Insulin downregulates M2-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 M2-muscarinic receptors and/or adenylyl cyclase activity. DESIGN AND METHODS:: After incubation with increasing concentrations of insulin, adult rat atrial cardiomyocytes were assayed for M2-muscarinic receptor binding density and affinity, and for M2R 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 M2-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 M2R mRNA, and in[3H]N-methylscopolamine binding by the receptor. These effects on the M2R mRNA levels and on[3H]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. CONCLUSION:: In rat atrial cardiomyocytes, insulin markedly alters both the M2-muscarinc 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.International Journal of Obesity (2005) 29, 176-182. doi:10.1038/sj.ijo.0802751 Published online 26 October 2004 [ABSTRACT FROM AUTHOR]