Adrenal cortex adenylate cyclase. In vitro acitivity of ACTH fragments and analogues

The anomalous effect of some ACTH-fragments missing the amino acid sequence 1-10 on the corticosteroidogenesis in purified isolated rat adrenal cells

The corticosteroidogenicity of ACTH-derived peptides was tested in a purified isolated rat adrenal cell system. The activity was related to the activity of standard ACTH (synthetic hACTH; potency fixed at 100). The peptides ACTH NH2 and ACTH showed steroidogenicity at pharmacological doses (potencies: 0.00067 and 0.00032, respectively). None of the peptides tested inhibited or potentiated the ACTH-induced steroidogenesis at low doses (0.5-50 000 pg). The results suggest the presence of a second center within the ACTH molecule capable of inducing steroidogenesis.

Responses of Y1 adrenocortical tumor cells to o-nitrophenyl sulfenyl ACTH

NPS(o-nitrophenyl sulfenyl)-ACTH stimulated steroidogenesis in Y1 cells to the same maximum level as did ACTH, but with a 60-fold lower potency than the native hormone. NPS-ACTH also stimulated the extracellular accumulation of cAMP in Y1 cells with lower potency than the unmodified hormone. The amount of cAMP accumulated in the presence of NPS-ACTH approached 75% of the maximum with ACTH. In Y1 plasma membranes, NPS-ACTH was a partial agonist of adenylate cyclase activity, witn an efficacy dependent upon the type of guanyl nucleotide present. The steroidogenic responses of two Y1(Kin) mutants and two Y1(Cyc) mutants to NPS-ACTH paralleled their responses to ACTH and reflected closely their defects in cAMP-dependent protein kinase and ACTH-sensitive adenylate cyclase activity. These data imply an obligatory role for adenylate cyclase and cAMP-dependent protein kinase activities in stimulation of steroidogenesis in Y1 cells by NPA-ACTH.

The regulation of adenylate cyclase of the adrenal cortex

This short review summarizes some of the data concerning the regulation of adrenocortical adenylate cyclase by ACTH and other putative effectors, such as guanosine and nucleotides, divalent cations and adenosine. The available information on ACTH-sensitive adenylate cyclase of the adrenal cortex is discussed in comparison to other cyclase systems and the possible biochemical mechanisms of action of ACTH on the adrenal cortex.

ACTH-induced lipolysis in rat adipocytes: structure-activity relationships

The lipolytic action of natural porcine ACTH1(-39) and of a number of highly purified synthetic ACTH peptide fragments was studied using rat adipocytes. Of the analogues tested, only ACTH1(-24) exhibited full lipolytic activity with respect to intrinsic activity and affinity. Several shorter fragments appeared to be full agonists but had lower affinity. Fragments ACTH5(-10) and ACTH7(-10) were inactive. No antagonistic effects against the lipolytic action of ACTH could be demonstrated with substimulatory doses of ACTH1(-16), ACTH1(-10), ACTH7(-24) and ACTH11(-24). Based on the relative potency derived from dose-response curves, a more refined model with respect to the active centers being encoded in various sequences of the hormone, is proposed.

Adrenal cortex adenylate cyclase. In vitro modification of the enzyme by cholera toxin

Pretreatment of rat adrenal particulate fractions with cholera toxin in vitro markedly changed the properties of the membrane-bound adenylate cyclase. The basal activity of the enzyme was increased after cholera toxin treatment. The main action of the toxin was on the Vmax of the enzyme. In the absence of added GTP Lineweaver-Burk plots indicate a deviation from normal Michaelis-Menten kinetics with respect to substrate, the slopes being concave downward for control and toxin-treated membranes. Although hormonal stimulation of the enzyme was diminished in toxin-treated membranes, the hormone receptors were still functionally active as revealed after addition of Gpp(NH)p, GTP or GTPgammaS. The response to NaF was decreased in the toxin-treated membranes. Whereas GTP behaves as an antagonist (or a partial agonist with low intrinsic activity) compared to Gpp(NH)p in control membranes, it has similar intrinsic activity as Gpp(NH)p in the toxin-treated membranes. The results indicate that cholera toxin modification of the adenylate cyclase complex is located at the guanyl nucleotide sites or factors controlling the turnover of GTP at these sites. Cholera toxin modification may be a useful tool to investigate the role of guanyl nucleotide sites in the regulation of adenylate cyclase activity.