TWO KINDS OF AFFINITIES OF MUSCARINIC RECEPTORS IN THE INTESTINAL SMOOTH MUSCLES OF MOUSE FOR ACETYLCHOLINE, ATROPINE AND ³H QNB

In the experiment of contraction of mouse small intestine, median effective doses of acetylcholine for contraction of longitudinal and circular smooth muscles from mouse small intestine were 10^-7 to 10^-6 M and 10^-4 to 10^-2 M, respectively. PA₂ values of atropine for these smooth muscles were approx. 9 and 7, respectively. This suggests that there were at least two kinds of muscarinic receptors of different affinities in the small intestine and these receptors played a role in contractile function in longitudinal and circular smooth muscles, respectively. Specific ³H-QNB binding to homogenates of whole and longitudinal smooth muscle layers of the small intestine were studied by the method of Yamamura et al. High and low affinities of the specific QNB binding were found in the homogenates of longitudinal and circular smooth muscles. KD values of various homogenates for high and low affinity bindings were 1 nM and 15 nM, respectively.