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) muscari nic acetylcholine receptors show a 40-50% reduction in the immunoreact ive G-proteins G(q) alpha and G(11)alpha when stimulated with the chol inergic agonist carbachol. This effect is seen after 9 h, is maximal a fter 24 h, and occurs over a range of carbachol concentrations that ac tivate phosphoinositide hydrolysis in these cells. The effect is speci fic for G(q) alpha family proteins as G(s) alpha was slightly increase d 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 o f which were down-regulated by carbachol. An M(3) receptor mutant, wit h C-terminal threonines changed to alanines as described previously, b inds ligand and activates phosphoinositide hydrolysis normally but is not down-regulated in response to carbachol. This receptor, however, i nduces G(q) alpha/G(11)alpha, down-regulation similarly to wild-type M (3) receptors, indicating that G-protein down-regulation is not direct ly coupled to receptor down-regulation. Thus down-regulation of G(q) a lpha and G(11)alpha may contribute to heterologous desensitization par ticularly at longer times of agonist exposure.