Received for publication February 11, 2008.
Revised May 29, 2008.
Accepted for publication June 2, 2008.

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

Abstract

We previously showed that activation of G_i/o-coupled histamine H₃-receptors (H₃R) is cardioprotective since it attenuates excessive norepinephrine release from cardiac sympathetic nerves. This action is characterized by a marked decrease in intraneuronal Ca²⁺ ([Ca²⁺]_i), as G_i impairs the adenylyl cyclase-cAMP-PKA pathway, and this decreases Ca²⁺ influx via voltage-operated Ca²⁺ channels (VOCC). Yet, the G_i/o-derived dimer could directly inhibit VOCC, and the subsequent reduction in Ca²⁺ influx would be responsible for the H₃R-mediated attenuation of transmitter exocytosis. Here, we tested this hypothesis in nerve-growth factor-differentiated rat pheochromocytoma cells (PC12) stably transfected with H₃R (PC12-H₃) and with the G scavenger -ARK1-(495-689)-polypeptide (PC12-H₃/-ARK1). Thus, we evaluated the effects of H₃R activation directly on: 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/-ARK1 cells. Moreover, a membrane permeable phosducin-like G scavenger also prevented the anti-exocytotic 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/-ARK1 cells. Our findings reveal that while G_i participates in the H₃-mediated anti-exocytotic effect when the adenylyl cyclase-cAMP-PKA pathway is stimulated, a direct G-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 anti-exocytotic effects. The discovery of this H₃R signaling step may offer new therapeutic approaches to cardiovascular diseases characterized by hyperadrenergic activity.

Key words: Gbetagamma dimer, cardiac sympathetic nerve endings, cardioprotection, histamine H3 receptors, norepinephrine exocytosis, voltage-operated calcium channels