Down-regulation of the G-proteins G(q)α and G₁₁α by transfected human M₃ muscarinic acetylcholine receptors in Chinese hamster ovary cells is independent of receptor down-regulation

Chinese hamster ovary cells stably transfected with human M₃ muscarinic acetylcholine receptors show a 40-50% reduction in the immunoreactive G-proteins G(q)α and G₁₁α when stimulated with the cholinergic agonist carbachol. This effect is seen after 9 h, is maximal after 24 h, and occurs over a range of carbachol concentrations that activate phosphoinositide hydrolysis in these cells. The effect is specific for G(q)α family proteins as G(S)α was slightly increased after carbachol treatment and G₁₃α was unchanged. Using a urea gel system, we were able to resolve G(q)α and G₁₁α, both of which were down-regulated by carbachol. An M₃ receptor mutant, with C-terminal threonines changed to alanines as described previously, binds ligand and activates phosphoinositide hydrolysis normally but is not down-regulated in response to carbachol. This receptor, however, induces G(q)α/G₁₁α down-regulation similarly to wild-type M₃ receptors, indicating that G-protein down-regulation is not directly coupled to receptor down-regulation. Thus down-regulation of G(q)α and G₁₁α may contribute to heterologous desensitization particularly at longer times of agonist exposure.