Department of Environmental Health, University of Washington, Seattle 98195.
The muscarinic agonist oxotremorine produced a concentration-dependent increase in membrane fluidity in intact viable rat splenic lymphocytes in vitro. This effect was antagonized by atropine, but only at high concentrations (1 mM), while scopolamine was ineffective. Two other muscarinic agonists, carbachol and pilocarpine, did not affect membrane fluidity in lymphocytes. The fluidizing effect of oxotremorine occurred at both 10 and 37 degrees C with a similar time-course. Oxotremorine also increased membrane fluidity in liposomes of DMPC in gel phase, although its effect was less pronounced than in lymphocytes. The data suggest that the fluidization caused by oxotremorine is primarily nonreceptor-mediated and associated with a nonspecific physicochemical effect.