Insulin downregulates M(2)-muscarinic receptors in adult rat atrial cardiomyocytes: a link between obesity and cardiovascular complications.
OBJECTIVE
To determine whether decreased cardiac parasympathetic activity observed in obesity is due to insulin-induced alterations in cardiac M(2)-muscarinic receptors and/or adenylyl cyclase activity.


METHODS
After incubation with increasing concentrations of insulin, adult rat atrial cardiomyocytes were assayed for M(2)-muscarinic receptor binding density and affinity, and for M(2)R mRNA expression using RT-PCR analysis. Forskolin-stimulated adenylyl cyclase activity and its inhibition by carbachol were also assayed, as was endothelial nitric oxide synthase mRNA expression. The effects of insulin on M(2)-muscarinic receptor density and mRNA expression levels were analyzed using the insulin signaling inhibitors rapamycin, wortmanin and PD 098059.


RESULTS
Insulin induces a concentration- and time-dependent decrease in expression of the M(2)R mRNA, and in [(3)H]N-methylscopolamine binding by the receptor. These effects on the M(2)R mRNA levels and on [(3)H]N-methylscopolamine binding were prevented by PD 98059, but not by wortmanin or rapamycin. Basal and forskolin-induced cAMP production did not differ, but the inhibition of forskolin-simulated enzyme activity by carbachol was blunted by insulin. No change in the mRNA levels for endothelial nitric oxide synthase was observed.


CONCLUSIONS
In rat atrial cardiomyocytes, insulin markedly alters both the M(2)-muscarinic receptor density, and its mRNA expression through transcriptional regulation and adenylyl cyclase activity. These data suggest that the obesity-associated decrease in cardiac parasympathetic tone may be related to hyperinsulinemia, which could directly contribute to cardiovascular morbidity in obese patients.
Pathak
A
A
Unité de Recherche sur les Obésités -Unité INSERM 586, Institut Louis Bugnard, Université Paul Sabatier, CHU Rangueil, Toulouse Cedex 4, France. pathak@cict.fr
Smih
F
F
Galinier
M
M
Verwaerde
P
P
Rouet
P
P
Philip-Couderc
P
P
Montastruc
J L
JL
Senard
J M
JM
D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D002318:Cardiovascular Diseases / Q000209:etiology / Q000378:metabolism; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation / Q000187:drug effects*; D007328:Insulin / Q000494:pharmacology*; D008297:Male; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D019001:Nitric Oxide Synthase / Q000235:genetics / Q000378:metabolism; D052250:Nitric Oxide Synthase Type III; D009765:Obesity / Q000150:complications / Q000378:metabolism*; D010275:Parasympathetic Nervous System / Q000187:drug effects; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D017208:Rats, Wistar; D043585:Receptor, Muscarinic M2 / Q000096:biosynthesis* / Q000187:drug effects / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000379:methods; D015398:Signal Transduction / Q000187:drug effects
D007328:Insulin; D012333:RNA, Messenger; D043585:Receptor, Muscarinic M2; D019001:Nitric Oxide Synthase; D052250:Nitric Oxide Synthase Type III; C496316:Nos3 protein, rat; D000262:Adenylyl Cyclases