Inhibitory regulation of adenylyl cyclases. Evidence inconsistent with beta gamma-complexes of Gi proteins mediating hormonal effects by interfering with activation of Gs.
The possible effect of cholera toxin (CTX) on hormonal inhibition of adenylyl cyclase in somatostatin (SST)-sensitive GH3 cells was quantitatively evaluated. The toxin treatment employed led to an essentially complete ADP ribosylation of all alpha s subunits of the stimulatory regulatory component (Gs) of the system and to ca. 5- to 7-fold increases in the activity measured, yet it failed to affect the inhibitory action of SST regardless of whether analyzed in terms of degree of inhibition (ca. 60%) that is attainable or in terms of the apparent Kact with which the inhibitory hormone elicits its action. In absolute terms the activity inhibited after CTX was ca. 6 times larger than that inhibited under control conditions, indicating that SST is equally effective in regulating control and CTX-stimulated adenylyl cyclase system and that interpretations are independent of possible intramembraneous compartmentalizations of adenylyl cyclase and its various regulatory components. Since CTX-mediated ADP ribosylation of the alpha-subunits of Gs has been demonstrated to result in an at least 10-fold decrease in the potency (i.e. EC50) with which the beta gamma-complexes of G proteins act to stabilize preactivated purified alpha-subunits of Gs and in an approximately 300-fold decrease in the potency with which exogenously added beta gamma-complexes act to prevent activation of Gs in intact membranes, the present data indicate that beta gamma-complexes cannot be mediating the inhibitory effects of hormones by interfering with activation of the Gs of adenylyl cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)
Toro
M J
MJ
Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030.
Montoya
E
E
Birnbaumer
L
L
D000246:Adenosine Diphosphate Ribose / Q000378:metabolism; D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D002772:Cholera Toxin / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism / Q000502:physiology*; D006728:Hormones / Q000502:physiology*; D010446:Peptide Fragments / Q000502:physiology; D015203:Reproducibility of Results; D014407:Tumor Cells, Cultured
D000067956:Adenylyl Cyclase Inhibitors; D006728:Hormones; D010446:Peptide Fragments; D000246:Adenosine Diphosphate Ribose; D002772:Cholera Toxin; D019204:GTP-Binding Proteins