Abstract
The negative inotropic effect of carbachol, as well as phosphoinositide hydrolysis, was measured in atria from neonatal and adult rats. Carbachol increased phosphoinositide hydrolysis and decreased dF dt of both neonatal and adult atria; however, the neonatal atria showed hyperactivity to carbachol as compared with adult atria. Inhibition of phospholipase C reduced the supersensitivity to carbachol upon contractility in neonatal atria producing values similar to those of the adult atria, suggesting that muscarinic acetylcholine receptor (mAchR) stimulation is secondary to receptor-mediated hydrolysis of phosphoinositides. Pharmacological analysis with mAchR antagonists tends to support the idea that ml and m2 subtypes are the most important mediators of the response to carbachol in neonatal atria. In adult atria, the effect of carbachol is coupled only to mAchR m2 subtypes.
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