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 incorporation into phosphatidylinositol (PI) by 35% (P < 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 2-fold increase in 32P 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 labeling by methoxamine did not occur in phospholipids other than PI, and was blocked by the .alpha.-adrenergic antagonist, phentolamine (20 .mu.M). The majority of the muscarinic receptors in rat atria are not coupled to phosphoinositide turnover. If indeed the observed enhancement in 32P labeling 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.