Heterologous desensitization and reduced G protein ADP-ribosylation following exposure to α₂-adrenoceptor and muscarinic receptor agonists

We investigated the acute and chronic effects of α₂-adrenoceptor and muscarinic receptor agonists on dihydropyridine-sensitive voltage-dependent Ca²⁺ channels in spinal cord-dorsal root ganglion cocultures. Clonidine and oxotremorine inhibited the voltage-dependent Ca²⁺ influx (42 ± 2% and 35 ± 6% with 100 μM, respectively). The respective antagonists, yohimbine and atropine, abolished these effects.Pertussis toxin attenuated the inhibitory effects of clonidine and oxotremorine on Ca²⁺ influx, demonstrating involvement of G proteins in the transduction process. Chronic treatment with clonidine or oxotremorine desensitized the Ca²⁺ channel response to the agonist applied as well as to other receptor agonist (heterologous desensitization). Such treatment with clonidine or oxotremorine decreased the pertussin toxin-catalyzed ADP-ribosylation of G_iα and G_oα subunits, an effect which could be largely reversed by the detergent Lubrol PX. Yohimbine and atropine blocked the effects of clonidine or oxotremorine on pertussis toxin-catalyzed ADP-ribosylation. Results suggest that α₂-adrenoceptor and muscarinic receptors couple to the dihydropyridine-sensitive voltage-dependent Ca²⁺ channels via pertussis toxin-sensitive G proteins. Chronic agonist treatment leads to heterologous desensitization and to a reduced capacity of G_i and G_o to undergo pertussis toxin-catalyzed ADP-ribosylation.