DOWN-REGULATION OF THE G-PROTEINS G(Q)ALPHA AND G(11)ALPHA BY TRANSFECTED HUMAN M(3) MUSCARINIC ACETYLCHOLINE-RECEPTORS IN CHINESE-HAMSTER OVARY CELLS IS INDEPENDENT OF RECEPTOR DOWN-REGULATION

Chinese hamster ovary cells stably transfected with human M(3) muscarinic acetylcholine receptors show a 40-50% reduction in the immunoreactive G-proteins G(q) alpha and G(11)alpha 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) alpha family proteins as G(s) alpha was slightly increased after carbachol treatment and G(i3)alpha was unchanged. Using a urea gel system, we were able to resolve G(q) alpha and G(11)alpha, both of which were down-regulated by carbachol. An M(3) 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) alpha/G(11)alpha, down-regulation similarly to wild-type M(3) receptors, indicating that G-protein down-regulation is not directly coupled to receptor down-regulation. Thus down-regulation of G(q) alpha and G(11)alpha may contribute to heterologous desensitization particularly at longer times of agonist exposure.