THE CLASS-III ANTIARRHYTHMIC DRUG AMIODARONE DIRECTLY ACTIVATES PERTUSSIS-TOXIN-SENSITIVE G-PROTEINS

The class III antiarrhythmic drugs amiodarone and bretylium tosylate are cationic/amphiphilic, and various substances with these physico-chemical properties are known to directly activate heterotrimeric regulatory G proteins. We asked the question of whether class III antiarrhythmic drugs are also direct G protein activators, using HL-60 leukemic cells and purified bovine brain G proteins as model systems. In HL-60 cell membranes, amiodarone increased high affinity GTP hydrolysis with an EC(50) of 7.5 mu M. The stimulatory effect of amiodarone on GIP hydrolysis was inhibited by pertussis toxin. Amiodarone stimulated binding of guanosine-5'-O-(3-thio)triphosphate to, and incorporation of GTP azidoanilide into, G(i) protein alpha subunits in HL-60 membranes. The drug increased the cytosolic Ca2+ concentration in HL-60 cells in the presence but not in the absence of extracellular Ca2+. Amiodarone-induced increases in the cytosolic Ca2+ concentration were reduced by pertussis toxin and by a blocker of nonselective cation channels, SKandF 96365. Amiodarone activated the GTPase of reconstituted G(i)/G(o) proteins and G(12) With EC(50) values of 20 mu M and 50 mu M, respectively. Bretylium tosylate did not increase GTP hydrolysis in HL-60 membranes or with G(i)/G(o) proteins. Our data suggest that amiodarone but not bretylium tosylate is a direct activator of G(i) and G(o) proteins and that amiodarone activates nonselective cation channels in HL-60 cells via G(i) proteins and independently of Ca2+ mobilization from intracellular stores. Future studies will have to test the hypothesis that direct G protein activation by amiodarone contributes to its toxic and/or therapeutic effects.