Phenobarbital selectively modulates the glucagon-stimulated activity of adenylate cyclase by depressing the lipid phase separation occurring in the outer half of the bilayer of liver plasma membranes.

The glucagon-stimulated (coupled) activity of rat liver plasma-membrane adenylate cyclase could be selectively modulated by the anionic drug phenobarbital, whereas the fluoride-stimulated (uncoupled) activity remained unaffected. It is suggested that the cationic drug phenobarbital preferentially interacts with the external half of the bilayer, as the negatively charged phospholipids are found at the cytosol-facing side. This results in a selective fluidization of the external half of the bilayer, leading to a depression in the high-temperature onset of the lipid phase transition (from 28 degree to 16 degree C) occurring there. This was detected both by e.s.r. analysis, using a fatty acid spin probe, and also by Arrhenius plots of glucagon-stimulated activity, where the enzyme forms a transmembrane complex with the receptor and is sensitive to the lipid environment of both halves of the bilayer. However, in the absence of hormone, adenylate cyclase only senses the lipid environment of the inner (cytosol) half of the bilayer. Thus its fluoride stimulated activity and Arrhenius plots of this activity remained unaffected by the presence of phenobarbital (less than 12 mM) in the assay. These results support the view that independent modulation of the fluidity or chemical constituents of each half of the bilayer can selectively affect the receptor-coupled and uncoupled activities of adenylate cyclase.