Function and localization of high and low affinity binding sites to muscarinic receptors in longitudinal and circular smooth muscles of human stomach.

The contractile response to acetylcholine (ACh) and the binding of [3H]-quinuclidinyl benzilate [( 3H]QNB) to muscarinic receptors in both longitudinal and circular muscles were examined in fundus, body and antrum of human stomach which was obtained by surgical operation for gastric cancer or ulcer. The values of pD2 and pA2 for ACh and atropine, respectively, on longitudinal muscle of fundus were similar to those of body but were larger than those of antrum. On the other hand, the values of pD2 and pA2 on circular muscles were not different among fundus, body, and antrum and were similar to those on longitudinal muscle of antrum. By Scatchard analysis of receptor binding of [3H]QNB in homogenate, at least two subclasses of binding sites, i.e. high and low affinity sites, were observed in fundus and body, while only high affinity binding site was found in antrum. Thus, we conclude that there are at least two subclasses of muscarinic receptors which regulate the contraction of smooth muscle in different regions of human stomach.