Histamine H 3-receptor signaling in cardiac sympathetic nerves: Identification of a novel MAPK-PLA 2-COX-PGE 2-EP 3R pathway

Abstract

We hypothesized that the histamine H 3-receptor (H 3R)-mediated attenuation of norepinephrine (NE) exocytosis from cardiac sympathetic nerves results not only from a Gα i-mediated inhibition of the adenylyl cyclase-cAMP-PKA pathway, but also from a Gβγ i-mediated activation of the MAPK-PLA 2 cascade, culminating in the formation of an arachidonate metabolite with anti-exocytotic characteristics (e.g., PGE 2). We report that in Langendorff-perfused guinea-pig hearts and isolated sympathetic nerve endings (cardiac synaptosomes), H 3R-mediated attenuation of K +-induced NE exocytosis was prevented by MAPK and PLA 2 inhibitors, and by cyclooxygenase and EP 3-receptor (EP 3R) antagonists. Moreover, H 3R activation resulted in MAPK phosphorylation in H 3R-transfected SH-SY5Y neuroblastoma cells, and in PLA 2 activation and PGE 2 production in cardiac synaptosomes; H 3R-induced MAPK phosphorylation was prevented by an anti-βγ peptide. Synergism between H 3R and EP 3R agonists (i.e., imetit and sulprostone, respectively) suggested that PGE 2 may be a downstream effector of the anti-exocytotic effect of H 3R activation. Furthermore, the anti-exocytotic effect of imetit and sulprostone was potentiated by the N-type Ca 2+-channel antagonist ω-conotoxin GVIA, and prevented by an anti-Gβγ peptide. Our findings imply that an EP 3R Gβγ i-induced decrease in Ca 2+ influx through N-type Ca 2+-channels is involved in the PGE 2/EP 3R-mediated attenuation of NE exocytosis elicited by H 3R activation. Conceivably, activation of the Gβγ i subunit of H 3R and EP 3R may also inhibit Ca 2+ entry directly, independent of MAPK intervention. As heart failure, myocardial ischemia and arrhythmic dysfunction are associated with excessive local NE release, attenuation of NE release by H 3R activation is cardioprotective. Accordingly, this novel H 3R signaling pathway may ultimately bear therapeutic significance in hyper-adrenergic states.