Relationship between ACTH secretion and corticoid binding to specific receptors in perifused adenohypophyses

Is the mineralocorticoid receptor in Brown Norway rats constitutively active?

In a previous study using corticosterone treatment of adrenalectomized rats, we hypothesized that mineralocorticoid receptor (MR)-related mechanisms are constitutively active and that glucocorticoid receptor (GR)-mediated mechanisms are more efficient in Brown Norway rats compared to Fischer 344 (F344) rats. In order to discriminate the mineralocorticoid from the glucocorticoid actions exerted by corticosterone, F344 and Brown Norway adrenalectomized rats were treated with increasing doses (1, 5 and 25 microg/ml of drinking water) of deoxycorticosterone (DOC, MR-specific ligand) or RU 28362 (GR-specific ligand). These rats were compared with long-term adrenalectomized (ADX) untreated rats and sham-ADX rats. This study confirms our previous results, notably the lack of effect of ADX on body weight and fluid intake in Brown Norway rats. Moreover, DOC treatment had no effect in Brown Norway rats whereas the higher dose restored fluid intake of the F344 ADX group to sham values. These results support the hypothesis of a constitutive activation of the MR and therefore the insensitivity of this receptor to its ligand in Brown Norway rats. Alternatively, RU 28362 treatment induced greater weight loss, decrease in food intake, anxiolysis, thymus involution, and decrease in plasma transcortin concentration and pituitary corticosteroid receptor densities in Brown Norway rats than in F344 rats, which is consistent with greater efficiency of GR mechanisms in Brown Norway rats than in F344 rats. Therefore, these strains are of great utility to disentangle MR and GR effects on complex phenotypes.

Indirect relationship between vasopressin-induced secretion of ACTH and cyclic nucleotides in cultured anterior pituitary cells

The present study examines whether a correlation exists between cyclic nucleotides and the mechanism of action of arginine vasopressin (AVP) on adrenocorticotropin hormone (ACTH) secretion from pituitary corticotrophs. Incubation of cultured anterior pituitary cells with 3-isobutyl-1-methylxanthine (IBMX) or Rolipram elevated the basal intracellular content of both adenosine 3':5'-cyclic monophosphate (cAMP) and guanosine 3':5'-cyclic monophosphate (cGMP) or cAMP alone, respectively. Both IBMX and Rolipram enhanced the AVP-stimulated secretion of ACTH in cultured anterior pituitary cells, but not in AtT-20 corticotrophs which lack functional AVP receptors. Rolipram was less potent than IBMX in this regard, which suggests a possible involvement of cGMP. In contrast, both drugs showed similar potency to stimulate CRF-induced ACTH secretion. Incubation of pituitary cells with atrial natriuretic factor elevated tissue cGMP levels and increased the ACTH response to AVP. The results of this study show that, although AVP fails to directly affect the levels of cAMP and cGMP in anterior pituitary cells, the stimulatory effect of AVP on ACTH secretion was modulated by the cellular cAMP content.

Effect of nivazol on ACTH secretion by human pituitary corticotrophic tumours in cell culture

The effects of nivazol, a novel steroid which lacks glucocorticoid activity, on ACTH secretion by cell cultures of human pituitary corticotrophic tumours has been investigated. Nivazol (0.002-20 mumol/l) inhibited ACTH secretion by 50-80%, after 4 and 24 h of incubation. (Trilostane (0.3-30 mumol/l) did not affect basal or stimulated ACTH secretion.) The potency of nivazol was comparable with that of hydrocortisone, and less than dexamethasone. The stimulatory effects of oCRF and AVP were reduced or completely blocked by nivazol in a similar manner to that previously described for hydrocortisone. It is concluded that nivazol may be of benefit in the direct treatment of Cushing's disease, particularly since it lacks glucocorticoid activity.

Cortisol inhibits the ACTH-releasing activity of urotensin I, CRF and sauvagine observed with superfused goldfish pituitary cells

The structurally homologous peptides urotensin I, ovine CRF and sauvagine stimulate the release of immunoreactive ACTH from a superfused dispersed goldfish anterior pituitary cell column. The addition of cortisol to the superfusion buffer resulted, following a latent period, in a decrease in basal release of ACTH from the pituitary cell column and a diminution in the ACTH-releasing activities of urotensin I, CRF and sauvagine. The removal of cortisol from the superfusion buffer resulted in a slow recovery of basal ACTH release and a recovery of the ACTH-releasing activities of urotensin I, CRF and sauvagine. These results are supportive of the view that urotensin I, or a urotensin I-like peptide, serves as a physiological regulator of ACTH release in teleost fishes.

Paradoxical involvement of glucocorticoid receptors in the aldosterone-induced impairment of ACTH secretion from perifused pituitary glands

The present investigation was aimed at examining whether interaction of aldosterone with specific mineralocorticoid receptors at the level of the pituitary gland may account for the inhibitory effect of that steroid on ACTH secretion. By using pituitaries from neonatal rats, which we show to completely lack specific mineralocorticoid receptors but to contain a functional glucocorticoid receptor system, we demonstrated the persistence of aldosterone-induced inhibition of ACTH release from perifused glands. Conversely, when the glucocorticoid receptors sites were blocked in pituitaries from mature rats by means of a potent antiglucocorticoid (RU 38486), thus leaving unaltered mineralocorticoid binder, aldosterone no longer dampened hormonal output. We conclude that the latter steroid affected corticotropic activity by interacting not with its proper and specific receptor, but rather with the glucocorticoid binding sites.

Intracellular CBG-like molecules in the rat pituitary

The localization of transcortin (CBG) in pituitary cells of the rat was investigated using the peroxydase-antiperoxydase (PAP) technique. A rabbit antiserum against purified rat plasma transcortin was used as the primary antiserum. Transcortin-like (CBG-like) immunoreactive products were found in the cytoplasma of certain cells in the anterior pituitary, but not in the intermediate lobe and weakly in the posterior pituitary. It is postulated that the CBG-like molecules participate in the cellular uptake process of corticosterone, thereby modulating the feedback signal of this steroid on pituitary function.

Physicochemical analysis of glucocorticoid receptor in the immature pituitary gland: evidence for lability and aggregation of the heat-activated form

The present study was aimed at characterizing the deficiency of the glucocorticoid-receptor system in the immature pituitary gland of neonatal rats. Under conditions of both constant protein and DNA concentrations, [3H]dexamethasone binding to nuclei was significantly lower in the immature, compared with the mature, glands, despite the comparable density of receptor sites. Chromatography on DEAE-Sephacel columns revealed the failure of the neonatal receptor to form stable activated complexes, although the dissociation of tracer from binding sites, as well as the magnitude of receptor activation, remained closely similar to those found for the adult binder. Also, a striking observation was that the immature receptor exhibited the property of forming more than twice as much inactive aggregate as the mature component, when heat-activated in presence of KCl. It thus seems that development of full expression of glucocorticoid-binding activity in the pituitary involves qualitative changes of both the receptor molecules and the cytoplasmic environment, that tend to enhance stability of the activated form of the binder.

In-vitro secretion of ACTH in Nelson's syndrome

Cell suspensions of ACTH cell adenomas of 10 patients with Nelson's syndrome were investigated for in-vitro secretion of ACTH. Two incubation systems, one using incubation beakers and the other a superfusion system, were employed. The cells were tested for their reactivity to lysine-vasopressin (LVP), cortisol, and combinations of both. LVP regularly provoked a rapid significant increase of ACTH secretion. The effect of cortisol was heterogeneous. Paradoxical initial stimulatory effects of cortisol were observed. There was a suppressive effect in some patients, which correlates to low proliferative activity in histological evaluation. In both systems a low secretory activity coincided with high proliferative activity in vivo.

Modulation by transcortin-like binding sites of uptake and distribution of glucocorticoids by dispersed pituitary cells

The influence of transcortin-like (TL) binding sites on uptake and distribution of glucocorticoids by dispersed pituitary cells was investigated. TL material, which combines corticosterone (CORT), but not dexamethasone (DEX), was previously found to be present on cell membranes and in cytosol of hypophysis. Exposure of cells at 0 degrees brought about striking differences in steroid binding, as labeled CORT was taken up more rapidly and to a significantly greater extent than DEX. This resulted from a higher concentration of binding sites and not from a difference in binding affinity. At 25 and 37 degrees, while the same relationship was apparent during the early events of steroid interaction with the cell, binding of DEX increased gradually with incubation time and finally exceeded that of the natural steroid. Also, time-course studies on nuclear translocation showed a biphasic pattern, which closely paralleled that of whole-cell binding. Interestingly, treatment of rats with transcortin antiserum caused a decrement of pituitary TL sites, as well as of CORT translocation. We conclude that the TL binder, which is probably of plasma origin, may be actively involved in the process of uptake and cellular distribution of corticosteroids in the pituitary gland.