Disruption of inhibitory G-proteins mediates a reduction in atrial β-adrenergic signaling by enhancing eNOS expression

Objective: Cardiac parasympathetic nerve activity is reduced in most cardiovascular disease states, and this may contribute to enhanced cardiac sympathetic responsiveness. Disruption of inhibitory G-proteins (Gi) ablates the cholinergic pathway and increases cardiac endothelial nitric oxide (NO) synthase (eNOS) expression, suggesting that NO may offset the impaired attenuation of beta-adrenergic regulation of supraventricular excitability. To test this, we investigated the role of endogenous NO production on beta-adrenergic regulation of rate (HR), contraction (CR) and calcium (Ca2+) handling in atria following blockade of Gi-coupled muscarinic receptors. Methods: Mice were administered pertussis toxin (PTx, n = 105) or saline (C, n = 100) intraperitoneally. After 3 days, we measured CR, HR, and NOS protein levels in isolated atria. Intracellular calcium (Ca2+) transients and Ca2+ current density (I-Ca) were also measured in atrial myocytes. Results: PTx treatment increased atrial myocyte eNOS protein levels compared to C (P < 0.05). This did not affect basal atrial function but was associated with a significant reduction in the CR and HR response to isoprenaline (ISO) compared with C. NOS inhibition nonnalized responses in PTx atria with respect to responses in C atria (P < 0.05), which were unaffected. Furthermore, PTx did not affect ISO-stimulated HR and CR in eNOS gene knockout mice (n=40). In agreement with these findings, the ISO-mediated increase in Ca2+ transient was suppressed in PTx-treated myocytes (P < 0.05), whereas I-Ca did not differ between groups. Conclusion: eNOS-derived NO inhibits beta-adrenergic responses following disruption of Gi signaling. This suggests that increased eNOS expression may be a compensatory mechanism which reduces beta-adrenergic regulation of heart rate when cardiac parasympathetic control is impaired. (c) 2005 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.