Muscarinic cholinergic receptors in brain and atrial membranes of adult brook trout (Salvelinus fontinalis) measured by radioligand binding techniques.

Muscarinic cholinergic receptors were measured by radioligand binding techniques in crude membrane particulate preparations of brain and atrial tissues from laboratory reared brook trout (Salvelinus fontinalis). The radioligand [3H]N-methyl scopolamine was used to determine number and affinity of receptors in saturation experiments. The affinity of the radioligand did not differ in brain and atrial preparations (96 +/- 8 and 60 +/- 4 pM, respectively). However, the number of binding sites was greater in atrium compared with brain (269 +/- 19 and 166 +/- 7 fmol/mg protein, respectively). The rank order of potency of competing drugs in inhibition experiments was similar for antagonists with atropine > or = scopolamine > pirenzepine. Pirenzepine, an M1-selective drug had a 3-fold higher affinity in brain than atrium. The agonists oxotremorine and carbachol each bound to two sites in both tissues. In contrast, pilocarpine bound to only one site in brain and two in atrial tissue. These results are compared with those observed in other nonmammalian species and discussed with reference to conservation of proteins that serve important cellular roles.