Inhibition of ACTH responsiveness of adrenocortical and fat cells adenylate cyclase by 4-methyl-4-aza-5α-cholestane

Abstract

Adrenocorticotropic hormone (ACTH)-induced steroidogenesis, but not cyclic AMP- or dibutyryl cyclic AMP- activated corticosterone formation, is competitively inhibited by 4-methyl-4-aza-5α-cholestane. This compound also inhibits ACTH-activated adrenal plasma membrane adenylate cyclase in a manner that is not competitive, whereas basal and sodium fluoride activated cyclase is only slightly inhibited. In contrast, cholesterol does not have any effect on hormones-stimulated or basal adenylate cyclase. Analogous to the situation in adrenal membranes, 4-methyl-4-aza-cholestane inhibits ACTH-activated fat cell membrane adenylate cyclase far more effectively than that stimulated by isoproterenol or sodium fluoride. On the other hand, 4-methyl-4-aza-cholestane does not interfere with binding of ACTH to adrenal membrane receptors. These results, therefore, indicate that the aza compound might be interfering with the effective coupling of ACTH-binding receptor with the adenylate cyclase. This provides another evidence for the multiple components of the adenylate cyclase system that need to be properly oriented before the hormone exhibits its activity. It is anticipated that the introduction of this compound will add another tool that will contribute to the investigations designed to better understand the molecular aspects of the assembly and functioning of adenylate cyclase system.