AJP - Gastrointestinal and Liver Physiology, Vol 245, Issue 2 270-G276, Copyright © 1983 by American Physiological Society
Contraction and [3H]QNB binding in collagenase isolated fundic smooth muscle cells
E. R. Seidel and L. R. Johnson
Smooth muscle cells from the guinea pig gastric fundus were isolated by
successive collagenase digestions. Tritiated quinuclidinyl benzilate [(
3H]QNB) was used to study the binding characteristics of the muscarinic
cholinergic receptors on these cells. Each cell bound 8.3 X 10(-19) mol of
QNB, and a concentration of QNB of 0.19 nM was required to label one-half
of the binding sites. This suggests a concentration of about 500,000
muscarinic cholinergic receptors per smooth muscle cell. The muscarinic
cholinergic receptor antagonists atropine and scopolamine inhibited QNB
binding with a 50% inhibiting concentration (IC50) in the nanomolar range,
whereas the agonists acetylcholine (ACh), oxotremorine, and carbamylcholine
had IC50S in the micromolar range. Hill coefficients (nH) for antagonists
approached unity, but agonists displayed fractional nH. Exposure of cells
to cholinergic muscarinic agonists resulted in dose-dependent decreases in
cell length. The concentration of agonist required to induce half-maximal
contractions (ED50) was 8.3 X 10(-12) M for ACh and 6.3 X 10(-13) M for
oxotremorine. Atropine (10(-9) M) decreased the sensitivity to ACh,
increasing the ED50 for ACh-induced contractions to 1.2 X 10(-10) M. These
results suggest the existence of muscarinic receptor heterogeneity for
cholinergic agonists but not for antagonists.
