gene-related peptide and neurokinin-1 receptors initiated NE release from cardiac synaptosomes, indicating that stimulation of sensory neurons in turn activates sympathetic nerve terminals. Thus, BK is likely to release NE in the heart in part by first liberating calcitonin gene-related peptide and Substance P from sensory nerve endings; these neuropeptides then stimulate specific receptors on sympathetic terminals. This action of BK is positively modulated by cyclooxygenase products, attenuated by activation of histamine H(3) receptors, and potentiated at a lower pH. The NE-releasing action of BK is likely to be enhanced in myocardial ischemia, when protons accumulate, C fibers become activated, and the production of prostaglandins and BK increases. Because NE is a major arrhythmogenic agent, the activation of this interneuronal signaling system between sensory and adrenergic neurons may contribute to ischemic dysrhythmias and sudden cardiac death.