Comparison of muscarinic and alpha-adrenergic receptors in rat atria based on phosphoinositide turnover.
The effects of muscarinic and alpha-adrenergic receptor stimulation on phosphoinositide turnover in rat atria have been compared. Despite the similar densities of muscarinic receptors in rat left and right atria, 0.1 mM carbachol increased [32P]phosphate incorporation into phosphatidylinositol (PI) by 35% (p less than 0.05) in left atria but had no effect in right atria. By contrast to the small muscarinic receptor effect, stimulation of alpha 1-adrenergic receptors by 0.1 mM methoxamine produced a more than two fold increase in [32P]phosphate incorporation into PI in both left and right atria, despite the reported smaller density of alpha-adrenergic receptors in rat atria compared to muscarinic receptors. Enhanced phosphate labelling by methoxamine did not occur in phospholipids other than PI, and was blocked by the alpha-adrenergic antagonist, phentolamine (20 microM). The results indicate that the majority of the muscarinic receptors in rat atria are not coupled to phosphoinositide turnover. If indeed the observed enhancement in [32P]-phosphate labelling by carbachol reflects phosphoinositide turnover, and assuming equal coupling efficiencies of muscarinic and adrenergic receptors, it is calculated that not more than 2% of the muscarinic receptors in rat left atria are coupled to this response.
D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D006325:Heart Atria / Q000187:drug effects / Q000378:metabolism; D006352:Heart Ventricles / Q000378:metabolism; D008729:Methoxamine / Q000494:pharmacology; D009206:Myocardium / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism; D010716:Phosphatidylinositols / Q000378:metabolism*; D010743:Phospholipids / Q000096:biosynthesis; D010761:Phosphorus Radioisotopes; D051381:Rats; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D011976:Receptors, Muscarinic / Q000502:physiology*
D010710:Phosphates; D010716:Phosphatidylinositols; D010743:Phospholipids; D010761:Phosphorus Radioisotopes; D011942:Receptors, Adrenergic, alpha; D011976:Receptors, Muscarinic; D002217:Carbachol; D008729:Methoxamine