Contraction and [3H]QNB binding in collagenase isolated fundic smooth muscle cells

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Abstract
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 .times. 10-19 mol of QNB, and a concentration of QNB of 0.19 nM was required to label 1/2 of the binding sites. This suggests a concentration of .apprx. 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 IC50 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 .times. 10-12 M for ACh and 6.3 .times. 10-13 M for oxotremorine. Atropine (10-9 M) decreased the sensitivity to ACh, increasing the ED50 for ACh-induced contractions to 1.2 .times. 10-10 M. The existence of muscarinic receptor heterogeneity for cholinergic agonists but not for antagonists is suggested.

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