Decreased intracellular calcium mediates the histamine H₃-receptor-induced attenuation of norepinephrine exocytosis from cardiac sympathetic nerve endings

Abstract

Activation of presynatic histamine H₃ receptors (H₃R) down-regulates norepinephrine exocytosis from cardiac sympathetic nerve terminals, in both normal and ischemic conditions. Analogous to the effects of α₂-adrenoceptors, which also act prejunctionally to inhibit norepinephrine release, H₃R-mediated antiexocytotic effects could result from a decreased Ca²⁺ influx into nerve endings. We tested this hypothesis in sympathetic nerve terminals isolated from guinea pig heart (cardiac synaptosomes) and in a model human neuronal cell line (SH-SY5Y), which we stably transfected with human H₃R cDNA (SH-SY5Y-H₃). We found that reducing Ca²⁺ influx in response to membrane depolarization by inhibiting N-type Ca²⁺ channels with ω-conotoxin (ω-CTX) greatly attenuated the exocytosis of [³H]norepinephrine from both SH-SY5Y and SH-SY5Y-H₃ cells, as well as the exocytosis of endogenous norepinephrine from cardiac synaptosomes. Similar to ω-CTX, activation of H₃R with the selective H₃R-agonist imetit also reduced both the rise in intracellular Ca²⁺ concentration (Ca_i) and norepinephrine exocytosis in response to membrane depolarization. The selective H₃R antagonist thioperamide prevented this effect of imetit. In the parent SH-SY5Y cells lacking H₃R, imetit affected neither the rise in Ca_i nor [³H]norepinephrine exocytosis, demonstrating that the presence of H₃R is a prerequisite for a decrease in Ca_i in response to imetit and that H₃R activation modulates norepinephrine exocytosis by limiting the magnitude of the increase in Ca_i. Inasmuch as excessive norepinephrine exocytosis is a leading cause of cardiac dysfunction and arrhythmias during acute myocardial ischemia, attenuation of norepinephrine release by H₃R agonists may offer a novel therapeutic approach to this condition.