Ethanol-induced desensitization of adenylate cyclase: Role of the adenosine receptor and GTP-binding proteins

Chronic exposure to ethanol decreases receptor stimulation of cyclic AMP production. In PC 12 cells, chronic treatment with ethanol decreased the maximal stimulation of cyclic AMP accumulation by 2-chloroadenosine without altering the concentration of drug required for half-maximal stimulation. This desensitization was shown to occur after a 7-day exposure to 25 mM ethanol, which is comparable to the legal limit for intoxication in most states. When adenylate cyclase activity was measured directly in PC 12 cells permeabilized with Staphylococcal α-toxin, chronic ethanol treatment also decreased enzyme activity. After chronic ethanol exposure, cholera toxin- dependent [³²P]ADP ribosylation of both the 44,000 and 52,000 dalton isoforms of the α-subunit of the stimulatory GTP-binding regulatory protein of adenylate cyclase (Gsα) was reduced. Similarly, an ethanol-induced decrease in the amounts of both isoforms of Gsα was found by immunoblot analysis. This decrease in Gsα levels was not observed after chronic ethanol exposure of A126-1B2-1 cells, which are functionally deficient in protein kinase A. Immunoblot analysis using an antiserum that recognizes both the α- subunit of the inhibitory GTP-binding regulatory protein of adenylate cyclase (Giα)₁ and Giα₂ indicated that chronic ethanol treatment did not alter membrane levels of these GTP-binding proteins. Chronic ethanol exposure of PC 12 cells did not alter the affinity of the adenosine A₂ receptor for the radioligand [³H]CGS 21680, nor was there a change in the density of binding sites. The present study indicates that ethanol-induced desensitization of adenylate cyclase activity results from a decrease in the membrane levels of Gsα rather than an increase in Giα_1,2 levels or a change in the properties of the adenosine A2 receptor.