Quinidine and melittin both decrease the fluidity of liver plasma membranes and both inhibit hormone-stimulated adenylate cyclase activity.

Increasing concentrations of either quinidine or melittin gave a dose-dependent inhibition of both the glucagon- and fluoride-stimulated activities of adenylate cyclase in the liver plasma membranes. At similar concentrations these agents increased the order of liver plasma membranes as detected by a fatty acid ESR probe, doxyl stearic acid. This increase in bilayer order (decrease in 'fluidity') is suggested to explain the inhibitory action of quinidine on adenylate cyclase activity but only in part contributes to the inhibitory action of melittin on adenylate cyclase. Arrhenius plots of fluoride-stimulated activity became non-linear in the presence of either quinidine or melittin, with a single well-defined break occurring at around 12 degrees C in each instance. Arrhenius plots of the glucagon-stimulated activity also exhibited such a novel break at around 12 degrees C when either quinidine or melittin were present as well as exhibiting a break at around 28 degrees C, as was seen in the absence of these ligands. The fatty acid spin probe inserted into liver plasma membranes detected a novel lipid phase separation occurring at around 12 degrees C when either quinidine or melittin was present and showed that the lipid phase separation occurring at around 28 degrees C in native membranes was apparently unaffected by these ligands.