Cardioprotective Effect of Histamine H₃-Receptor Activation: Pivotal Role of Gβγ-Dependent Inhibition of Voltage-Operated Ca²⁺ Channels

Abstract

We previously showed that activation of G_i/o-coupled histamine H₃-receptors (H₃R) is cardioprotective because it attenuates excessive norepinephrine release from cardiac sympathetic nerves. This action is characterized by a marked decrease in intraneuronal Ca²⁺ ([Ca²⁺]_i), as Gα_i impairs the adenylyl cyclase-cAMP-protein kinase A (PKA) pathway, and this decreases Ca²⁺ influx via voltage-operated Ca²⁺ channels (VOCC). Yet, the G_i/o-derived βγ dimer could directly inhibit VOCC, and the subsequent reduction in Ca²⁺ influx would be responsible for the H₃R-mediated attenuation of transmitter exocytosis. In this study, we tested this hypothesis in nerve-growth factor-differentiated rat pheochromocytoma cells (PC12) stably transfected with H₃R (PC12-H₃) and with the Gβγ scavenger β-adrenergic receptor kinase 1 (β-ARK1)-(495-689)-polypeptide (PC12-H₃/β-ARK1). Thus, we evaluated the effects of H₃R activation directly on the following: 1) Ca²⁺ current (I_Ca) using the whole-cell patch-clamp technique; and 2) K⁺-induced exocytosis of endogenous dopamine. H₃R activation attenuated both peak I_Ca and dopamine exocytosis in PC12-H₃ but not in PC12-H₃/β-ARK1 cells. Moreover, a membrane permeable phosducin-like Gβγ scavenger also prevented the antiexocytotic effect of H₃R activation. In contrast, the H₃R-induced attenuation of cAMP accumulation and dopamine exocytosis in response to forskolin were the same in both PC12-H₃ and PC12-H₃/β-ARK1 cells. Our findings reveal that although Gα_i participates in the H₃-mediated antiexocytotic effect when the adenylyl cyclase-cAMP-PKA pathway is stimulated, a direct Gβγ-induced inhibition of VOCC, resulting in an attenuation of I_Ca, plays a pivotal role in the H₃R-mediated decrease in [Ca²⁺]_i and associated cardioprotective antiexocytotic effects. The discovery of this H₃R-signaling step may offer new therapeutic approaches to cardiovascular diseases characterized by hyperadrenergic activity.