Disruption of inhibitory G-proteins mediates a reduction in atrial beta-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 normalized 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.


CONCLUSIONS
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.
Danson
E J F
EJ
University Laboratory of Physiology, Parks Road, Oxford, OX1 3PT, UK. edward.danson@physiol.ox.ac.uk
Zhang
Y H
YH
Sears
C E
CE
Edwards
A R
AR
Casadei
B
B
Paterson
D J
DJ
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D015153:Blotting, Western / Q000379:methods; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism; D051244:Caveolin 3 / Q000378:metabolism; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000037:antagonists & inhibitors*; D015870:Gene Expression / Q000187:drug effects; D006325:Heart Atria; D007150:Immunohistochemistry / Q000379:methods; D007545:Isoproterenol / Q000494:pharmacology*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018613:Microscopy, Confocal; D009200:Myocardial Contraction / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism; D052250:Nitric Oxide Synthase Type III / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D037342:Pertussis Toxin / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects
D000318:Adrenergic beta-Agonists; D015220:Calcium Channels; D051244:Caveolin 3; D009569:Nitric Oxide; D052250:Nitric Oxide Synthase Type III; D037342:Pertussis Toxin; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D007545:Isoproterenol; D002118:Calcium