Cardioprotective Effect of Histamine H 3 -Receptor Activation: Pivotal Role of G␤␥-Dependent Inhibition of Voltage-Operated Ca 2ϩ Channels

ABSTRACT We previously showed that activation of G i/o -coupled histamine H 3 -receptors (H 3 R) is cardioprotective because it attenuates excessive norepinephrine release from cardiac sympathetic nerves. This action is characterized by a marked decrease in intraneuronal Ca 2ϩ ([Ca 2ϩ ] i ), as G␣ i impairs the adenylyl cyclasecAMP-protein kinase A (PKA) pathway, and this decreases Ca 2ϩ influx via voltage-operated Ca 2ϩ channels (VOCC). Yet, the G i/oderived ␤␥ dimer could directly inhibit VOCC, and the subsequent reduction in Ca 2ϩ influx would be responsible for the H 3 Rmediated attenuation of transmitter exocytosis. In this study, we tested this hypothesis in nerve-growth factor-differentiated rat pheochromocytoma cells (PC12) stably transfected with H 3 R (PC12-H 3 ) and with the G␤␥ scavenger ␤-adrenergic receptor kinase 1 (␤-ARK1)-(495-689)-polypeptide (PC12-H 3 /␤-ARK1). Thus, we evaluated the effects of H 3 R activation directly on the following: 1) Ca 2ϩ current (I Ca ) using the whole-cell patchclamp technique; and 2) K ϩ -induced exocytosis of endogenous dopamine. H 3 R activation attenuated both peak I Ca and dopamine exocytosis in PC12-H 3 but not in PC12-H 3 /␤-ARK1 cells. Moreover, a membrane permeable phosducin-like G␤␥ scavenger also prevented the antiexocytotic effect of H 3 R activation. In contrast, the H 3 R-induced attenuation of cAMP accumulation and dopamine exocytosis in response to forskolin were the same in both PC12-H 3 and PC12-H 3 /␤-ARK1 cells. Our findings reveal that although G␣ i participates in the H 3 -mediated antiexocytotic 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 H 3 R-mediated decrease in [Ca 2ϩ ] i and associated cardioprotective antiexocytotic effects. The discovery of this H 3 R-signaling step may offer new therapeutic approaches to cardiovascular diseases characterized by hyperadrenergic activity