AJP - Gastrointestinal and Liver Physiology, Vol 251, Issue 4 546-G552, Copyright © 1986 by American Physiological Society
Dissociation of contraction and muscarinic receptor binding to isolated smooth muscle cells
S. M. Collins and D. J. Crankshaw
We examined changes in [3H]QNB binding and cell length induced by
muscarinic ligands in a suspension of single smooth muscle cells isolated
from the canine stomach. Cells contracted following a brief (30 s) exposure
to picomolar concentrations of muscarinic agonists and yielded ED50 values
of 1.0 +/- 0.7 pM for oxotremorine, 12.5 +/- 1.8 pM for carbachol, and 16.0
+/- 2.9 pM for metacholine. Contraction was inhibited by atropine with a
pA2 value of 10.2 +/- 1.1. The binding of [3H]QNB was rapid and reversible
and was stereospecific and pharmacologically appropriate. Specific binding
of [3H]QNB was saturable and bound with high affinity (KD 1.04 +/- 0.23 nM)
to a single class of sites, of which there were approximately 200,000/cell.
In competition experiments antagonist binding was generally homogeneous,
whereas that of agonists was heterogeneous and subpopulations of binding
sites with different affinities for agonists were identified. The Ki value
of 8.1 +/- 1.1 nM for inhibition of QNB binding by atropine was greater
than the pA2 of 10.2 +/- 1.1 derived from contraction studies. Furthermore,
whereas picomolar concentrations of agonists induced cell contraction,
substantially higher concentrations (10 nM to 10 mM) were required to
inhibit [3H]QNB binding to the isolated cells.
