Dissociation of contraction and muscarinic receptor binding to isolated smooth muscle cells.
We examined changes in [3H]QNB binding and cell length induced by muscarinic ligands in a suspension of single smooth muscle cells isolated from the canine stomach. Cells contracted following a brief (30 s) exposure to picomolar concentrations of muscarinic agonists and yielded ED50 values of 1.0 +/- 0.7 pM for oxotremorine, 12.5 +/- 1.8 pM for carbachol, and 16.0 +/- 2.9 pM for metacholine. Contraction was inhibited by atropine with a pA2 value of 10.2 +/- 1.1. The binding of [3H]QNB was rapid and reversible and was stereospecific and pharmacologically appropriate. Specific binding of [3H]QNB was saturable and bound with high affinity (KD 1.04 +/- 0.23 nM) to a single class of sites, of which there were approximately 200,000/cell. In competition experiments antagonist binding was generally homogeneous, whereas that of agonists was heterogeneous and subpopulations of binding sites with different affinities for agonists were identified. The Ki value of 8.1 +/- 1.1 nM for inhibition of QNB binding by atropine was greater than the pA2 of 10.2 +/- 1.1 derived from contraction studies. Furthermore, whereas picomolar concentrations of agonists induced cell contraction, substantially higher concentrations (10 nM to 10 mM) were required to inhibit [3H]QNB binding to the isolated cells.
D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001667:Binding, Competitive; D002217:Carbachol / Q000494:pharmacology; D004285:Dogs; D007700:Kinetics; D016210:Methacholine Chloride; D008688:Methacholine Compounds / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000502:physiology*; D010095:Oxotremorine / Q000494:pharmacology; D011813:Quinuclidinyl Benzilate / Q000378:metabolism; D011976:Receptors, Muscarinic / Q000502:physiology*; D013270:Stomach / Q000502:physiology; D013696:Temperature
D008688:Methacholine Compounds; D011976:Receptors, Muscarinic; D016210:Methacholine Chloride; D010095:Oxotremorine; D011813:Quinuclidinyl Benzilate; D001285:Atropine; D002217:Carbachol