Comparison of the muscarinic cholinoceptors in the rabbit ciliary body and the guinea-pig ileum.
Interactions of several muscarinic drugs with their receptors were studied in the ciliary body smooth muscle of rabbits and the ileal longitudinal muscle of guinea-pigs, using pharmacological and biochemical procedures. The dissociation constants of carbachol, pilocarpine, atropine and pirenzepine estimated by these procedures indicate no heterogeneity of muscarinic receptors in either tissue; thus both are probably of the M2 type. However, the density of the receptors in the ciliary body is lower than in the ileum. Pilocarpine, with lower intrinsic efficacy, demonstrated a pronounced organ selectivity when compared to carbachol as it was a potent full agonist in the ileum and a competitive antagonist in the ciliary body. These results suggest the importance of both receptor density and threshold as determinants of agonist potency.
D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D002924:Ciliary Body / Q000502:physiology*; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects; D066298:In Vitro Techniques; D007498:Iris / Q000187:drug effects; D007700:Kinetics; D008297:Male; D009130:Muscle, Smooth / Q000502:physiology*; D011813:Quinuclidinyl Benzilate; D011817:Rabbits; D011976:Receptors, Muscarinic / Q000502:physiology*
D011976:Receptors, Muscarinic; D011813:Quinuclidinyl Benzilate