Interaction of halothane with inhibitory G-proteins in the human myocardium.
BACKGROUND
Halothane has been reported to possess a catecholamine-sensitizing effect in laboratory animals and in anesthetized patients and to enhance the positive inotropic effect of isoproterenol in human papillary muscle strips. The current study was designed to investigate further the underlying subcellular mechanisms on human myocardium, in particular the mechanism of action of halothane on G-proteins.


METHODS
To investigate the effect of halothane on adenylyl cyclase activity, isoproterenol-, guanylylimidodiphosphate (Gpp(NH)p)-, and forskolin-activated enzyme activities were studied alone and in the presence of halothane in native and manganese-treated membranes. The mechanisms of halothane interaction with inhibitory G-proteins (G1) were studied in adenosine diphosphate-ribosylation studies with pertussis toxin and immunochemical techniques.


RESULTS
Halothane (1%) augmented isoproterenol- and Gpp(NH)p-stimulated adenylyl cyclase activity but had no effect on forskolin-stimulated enzyme activity. Manganese ions inhibited the stimulating effect of isoproterenol and Gpp(NH)p on adenylyl cyclase activity, but the effect of forskolin remained unchanged in control and halothane-treated membranes. In the presence of pertussis toxin, the effect of isoproterenol and Gpp(NH)p on adenylyl cyclase activity was enhanced, but further stimulation by halothane was abolished. Halothane did not influence the attachment of Gi alpha to the membrane. No effect of halothane on adenosine diphosphate-ribosylation of Gi alpha by pertussis toxin was observed.


CONCLUSIONS
Halothane stimulates adenylyl cyclase activity by inhibiting the function of the inhibitory G-proteins by interfering with the effects of the alpha subunits or beta gamma subunits with the effector. Decreased membrane attachment of Gi alpha in the presence of halothane does not occur. The interaction of alpha and beta gamma subunits is not affected by halothane. Halothane does not impair the binding of pertussis toxin to the Gi alpha-protein.
Schmidt
U
U
Klinik III für Innere Medizin, Universität zu Köln, Germany.
Schwinger
R H
RH
Böhm
M
M
D000246:Adenosine Diphosphate Ribose / Q000378:metabolism; D000262:Adenylyl Cyclases / Q000378:metabolism; D005260:Female; D019204:GTP-Binding Proteins / Q000187:drug effects*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006221:Halothane / Q000494:pharmacology*; D006321:Heart / Q000187:drug effects*; D006801:Humans; D008297:Male; D008875:Middle Aged
D000246:Adenosine Diphosphate Ribose; D006165:Guanylyl Imidodiphosphate; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases; D006221:Halothane