Nicotine Diminishes Alpha2-Adrenergic Receptor-Dependent Protection Against Oxidative Stress in H9c2 Cardiomyocytes.

Introduction: Nicotine is a major component of cigarette smoke with various detrimental cardiovascular effects, including increased oxidative stress in the heart. Agonism of α-adrenergic receptors (ARs), such as with dexmedetomidine, has been documented to exert cardioprotective effects against oxidative stress and related apoptosis and necroptosis. α-ARs are membrane-residing G protein-coupled receptors (GPCRs) that primarily activate Gi/o proteins. They are also subjected to GPCR-kinase (GRK)-2-dependent desensitization, which entails phosphorylation of the agonist-activated receptor by GRK2 to induce its decoupling from G proteins, thus terminating αAR-mediated G protein signaling.

Objective: In the present study, we sought to examine the effects of nicotine on αAR signaling and effects in H9c2 cardiomyocytes exposed to HO to induce oxidative cellular damage.

Methods And Results: As expected, treatment of H9c2 cardiomyocytes with HO significantly decreased cell viability and increased oxidative stress, as assessed by reactive oxygen species (ROS)-associated fluorescence levels (DCF assay) and superoxide dismutase activity. Both HO effects were partly rescued by αAR activation with brimonidine in control cardiomyocytes but not in cells pretreated with nicotine for 24 hours, in which brimonidine was unable to reduce HO-induced cell death and oxidative stress. This was due to severe αAR desensitization, manifested as very low Gi protein activation by brimonidine, and accompanied by GRK2 upregulation in nicotine-treated cardiomyocytes. Finally, pharmacological inhibition of adenylyl cyclase (AC) blocked HO-dependent oxidative damage in nicotine-pretreated H9c2 cardiomyocytes, indicating that αAR activation protects against oxidative injury via its classic coupling to Gai-mediated AC inhibition.

Discussion/conclusions: Nicotine can negate the cardioprotective effects of αAR agonists against oxidative injury, which may have important implications for patients treated with this class of drugs that are chronic tobacco smokers.