Interactions of acth with its adrenal receptors: Specific binding of ACTH1–24, its O-nitrophenyl sulfenyl derivative and ACTH11–24

Adrenocorticotropin binding sites in rat cardiac tissue

Using the ACTH analog [125I-Tyr23,Phe2,Nle4] ACTH(1-24), the existence of specific binding sites for ACTH in atrial membrane preparations was demonstrated. The dissociation constants (Kd), determined by Scatchard analysis were not significantly different for membrane preparations of adrenal gland or atrial tissue (being 6.40 x 10(-12)M and 8.86 x 10(-12)M respectively). No binding was observed to membrane preparations from kidney or lung. While the binding of the ACTH(1-24) analog to atrial membranes was inhibited by ACTH(1-24), it was not affected by norepinephrine or epinephrine. It was proposed that the ACTH(1-24) analog may bind to sites located on the adrenergic nerve endings associated with the cardiac tissue, and that such binding would interfere with the neuronal reuptake of the catecholamines.

Neuropeptides modulate beta-adrenergic agonist binding and receptor downregulation in slices of rat brain

Displacement curves of the beta-adrenergic agonist (-)-isoproterenol, on the binding of [3H]-dihydroalprenolol (DHA) in homogenates from the brain of rat defined a two-site model of high (KH; 8.5 +/- 1.6 nM) and low (KL; 771 +/- 111 nM) affinity. The KL/KH ratio was 84.5 +/- 4.5. Somatostatin, ACTH, dynorphin and mastoparan, but not vasoactive intestinal peptide (VIP), substance P, met-enkephalin or leu-enkephalin, each at 1 microM, decreased the KL/KH ratio to 35.2 +/- 4.2, 43.0 +/- 5.0, 52.0 +/- 2.3 and 60.3 +/- 8.0, respectively, without significant effect on either the Kd or Bmax of antagonist binding. Calcium was required for this effect of the neuropeptides. In slices from whole brain of rat, incubated for 2 hr, ACTH attenuated the isoproterenol-induced downregulation of beta-adrenoceptors. For beta-adrenergic agonists, there appeared to be a close correlation between the KL/KH ratio and intrinsic activity. These results suggest that certain neuropeptides may influence beta-adrenergic neurotransmission by modulating the intrinsic activity of beta-agonists. This modulation is manifested, at least in part, by changes in both binding affinity and receptor down-regulation.

Effects of ACTH-(11-24) on the corticosteroid production of isolated adrenocortical cells

The steroidogenic action of ACTH-(11-24) was studied on isolated zona glomerulosa and zona fasciculata cells dispersed by collagenase. ACTH-(11-24) stimulated the corticosterone production of zona fasciculata cells and the aldosterone production of zona glomerulosa cells; in addition, it potentiated the effects of ACTH-(1-39) on both cell systems. It is suggested that the ACTH molecule contains more active sites for steroidogenesis than usually acknowledged, as has been found for lipolysis and behavior.

Immunocytochemical localization of ACTH-like immunoreactivity in rat adrenal glomerulosa and fasciculata cells

The role of ACTH in the synthesis of the adrenocortical hormones has been largely described. In order to investigate the localization of this peptide at the subcellular level of the adrenal glomerulosa and fasciculata cells, an immunocytological method was used. Rat adrenals were fixed and frozen. Ultrathin sections obtained by cryoultramicrotomy, were incubated with anti-beta (1-24) ACTH or anti-alpha (17-39) ACTH sera. The antigen-antibody reaction was detected by PAP complexes (revealed by 4-chloro-1-naphthol) or with protein A-colloidal gold or IgG-colloidal gold. The results obtained were the same whatever the antisera of the technique employed. All the cells of the adrenal zona glomerulosa and zona fasciculata were labelled. ACTH-like immunoreactivity in zona glomerulosa and zona fasciculata cells was observed at the plasma membrane level, in cytoplasmic matrix, mitochondria and nucleus (in the euchromatin close to the heterochromatin aggregations and, occasionally, associated with the nucleolus). No immunoreactivity was observed when non-immune serum or anti-ACTH serum preincubated with ACTH were used, nor there was any modification of the immunocytochemical reaction when anti-ACTH serum incubated with heterologous antigens was employed. These data, demonstrate the presence of endogenous ACTH in both adrenal glomerulosa and fasciculata cells, and suggest that the peptide is internalized after binding to the plasma membrane.

Stimulation of brain lipase activity by polyamines. Comparison with the effect of ACTH

Polyamines, as well as ACTH, strongly stimulate at pH 5.75 triacylglycerol lipase (TAGL) activity from rat brain. Whether the activating potency is expressed in terms of molar concentration or amount of positive charges, polyarginine, polylysine, spermine and spermidine exhibit, in this order, decreasing potencies. By contrast to other lipases, heparin (25 micrograms/ml) inhibits brain TAGL. Polyarginine, polylysine and spermine reverse the heparin-dependent inhibition and further stimulate TAGL activity above basal values; spermidine is much less potent. In the presence of heparin, ACTH has the greatest stimulating effect, being 1.6-fold and 3.3-fold more potent than polyarginine and polylysine, respectively. Taken together, the data suggest that polybasic effectors modify the interaction of TAGL with its substrate, resulting in increased levels of TAGL activity. In the presence of heparin, the enzyme charge density is mandatory for determining the stimulation process. Such cationic interactions appear to be specific of brain TAGL and should be considered in assessing any direct neuro-hormonal role to ACTH or physiological polyamines in brain.

Ontogeny of the ACTH receptor, adenylate cyclase and steroidogenesis in adrenal

The steroidogenic responsiveness to ACTH of the fetal adrenal gland of several species increases strikingly during the last weeks of gestation. Using the ovine fetus model it has been shown that this spontaneous development involves modifications at different steps in the mechanism by which ACTH stimulates the cells: one located at the cell membrane, another beyond cAMP formation, namely, in the steroidogenic pathway. The first phenomenon is related to at least three processes: an increased number of ACTH receptors, an increased activity of adenylate cyclase, and an enhanced coupling of these entities. The enhanced steroidogenic capacity is related mainly to an increased ability to produce pregnenolone and to an enhanced activity in both 3 beta-hydroxysteroid dehydrogenase and 17 alpha-hydroxylase. Infusion of 115-120-day-old fetuses with ACTH for only 5 days induced development of most of these biochemical steps, indicating that ACTH is an important regulating hormone for the maturation of fetal adrenocortical function. However, when ovine fetal adrenal cells were cultured in the absence of ACTH, there was a spontaneous development of the hormone-sensitive adenylate cyclase system and of the steroidogenic pathway. Addition of ACTH to the culture medium accelerated and further enhanced these maturation processes. These results suggest that the in vivo maturation of ovine fetal adrenal is blocked by the presence in the fetal circulation of some inhibiting factors which are likely to be of extrapituitary origin. Therefore, the 'maturation' of ovine fetal adrenocortical function appears to be regulated in a coordinate fashion by stimulatory, ACTH and unknown inhibitory factors.

ACTH stimulation on cholesterol side chain cleavage activity of adrenocortical mitochondria. Transfer of the stimulus from plasma membrane to mitochondria

Adrenocortical mitochondrial cholesterol side chain cleavage reactions are regulated by the influence of pituitary ACTH. The mechanism of the stimulation involves adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase, and ribosomal labile protein synthesis. Through these reactions the stimulus reaches the mitochondrial side chain cleavage enzyme system. In this review article, the current implications on the stimulus transfer from the plasma membrane to the mitochondrial inner membrane are summarized. In particular the availability of cholesterol to P-450scc was discussed in terms of the distribution of cholesterol molecules in the membranes.