Effect of urea-treatment on agonist binding affinity of the muscarinic acetylcholine receptor.

Urea-treatment of the microsome fraction of the heart of guinea-pigs caused selective reduction in the apparent affinity of an agonist (carbachol), but not an antagonist (atropine), to muscarinic acetylcholine receptors (mAChR), measured as inhibition of binding of 3H-quinuclidinyl benzilate (3H-QNB). This effect was similar to that of Gpp (NH)p. The effects of urea-treatment and Gpp (NH)p were not additive. On the other hand, treatment of the microsome fraction with 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) increased the apparent affinity of agonist, but not antagonist. The effect of DTNB predominated over those of urea-treatment and Gpp (NH)p, when these treatments were combined with DTNB.