Biomedical subjects

M J Fullerton

Publications and source records attributed to M J Fullerton.

23 records · Page 2Linked to original sources

PubMed1988

Studies of the regulation of the hypothalamic-pituitary-adrenal axis in sheep with hypothalamic-pituitary disconnection. I. Effect of an audiovisual stimulus and insulin-induced hypoglycemia.

These studies were undertaken to characterize the secretion of adrenocorticotropin (ACTH), immunoreactive (ir) beta-endorphin (ir-beta-EP) and ir alpha-melanocyte-stimulating hormone (ir-alpha-MSH) from the surgically isolated ovine pituitary in response to an audiovisual stress (barking dog, 3 min) and insulin hypoglycemia. The studies were performed in 4 ovariectomized, hypothalamo-pituitary-disconnected (HPD) and 4 sham-HPD ewes bearing indwelling jugular venous catheters. Basal concentrations of the three pro-opiomelanocortin (POMC) peptides and plasma cortisol were significantly increased in the HPD animals. When the control ewes were exposed to the audiovisual stimulus, plasma ACTH, ir-beta-EP and ir-alpha-MSH levels were increased 2.5-, 10-, and 5-fold 1 min after the stress; plasma cortisol attained maximal values at 5 min. In contrast, plasma levels of the three POMC peptides were not significantly increased in the HPD animals, although a rise in plasma cortisol occurred. The administration of regular insulin (5 units/kg i.v.) to control ewes caused plasma ACTH, ir-beta-EP, and ir-alpha-MSH levels to increase 17-, 22-, and 67-fold at 50 min; plasma cortisol values were maximal at 60 min. In contrast, the elevated basal levels of POMC peptides in the HPD animals were not significantly increased by the hypoglycemia, but a significant elevation of plasma cortisol was seen. We conclude that: (1) the increase in ACTH in intact animals after an audiovisual emotional stress and hypoglycemia, and the abolition of this increase by HPD, indicates that both stimuli, each acting through distinct neuroanatomical pathways, increase the net corticotropin-releasing activity of the hypothalamus; (2) the rise in plasma cortisol in HPD animals after stress suggests that peripheral humoral factors may release additional small amounts of ACTH from the anterior pituitary, and (3) the finding of increased basal ACTH levels after HPD suggests that POMC peptide synthesis and secretion by the anterior pituitary is tonically regulated by an inhibitory factor of hypothalamic origin.

Adrenocorticotropic Hormone↗

PubMed1987

Posttranslational processing of corticotropin-releasing factor in the ovine tuberoinfundibular system and pituitary.

The present studies were undertaken to characterize the immunoreactive-corticotropin-releasing factor (ir-CRF) in two areas of the ovine tuberoinfundibular system, hypophysial portal blood, and pituitary. With an antiserum raised against synthetic ovine (o)CRF(1-41) and 125I-Tyro-oCRF(1-41) as the tracer, concentrations of ir-CRF (pg/mg wet weight, n = 5) were: paraventricular hypothalamus (PVN), 11.7 +/- 2.5; median eminence (ME), 2276 +/- 296; anterior pituitary (AP), less than 0.5; posterior pituitary (PP), 10.0 +/- 2.2. Analysis of the ir-CRF in these areas on G-75 Sephadex chromatography revealed two main peaks--a 'major' peak which coeluted with synthetic oCRF(1-41) and a 'minor' peak which eluted eight fractions later. These two immunoreactive species of CRF were also found in hypophysial portal blood. When ME extract was analyzed by reverse-phase high performance liquid chromatography (HPLC), the 'minor' peak of ir-CRF eluted before that of CRF(1-41). Since CRF contains Arg35-Lys36 within its sequence, we tested the hypothesis that the 'minor' peak of ir-CRF represented a fragment, or fragments, of the molecule derived by proteolytic cleavage at this site. Tyro-oCRF(34-41) was digested with trypsin and the reaction products were identified by amino acid analysis. Two of these products were CRF(36-41) and CRF(37-41), and both migrated in the 'minor' peak area on G-75 Sephadex chromatography and HPLC. In the CRF(1-41) RIA, serial dilution of both fragments yielded nonparallel displacement curves. However, with 125I-Tyro-oCRF(34-41) as the radiolabeled ligand and Tyro-oCRF(34-41) as the standard, serial dilutions of CRF(1-41), CRF(36-41), and CRF(37-41) generated parallel displacement curves, and the molar cross-reactivities were 90%, 45% and 10% respectively. When the ir-CRF in HPLC fractions of ovine ME was measured in the Tyro-oCRF(34-41) RIA, the molar abundance of the hexapeptide and pentapeptide could be obtained. Calculations based on the premise that the 'minor' peak was solely composed of either the hexapeptide or pentapeptide indicated that CRF(36-41) could account for up to 37% of the total ir-CRF, or that CRF(37-41) could account for up to 73% of the total immunoreactivity. On more discriminating HPLC systems, immunoreactive (ir-) oCRF in the sheep median eminence (ME) could be resolved into five different molecular forms. On three distinct chromatographic systems, four of these immunoreactive species shared retention characteristics identical with synthetic oCRF(37-41), oCRF(36-41), oCRF(16-41) and oCRF(1-41), with the fifth immunoreactive peak as yet unidentified.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1986

Mechanisms of biphasic action of glucocorticoids on alpha-lactalbumin production by rat mammary gland explants.

The highly selective Type II glucocorticoid ligand RU28362 showed a clear biphasic effect on alpha-lactalbumin (alpha-LA) production in rat mammary gland explants, with a peak at 1 nM and a return to basal levels at 30-300 nM; dexamethasone showed a similar profile. Corticosterone, which has a higher affinity for Type I than Type II receptors, produced a variable response. In six out of eleven studies this was biphasic, with a maximum at 300 nM; in five no increase above baseline was seen. Classical Type I receptor ligands--aldosterone and deoxycorticosterone--showed responses parallel to their Type II agonist activity. We interpret these data as follows occupancy of Type I receptors does not increase alpha-LA production the response to selective Type II receptor ligands is truly biphasic and one explanation of this pattern may be the existence of both "turn-on" and "turn-off" acceptor sites in the nucleus.

Aldosterone↗

PubMed1986

Specificity and mechanism of biphasic action of glucocorticoids on alpha-lactalbumin production by rat mammary gland explants.

RU26988 [11 beta, 17 beta-dihydroxy-17 alpha-(1-propynyl)androsta-1,4,6-trien-3-one], a specific type II (glucocorticoid) ligand with negligible affinity for type I (mineralocorticoid) receptors, increased alpha-lactalbumin production in mammary gland explants from midpregnant rats when cultured in vitro in the presence of insulin (5 micrograms/ml) and rat PRL (1 microgram/ml). The dose-response curve was biphasic, over a 3-300 nM dose range, with maximum at 3-10 nM, followed by a progressive decline toward or to control levels at 300 nM. A similar dose-response curve was obtained with another type II-specific ligand, RU28362 [11 beta, 17 beta-dihydroxy-6-methyl-17 alpha-(1-propynyl)androsta-1,4,6-trien-3-one], and with dexamethasone. This biphasic response had been previously reported for mouse mammary gland explants using cortisol, which binds to both type I and type II receptors. Our experiments, therefore, show that occupancy of type II receptors alone can be responsible for the stimulation at low steroid concentrations, followed by the decrease from peak levels observed at higher steroid concentrations. On the basis of these data, we propose a model to account for these findings, based on ligand binding to a single class of receptors and the putative existence of both turn-on and turn-off glucocorticoid regulatory elements in the nucleus.

Androstanols↗

PubMed1985

Progestins specifically suppress alpha-lactalbumin synthesis and secretion.

Mammary gland explants from pregnant (day 12-15) rats were cultured with insulin and prolactin, and their content and secretion of alpha-lactalbumin determined after exposure to a wide range of doses (0.01-300 nM) of the specific synthetic progestin (ORG2058), alone or with a maximally stimulatory dose of the highly specific glucocorticoid RU26988. ORG2058 alone suppressed alpha-lactalbumin synthesis below baseline, with a half-maximal effect at a concentration of less than 0.1 nM; RU26988-stimulated secretion was similarly abrogated by ORG2058, similarly with a half maximally effective dose of less than 0.1 nM. We interpret these data as suggesting that (i) given the specificity and doses of the steroids used the effect of progestins on alpha lactalbumin synthesis is directly via progesterone receptor occupancy, and not by competing with glucocorticoids for glucocorticoid receptors and (ii) given the shift to the left in the alpha-lactalbumin response (half maximal less than 0.1 nM ORG2058) compared with receptor binding (Kd (37 degrees C) greater than 1 nM), one possible model for such sensitivity is that of multiple, independent regulatory elements on the chromatin controlling alpha-lactalbumin gene expression, occupancy of any one of which by an activated progesterone receptor is sufficient to abrogate transcription.

Androstanols↗