Distribution of (3H)QNB binding in dog gastric smooth muscle pre- and post vagotomy.

Tritiated quinuclidinyl benzilate [(3H) QNB] was used to characterize muscarinic cholinergic receptors in membrane fragments prepared from the circular smooth muscle of the dog stomach. In preliminary experiments the effect of protein, incubation time, temperature and pH on QNB binding were evaluated. Muscarinic cholinergic antagonists and agonists inhibited QNB binding in a concentration-dependent manner, but the nicotinic antagonist hexamethonium and adrenergic compounds were not effective in displacing QNB from binding sites. Scatchard plot analysis of binding data showed an asymmetric receptor distribution in the stomach. The cardia bound 425fmol of QNB/mg protein with a Kd of 0.05nM, the fundus 267fmol/mg protein with a Kd of 0.09nM and the antrum 147 fmol/mg protein with a Kd of 0.14nM. In a second series of experiments, binding of QNB was measured in dogs which had been vagotomized three weeks earlier. Vagotomy had no effect on the apparent Kd but disrupted the asymmetric receptor distribution seen in the normal dog such that the Bmax of the cardia fell to a value of 222fmol/mg protein.