Studies on adrenocorticotropic hormone receptor using isolated rat adrenocortical cells

Adenosine 5'-(gamma-thio) triphosphate (ATPgammaS) stimulates both P2Y receptors linked to inositol phosphates production and cAMP accumulation in bovine adrenocortical fasciculata cells

The study was aimed to investigate the existence of at least two kinds of P2Y receptors linked to steroidogenesis in bovine adrenocortical fasciculata cells (BAFCs). Extracellular nucleotides facilitated steroidogenesis in BAFCs. The potency order was UTP > adenosine 5'-(gamma-thio) triphosphate (ATPgammaS) > ATP > 2-methylthio ATP (2MeSATP) > adenosine 5'-(beta-thio) diphosphate (ADPbetaS) > alpha,beta-methylene ATP (alpha,beta-me-ATP), beta,gamma-methylene ATP (beta,gamma -me-ATP). ATPgammaS (10-100 microM) remarkably stimulated both total inositol phosphates (IPs) production and cyclic AMP (cAMP) accumulation. Competitive displacement experiments by using [35S]ATPgammaS as a radioactive ligand in BAFCs showed that the potency under these unlabelled ligands was ATPgammaS > ATP > ADPbetaS > 2MeSATP > UTP > alpha,beta-me-ATP, beta,gamma-me-ATP. These suggest that two different binding sites of [35S]ATPgammaS, namely P2Y receptors, exist in BAFCs, and that these receptors are linked to steroidogenesis via distinct second messenger systems in the cells.

Comparative involvement of cyclic nucleotide phosphodiesterases and adenylyl cyclase on adrenocorticotropin-induced increase of cyclic adenosine monophosphate in rat and human glomerulosa cells

The present study investigated the role and identity of cyclic nucleotide phosphodiesterases (PDEs) in the regulation of basal and ACTH-stimulated levels of intracellular cAMP in human and rat adrenal glomerulosa cells. Comparative dose-response curves indicated that maximal hormone-stimulated cAMP accumulation was 11- and 24-fold higher in human and rat cells, compared with cAMP production obtained in corresponding membranes, respectively. Similarly to 3-isobutyl-1-methyl-xanthine, 25 microM erythro-9-[2-hydroxy-3-nonyl]adenine (EHNA, a specific PDE2 inhibitor), caused a large increase in ACTH-stimulated cAMP accumulation; by contrast, it did not change cAMP production in membranes. Moreover, in membrane fractions, addition of 10 microM cGMP inhibited ACTH-induced cAMP production, an effect completely reversed by addition of 25 microM EHNA. These results indicate that PDE2 activity is involved in the regulation of cAMP accumulation induced by ACTH, and suggest that ACTH inhibits this activity. Indeed, time-course studies indicated that ACTH induced a rapid decrease in cGMP production, resulting in PDE2 inhibition, which in turn, contributed [with adenylyl cyclase (AC) activation] to an accumulation in cAMP for 15 min. Thereafter, cAMP content decreased, because of cAMP-stimulated PDE2, as confirmed by measurement of PDE activity that was activated by ACTH, but only after a 10-min incubation. Hence, we demonstrate that the ACTH-induced increase in intracellular cAMP is the result of a balance between activation of AC and direct modulation of PDE2 activity, an effect mediated by cGMP content. Although similar results were observed in both models, PDE2 involvement is more important in rat than in human adrenal glomerulosa cells, whereas AC is more stimulated in human than in rat glomerulosa cells.

Different signaling pathways mediate stimulated secretions of endogenous ouabain and aldosterone from bovine adrenocortical cells

Angiotensin II stimulates secretion of corticosteroids and an ouabain-like compound from adrenocortical cells. The angiotensin AT1 and AT2 receptor subtypes have been linked with stimulated secretion of aldosterone and endogenous ouabain, respectively, but the second messenger mechanisms involved in the latter secretion are not known. Accordingly, we investigated the effects of several pharmacological agents that affect signaling pathways on the basal and stimulated secretions of aldosterone and endogenous ouabain from primary cell cultures of bovine adrenocortical cells. The AT2 receptor antagonist, PD 123319, blocked the effects of angiotensin II on secretion of endogenous ouabain but not aldosterone. Treatment of the cells with either dibutyryl cAMP, a membrane permeant analog, or the phorbol ester tetradecanoyl phorbol acetate stimulated aldosterone secretion but had no effect on the secretion of endogenous ouabain. On the other hand, the membrane permeant analog, 8BcGMP, maximally activated secretion of endogenous ouabain whereas incubation of cells with sodium orthovanadate blocked angiotensin II stimulated secretion of endogenous ouabain. Neither 8BcGMP nor sodium orthovanadate affected the basal or stimulated components of aldosterone secretion. These results show that the secretions of aldosterone and endogenous ouabain from bovine adrenocortical cells are mediated by different intracellular signaling mechanisms and provide evidence that the adrenal secretions of these steroids are regulated differently.

Voltage dependent calcium and potassium currents in Y-1 adrenocortical cells are unresponsive to ACTH

In this report we use both whole cell and perforated patch clamp recording techniques to characterize calcium and potassium channels in Y-1 adrenocortical cells in order to assess their responsiveness to ACTH. Both transient and long-lasting components of an inward calcium current were identified which were similar to T and L-type Ca2+ currents. With Ba2+ as the charge carrier, the transient current activated at voltages more hyperpolarized than -50 mV with V1/2 for activation at -78.1 mV, and for steady state inactivation at -52.3 mV. The L-type current activated at -20 mV, with a V1/2 for activation at -29.9 mV and steady state inactivation at -44.2 mV. Under perforated patch conditions the response was shifted to more depolarized voltages. Both currents were responsive to agents which usually affect T- or L-type Ca2+ currents. The transient current was completely blocked by 50 microM lanthanum or 200 microM nickel and partially blocked by 300 mM amiloride. Cadmium (100 microM) and nifedipine (300 nM) completely blocked the long-lasting current while omega-conotoxin GVIA (1992 nM) inhibited the current by only 20-25%. The agonist, Bay K 8644 was stimulatory at 50 nM. Both transient and sustained outward potassium currents similar to A-type and delayed rectifier currents, respectively, were present. The transient current demonstrated fast activation at voltages more positive than -10 mV, inactivation with continued depolarization and steady state inactivation at V1/2 = -50 mV. The sustained current activated rapidly and had minimal inactivation with continued depolarization. The transient current was blocked by 5 mM 4AP and the sustained by 25 mM TEA. While Y-1 cells contain both calcium and potassium currents similar to those found in other adrenocortical cells, none of the currents were affected by ACTH or AII, secretagogues which stimulate steroidogenesis. These data, combined with the inability of both Ca2+ and K+ channel blockers to alter ACTH-induced steroidogenesis as reported earlier, suggests that neither calcium nor potassium currents are responsive to ACTH in Y-1 cells.

Recent progress in understanding aldosterone secretion

1. The synthesis and secretion of aldosterone in the adrenal zona glomerulosa in physiologic conditions is controlled by adrenocorticotropin (ACTH), angiotensin II (AII), and extracellular (K+). 2. ACTH effects on aldosterone output are explained by cyclic AMP-(cAMP)- and Ca(2+)-dependent mechanisms. 3. All effects on aldosterone secretion are initiated by an increase in Ca2+ influx through hormone-operated Ca2+ channels and G-protein- and phospholipase C-(PLC) dependent hydrolysis of phosphoinositides leading to the generation of inositol 1,4,5 trisphosphate (IP3) and DAG that induce intracellular Ca2+ release and PKC activation, respectively. 4. ACTH increases DAG formation with marginal or undetectable IP3 generation. The effect of ACTH on DAG levels is discussed. 5. The requirement of external Ca2+ in PLC activation and aldosterone secretion also is discussed.

Adrenocortical ACTH receptors in pigs of differing in vivo response to adrenocorticotropin

1. Adrenocortical membrane protein was isolated from the adrenal glands of 12 Large White x Landrace male pigs, six with high adrenocortical response to adrenocorticotropic hormone (ACTH) and six with low response. 2. The peptide (Phe2, Nle4) ACTH was iodinated by the chloramine-T method and served as the radioligand in receptor binding studies. 3. Only one class of ACTH receptor was detected, with Kd = 2.57 +/- 0.35 x 10(9) M and Bmax = 1.59 +/- 0.06 pmol/mg protein in high responders and Kd = 1.68 +/- 0.18 x 10(-9) M and Bmax = 1.17 +/- 0.11 pmol/mg protein in low responders. 4. The difference in the Bmax between high and low responders was significant (P < 0.05), the difference in Kd was not statistically significant.

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.

Role of calcium ion in acth-induced steroidogenesis in humans

In order to examine the role of calcium ion in ACTH-induced steroidogenesis in humans, we carried out infusion of a pharmacological dose of ACTH (4.2 micrograms/kg) and a physiological dose of ACTH (0.0084 microgram/kg) for 120 min, and infusion of dibutyryl cyclic AMP (DBcAMP) [0.33 mg/kg/min] for 20 min, in 22 normal subjects with or without verapamil treatment (360 mg/day, orally) for 5 days. The subjects were pretreated with 1.0 mg of dexamethasone and 5.0 mg of enalapril daily for 2 days before each infusion test to inhibit endogenous ACTH and angiotensin II. Following infusion of 0.0084 microgram/kg of ACTH, plasma levels of corticosterone (P less than 0.02) and cortisol (P less than 0.01) were significantly increased; with chronic verapamil treatment plasma levels of corticosterone (P less than 0.05) and cortisol (P less than 0.02) were significantly lower than those without verapamil. On the other hand, 4.2 micrograms/kg of ACTH for 120 min significantly increased the plasma levels of several steroid hormones, although there were no differences between the infusion with and without verapamil. Infusion of DBcAMP for 20 min significantly increased plasma levels of corticosterone (P less than 0.02) and cortisol (P less than 0.01), but verapamil did not affect the steroidogenic response to the DBcAMP infusion. The present results suggest that steroidogenesis induced by a physiological dose of ACTH differs from that after a pharmacological dose of ACTH or DBcAMP, and is mediated mainly by calcium ion as an intracellular messenger in man.

Sources of extramitochondrial corticoidogenic cholesterol in the adrenal cortex

Intracellular sources of extramitochondrial corticoidogenic cholesterol in bovine, rat and hamster adrenocortical cells were examined in vitro by comparing the species differences in the effects of various inhibitors on the adrenocorticotropic hormone (ACTH)-induced corticoidogenesis. The inhibitors were ML-236B (3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor), W-7 (N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide; calmodulin inhibitor), dichlorvos (O,O-dimethyl-2,2-dichlorovinyl phosphate; organic phosphorylation inhibitor), chloroquine [7-chloro-4-4-diethylamino-1-methyl-butylamino) quinoline; lysosomal enzyme inhibitor) and cycloheximide (protein synthesis inhibitor). During 2 to 3 hr incubation periods, the ACTH-induced corticoidogenesis was not inhibited by ML-236B (100 microM) in the bovine and rat adrenocortical cells. In the hamster adrenocortical cells, ML-236B (100 microM) did not affect the ACTH-induced corticoidogenesis during the initial 1 hr incubation periods; but thereafter, the ACTH-induced corticoidogenesis during the subsequent 2 hr incubation periods was completely blocked by ML-236B. The ACTH-induced corticoidogenesis was inhibited by W-7 (up to 25 microM) in the bovine and rat adrenocortical cells, but this was not the case in the hamster cells. Chloroquine (up to 400 microM) inhibited the ACTH-induced corticoidogenesis in the adrenocortical cells of three different species, but the hamster adrenal cells were much more vulnerable than the bovine and rat cells. The ACTH-induced corticoidogenesis in the adrenocortical cells of three different species were equally inhibited by cycloheximide (up to 1 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of adrenaline on steroidogenesis in primary cultured bovine adrenocortical cells

In primary 2-day cultured bovine adrenocortical cells, adrenaline stimulated the steroidogenesis, while the effect of adrenaline did not appear in the freshly isolated cells. Thus the primary 2-day cultured cells were used to study the effect of adrenaline on steroidogenesis. Adrenaline showed the steroidogenesis-stimulating effect at concentrations higher than 10(-9) M, and the maximum effect was obtained between 10(-6) M and 10(-5) M in the primary 2-day cultured cells. The maximum effect of adrenaline was 50-70% of that of adrenocorticotropic hormone (ACTH). Noradrenaline, isoproterenol and phenylephrine also stimulated the steroidogenesis. However, the order of the potency was isoproterenol much greater than adrenaline = noradrenaline much much greater than phenylephrine. Propranolol and alprenolol inhibited the effect of adrenaline, but phenoxybenzamine and phentolamine did not inhibit the effect. Moreover, adrenaline stimulated the cyclic AMP production dose-dependently at concentrations higher than 10(-8) M. These results suggest that there are steroidogenesis-linked adrenergic receptors in primary 2-day cultured bovine adrenocortical cell membrane and that the steroidogenesis-stimulating effect of adrenaline occurs through the beta-adrenergic receptor.

Cortisol production by dispersed guinea-pig adrenal cells; a specific, sensitive and reproducible response to ACTH....and its fragments

Naturally occurring steroids and peptide hormones, tested at supraphysiological concentrations, were without effect on basal and human (h) 1-39 ACTH (NIBSC code 74/555, 25 ng/l (5.5 X 10(-12) mol/l] stimulated cortisol production. Further, low concentrations of angiotensin II, N-pro-opiocortin (N terminal fragment 1-76) and gamma-MSH all of which have been reported to synergise with ACTH with regard to cortisol production, were without significant effect alone or in combination with ACTH over the range 2.2 X 10(-13) to 5.5 X 10(-12) mol/l. The activity of h 1-39 was compared with that of the ACTH related peptides 1-24, 1-18, 1-17, 1-16, 1-13-NH2 (alpha MSH), 1-10 and 4-10. The dose responses were parallel and the same maximal cortisol output was observed with all the peptides except the 1-10 fragment. Half maximal stimulation occurred at 3.1 X 10(-12) (1-24), 4.4 X 10(-12) (h 1-39), 1.5 X 10(-11) (1-39), 3.3 X 10(-10) (1-18), 5 X 10(-9) (1-13-NH2), 8 X 10(-9) (1-17), 2 X 10(-7) (1-16) and 1 X 10(-5) (4-10) mol/l respectively. Interference by the above ACTH-derived peptides in cortisol secretion by the cells in response to 5.5 X 10(-12) mol/l h 1-39 ACTH was minimal over the range 5.2 X 10(-12)-2.2 X 10(-6) mol/l. The sensitivity of the adrenal cells to h 1-39 ACTH was such that 2 ng/l (4.4 X 10(-13) mol/l) provoked cortisol secretion over the control (P less than 0.05, n = 17). The coefficient of variation within assay for each dose on the full standard curve (2.2 X 10(-13)-1.1 X 10(-10) mol/l) was less than 10% (n = 6). Half maximal stimulation was given by 14.5 ng/l (3.2 X 10(-12) mol/l). Between control and 1.1 X 10(-10) mol/l ACTH there was a 32 +/- 8 (mean +/- SD, n = 9) fold change in cortisol production.

Corticotropin-induced desensitization: steroidogenic and cyclic AMP responses in superfused adrenocortical cells

The characteristics of the sensitization and desensitization of superfused, rat dissociated adrenal fasciculata cells were examined. The dynamic output of steroids and cyclic AMP was determined following pulsed treatment with various agonists. Repeated doses of identical small amounts of ACTH or ACTH gave gradually increasing responses which were maximal after 3-4 injections, but then desensitized the adrenal cells. An initial dose of 2.5 X 10(-11) moles of ACTH, itself insufficient to stimulate steroidogenesis in this system, had the effect of priming the cells, which showed an enhanced initial response and achieved maximum responsiveness on the second injection of 4 X 10(-13) moles ACTH. Thereafter, although the cells exhibited a diminishing steroid output, a dose at the end of the experiment of 8 X 10(-12) moles ACTH restored the maximum responsiveness, and demonstrated that cell loss or death could not account for the desensitization effect. Only a sensitization of the cells was observed following repeated doses of 5 X 10(-6) moles cyclic AMP. Since no desensitization effect was discernible for this agonist, it was concluded that in this system the lesion giving rise to the desensitization effect occurred prior to the adenylate cyclase catalytic unit for the generation of cyclic AMP within the cell and the receptor-nucleotide-regulatory protein complex is thus implicated in the desensitization mechanism for adrenal steroidogenesis. The studies demonstrate the exquisite sensitivity of adrenal cells to the desensitizing effects of even brief intermittent pulses of ACTH.

Corticotropin receptors, cyclic AMP and steroidogenesis

The detection and characterization of the physiologically relevant receptors for corticotropin (ACTH) in rat adrenocortical cells is described. By the use of a radioligand with full biological potency and high specific radioactivity (1800 +/- 75 Ci/mmol), a single class of receptors with an apparent Kd of 1.41 +/- 0.21 nM was detected and the number of sites was estimated to be 3840 +/- 1045 per cell. The binding curve was superimposable on the concentration-response curve for cAMP but dissociated from that for steroidogenesis. These data are best explained by the receptor-reserve model in which occupancy of a small fraction of the receptors is sufficient for inducing maximal steroidogenesis.

Characterization of corticotropin receptors on adrenocortical cells

The binding of corticotropin (ACTH) to receptors on isolated rat adrenocortical cells was investigated with the aid of [[125I]ITyr23, Phe2, Nle4]ACTH-(1-38) (125I-ACTH analog) which retained full biological potency and had a specific radioactivity of 1800 +/- 75 Ci/mmol. Binding was highly specific to adrenocortical cells, and the radioactive peptide was displaced by low concentrations of ACTH but not by other basic peptides. Binding was rapid, reversible, and linearly related to the number of cells. 125I-ACTH analog was not significantly degraded by incubation with the cells at 23 degrees C for 1 hr. Scatchard analysis of the binding was compatible with a single class of binding sites with Kd = 1.41 +/- 0.21 nM, and the number of sites was estimated to be 3840 +/- 1045 per cell. The binding curve was superimposable on the concentration-response curve for cyclic AMP. Small, but significant amounts of 125I-ACTH analog were bound at concentrations sufficient for maximal stimulation of steroidogenesis. For a series of ACTH analogs, the concentrations of the peptides required for half-maximal stimulation of cyclic AMP production were in excellent agreement with the concentration required for half-maximal inhibition of binding. These results suggest that the adrenocortical cells contain only one class of ACTH receptors and that stimulation of a small fraction of these receptors (less than 3%) is sufficient for maximal steroidogenesis.

Comparison of the structural features of corticotropin required for stimulation of steroidogenesis and cAMP production in rat and rabbit adrenocortical cells

The steroidogenic activities of ACTH, alpha-MSH, beta-MSH as well as analogs of the hormones have been compared in rat and rabbit adrenocortical cells. ACTH is equally active in both species and the melanotropins have very low steroidogenic potency in either species. The steroidogenic potencies of the peptide analogs are strikingly similar in the two species, suggesting that the structural requirements for eliciting steroidogenesis are the same in rat and rabbit adrenocortical cells. The analog NPS-ACTH has low, comparable steroidogenic activity in both species. NPS-ACTH is a potent antagonist of ACTH-induced cAMP production in rat adrenocortical cells but acts as a weak partial agonist in rabbit adrenocortical cells. These results suggest that steroidogenesis may be mediated by receptors different from those involved in the cAMP response observed at supraphysiological concentrations of ACTH.

Steroidogenic action of calcium ions in isolated adrenocortical cells

The corticotropin-induced increase of total intracellular and receptor-bound cyclic AMP in isolated rat adrenocortical cells was strictly dependent on extracellular Ca(2+). A rise in bound cyclic AMP with rising Ca(2+) concentrations was accompanied by a decrease in free cyclic AMP-receptor sites. A Ca(2+)-transport inhibitor abolished the rise in bound cyclic AMP induced by corticotropin. These data suggested that during stimulation by corticotropin some Ca(2+) has to be taken up in order to promote the rise of the relevant cyclic AMP pool. In agreement with this view, adenylate cyclase activity from isolated cells proved also to be dependent on a sub-millimolar Ca(2+) concentration in the presence of corticotropin and GTP. When cells were treated under specific conditions, corticosterone production could be activated by Ca(2+) in the absence of corticotropin (cells primed for Ca(2+)). Ca(2+)-induced steroidogenesis of these cells, in the absence of corticotropin, was also accompanied by an increase in total intracellular and receptor-bound cyclic AMP, as was found previously with corticotropin-induced steroidogenesis in non-primed cells. Calcium ionophores increasing the cell uptake of Ca(2+) were not able, however, to increase the cyclic AMP pools in non-primed cells, unlike corticotropin in nonprimed cells or Ca(2+) in cells primed for Ca(2+). It was concluded that during stimulation by either corticotropin or Ca(2+) a possible cellular uptake of Ca(2+) must be very limited and directed to a specific site which may affect the coupling of the hormone-receptor-adenylate cyclase complex.