Loss of pirenzepine regional selectivity following solubilization and partial purification of the putative M1 and M2 muscarinic receptor subtypes.

Pirenzepine inhibition of [3H]N-methylscopolamine ([3H]NMS) binding was studied in membranous, digitonin-solubilized, and partially purified muscarinic receptors from bovine cortex, an area of the brain rich in the putative M1 muscarinic receptor subtype, and from pons-medulla, an area rich in the putative M2 subtype. In accord with previous work, we found that pirenzepine bound to membranous receptors from cortex with an IC50 that was over one order of magnitude lower than to receptors from pons-medulla. After digitonin solubilization, however, this regional selectivity was significantly reduced. In receptors from pons-medulla, the IC50 for pirenzepine inhibition of [3H]NMS was reduced from 2.1 +/- 0.7 X 10(-6) M in membrane-bound receptors, to 4.3 +/- 0.3 X 10(-7) M after solubilization, whereas in receptors from cortex, the IC50 remained unchanged after solubilization. The solubilized receptors from both brain areas maintained their binding characteristics after partial purification over an ABT-Sepharose affinity column and a hydroxylapetite column. These findings raise the possibility that the different pirenzepine binding characteristics used to define M1 and M2 receptor subtypes are not inherent in the receptor protein itself, but may be due to coupling factors such as effector proteins, phospholipids or cytoskeletal proteins which could be associated with the membranous receptor and become dissociated from the receptor after solubilization.