Search PubMed⌕ Search

Biomedical subjects

A Enjalbert

Publications and source records attributed to A Enjalbert.

At least 109 records · Page 6Linked to original sources
PubMed1986

Involvement of endogenous somatostatin in the regulation of thyrotroph secretion during acute and chronic changes in diet.

The aim of this study was to investigate the involvement of somatostatin (SRIF) in the thyrotroph adaptation to nutritional changes. For this purpose, we studied the effects of passive immunization with SRIF antiserum (A-SRIF) on the reduced basal TSH secretion in rats starved for 72 h and on the plasma TSH surge following carbohydrate (CHO) refeeding. This latter experiment was performed at two different times of the day in order to elucidate whether SRIF may participate in the regulation of the plasma TSH circadian rhythm. In chronically catheterized rats, we observed that A-SRIF injection induced a similar pattern of plasma TSH rise over a sampling period of 5 1/2 h in both fed and rats starved for 72 h (3-way analysis of variance). In morning experiments, CHO refeeding or A-SRIF injection elicited a significant rise in plasma TSH. The amplitude and duration of the response was proportional to the injected dose. In evening experiments, although basal TSH values were significantly lower than those observed in the morning ones, maximal plasma TSH values after A-SRIF injection were not significantly different. At both times of the day, association of refeeding and A-SRIF injection did not stimulate TSH further than either refeeding alone or A-SRIF alone. In conclusion, our data suggest that SRIF cannot account for the differences in serum TSH levels between fed and starved rats; is not responsible for the diurnal difference in basal serum TSH in starved rats, and seems to be involved in the TSH response to refeeding.

Animals↗
PubMed1986

Sensitivity of thyrotropin (TSH) secretion to 3,5,3'-triiodothyronine and TSH-releasing hormone in rat during starvation.

The mechanisms by which plasma T3 and TSH decrease after a 3-day starvation period are not completely understood. In this study we tested the hypothesis of a possible modification in the sensitivity of thyrotroph cell to T3 and/or TRH. For that purpose, TRH tests were performed before and after a 3-day starvation in euthyroid, thyroidectomized, and T3-treated (75 or 175 ng/100 g BW) thyroidectomized male Wistar rats. TRH (10 to 500 ng/100 g BW) was injected iv through a chronically-implanted catheter. In another set of experiments, hypophyseal TSH content was also determined. Our results showed that after a 3-day-starvation plasma TSH decreased in all except hypothyroid rats; TSH responsiveness to TRH was unchanged in euthyroid rats but was increased in hypothyroid rats; and the T3-dependent increase in TSH responsiveness to TRH was significantly amplified. Moreover, there was a significant positive correlation between TSH responsiveness to TRH and hypophyseal TSH content. These results suggest that starvation induces an increased sensitivity of thyrotroph cell to T3.

Animals↗
PubMed1986

Differential effects of passive immunization with somatostatin antiserum on adenohypophysial hormone secretions in starved rats.

The role of somatostatin (SRIF) on adenohypophysial hormone secretion in starved rats was reassessed by passive immunization. Because of the absence of pulsatile GH secretion in starved rats, the effects of the injection of SRIF antiserum on GH levels can be clearly demonstrated. To determine whether starvation modifies the sensitivity of the adenohypophysis to SRIF, we measured 125I-labelled iodo-N-Tyr-SRIF binding. There was no difference in the dissociation constant (Kd) nor in the maximal binding capacity (Bmax) in fed (n = 15) and starved (n = 15) animals (Kd = 0.38 +/- 0.09 (S.E.M.) and 0.45 +/- 0.09 nmol; Bmax = 204 +/- 39 and 205 +/- 30 fmol/mg protein respectively). Administration of SRIF antiserum resulted in a dose-dependent increase in plasma concentrations of GH, TSH and prolactin. The minimal effective dose of SRIF antiserum was 50 microliters for GH, 100 microliters TSH and 200 microliter for prolactin. Our results show that: starvation does not modify adenohypophysial SRIF-binding sites, in starved male rats endogenous SRIF exerts a negative control on prolactin secretion in vivo and sensitivity to endogenous SRIF seems to be different for each hypophysial cell type.

Animals↗
PubMed1985

Somatostatin receptors in human growth hormone and prolactin-secreting pituitary adenomas.

[125I-Tyr]Somatostatin [( 125I-Tyr]SRIH) binding was found in 11 GH-secreting pituitary adenomas [Kd = 0.46 +/- 0.15 (+/- SE) nM; maximum binding, 165 +/- 35 fmol/mg protein). This binding was specific, since it was displaced by somatostatin-14 (SRIH-14), N-Tyr-SRIH-14, and SRIH-28. In contrast, a number of peptides and drugs not structurally related to SRIH, such as bombesin, dopamine, LHRH, met-enkephalin, naloxone, neurotensin, secretin, substance P, TRH, or vasoactive intestinal peptide, did not affect [125I-Tyr]SRIH binding. [125I-Tyr]SRIH specific binding also was found in PRL-secreting pituitary adenomas. The kinetic characteristics of the specific binding were similar to those of GH-secreting adenomas. However, maximal binding was one quarter that of GH-secreting adenomas (37 +/- 9 fmol/mg protein). In contrast, nonsecreting (chromophobe) tumors were devoid of any specific binding. Finally, in acromegaly, the density of [125I-Tyr]SRIH-binding sites in the adenomas was negatively correlated with plasma GH levels before surgery (r = -0.80). This suggests that somatostatinergic control is involved in GH secretion in acromegalic patients.

Acromegaly↗
PubMed1984

Subcellular distribution of somatostatin-14, somatostatin-28 and somatostatin-28 (1-12) in rat brain cortex and comparisons of their respective binding sites in brain and pituitary.

Subcellular distribution and binding characteristics of the three endogenous peptides somatostatin-14 (SRIF-14), somatostatin-28 (SRIF-28) and somatostatin-28(1-12) (SRIF-28(1-12] derived from preprosomatostatin were investigated in the rat brain cortex. The three peptides are predominantly recovered from a crude mitochondrial pellet (P2), containing the pinched off nerve endings. Specific high affinity binding sites for 125I-N-Tyr-SRIF-14 and 125I-N-Tyr-SRIF-28 are present on pituitary and brain membranes. Under the same conditions, 125I-N-Tyr-SRIF-28(1-12) binding is undetectable. Moreover, SRIF-28(1-12) does not displace 125I-N-Tyr-SRIF-14 or 125I-N-Tyr-SRIF-28 binding. SRIF-28 is more potent than SRIF-14 to displace 125I-N-Tyr-SRIF-28 binding to brain and pituitary membranes, while both peptides are equipotent to displace 125I-N-Tyr-SRIF-14 binding. Finally, the regional distribution of 125I-N-Tyr-SRIF-14 and 125I-N-Tyr-SRIF-28 binding sites in the brain is identical. In conclusion, the present results are consistent with a neurotransmitter and neurohormonal role for SRIF-14 and SRIF-28. The function of SRIF-28(1-12) in brain remains to be elucidated. Additionally, a differential role for SRIF-14 and SRIF-28 both in adenohypophysis and brain cannot be ascertained at the present time.

Animals↗
PubMed1983

Guanine nucleotide sensitivity of [125I]-Iodo NTyr somatostatin binding in rat adenohypophysis and cerebral cortex.

Specific [125I]-Iodo-NTyr somatostatin binding sites are present in adenohypophyseal and cerebral cortical membranes. Guanine nucleotides reduce the maximal binding capacity of adenohypophyseal binding sites without significantly affecting their apparent affinity. In pituitary as well as in cortex, GTP is the most potent nucleotide followed by GDP and guanylyl imidodiphosphate (GMP-PNP). The effect appears specific of guanine nucleotides since ATP, ADP and AMP are inactive on [125I]-Iodo-NTyr somatostatin binding. These results, showing the nucleotide sensitivity of [125I]-Iodo-NTyr somatostatin binding in pituitary and cerebral cortex, are compatible with a coupling of somatostatin receptors with adenylate cyclase.

Animals↗
PubMed1983

Pharmacological characterization of the D2 dopamine receptor negatively coupled with adenylate cyclase in rat anterior pituitary.

In male and female rat anterior pituitary homogenates dopamine inhibited basal adenylate cyclase by 30% and 50%, respectively. Dopamine also inhibited vasoactive intestinal peptide-stimulated adenylate cyclase by 50% in both sexes. Sulpiride, a specific D2 antagonist, stereospecifically blocked with high affinity the dopamine inhibition in both males and females. RU 24926, a specific, non-catechol, non-ergot D2 agonist, also inhibited basal adenylate cyclase of female pituitary with a higher apparent affinity than dopamine (KDapp 20 nM and 450 nM, respectively). This effect was also stereospecifically antagonized by sulpiride. Apomorphine was also more potent (KDapp 100 nM) than dopamine, whereas norepinephrine and SKF 38393, a specific D1 agonist, were poorly active; isoproterenol and clonidine were inactive. Ergots derivatives such as CB 154, LY 14865, pergolide, and lergotrile were potent agonists. alpha-Dihydroergocryptine was a partial agonist of the dopamine receptor negatively coupled with an adenylate cyclase. Because of the slow association kinetics of this drug with the dopamine receptor, its KDapp (0.7 nM) for adenylate cyclase inhibition could be correctly determined only after a 30-min incubation period. All classical dopaminergic antagonists blocked dopamine inhibition of pituitary adenylate cyclase, pimozide (KI 1 nM) and spiperone (KI 0.8 nM) being the more potent. There were good correlations between the affinities of large series of agonists and antagonists for the anterior pituitary dopamine receptors negatively coupled with an adenylate cyclase on one hand, and for either D2 dopamine receptors labeled with [3H] dihydroergocryptine or [3H]spiroperidol in both pituitary and striatum, or D2 pituitary receptors involved in prolactin secretion on the other hand. It is concluded that the pituitary dopamine receptors negatively coupled with an adenylate cyclase are the classical D2 receptors involved in prolactin secretion.

Adenylyl Cyclases↗
PubMed1982

Neurotensin stimulation of prolactin secretion in vitro.

Neurotensin stimulated prolactin (PRL) secretion from incubated rat hemipituitaries. Under the same conditions, the secretion of growth hormone, luteinizing hormone and follicle-stimulating hormone was not affected. The stimulation of PRL was dose dependent, with an apparent affinity of neurotensin of 0.56 +/- 0.12 nM and a maximal stimulation of 56.5 +/- 6.7%. The effect of neurotensin seemed to be independent of that of other PRL releasing factors. In fact, the stimulation of neurotensin and thyrotropin-releasing hormone (TRH) and also of neurotensin and vasoactive intestinal peptide were additive. The action of neurotensin on PRL cells does not appear to involve either dopamine or gamma-aminobutyric acid receptors, since antagonists to these transmitters were found ineffective on PRL stimulation by neurotensin. PRL-releasing factor activity distinct from TRH has been described in fractions of hypothalamic extracts. Neurotensin, which is present in high amounts in the median eminence and has been measured in the adenohypophysis, is a candidate as a physiological PRL-releasing factor distinct from TRH.

Animals↗
PubMed1982

Somatostatin receptors on rat anterior pituitary membranes.

[125I]Iodo-Tyr1-somatostatin (SRIF) binds with high affinity to one class of sites in the rat anterior pituitary with a KD of 0.91 +/- 0.22 nM and a receptor concentration of 104.4 +/- 1.9 fmol/mg protein. This binding is saturable with respect to tissue concentration and is time-, temperature-, pH-, and calcium-dependent. It is also reversible as a function of time. The rates of association and dissociation were calculated to be 5.98 X 10(7) M-1 min-1 and 0.578 min-1, respectively. Binding of [125I]iodo-Tyr1-SRIF is not inhibited by morphine, beta-endorphin, [D-Ala2]Met-enkephalin, LHRH, TRH, histidylproline diketopiperazine, neurotensin, substance P, bombesin or vasoactive intestinal peptide. In contrast SRIF, [Tyr1]SRIF, and [D-Trp8,D-Cys14]SRIF displace [125I]iodo-Tyr1-SRIF binding with Ki values 0.10 +/- 0.05, 0.46 +/- 0.18, 0.05 +/- 0.01 nM, respectively. The constants of inhibition of a series of alanine monosubstituted analogs of SRIF are correlated (r = 0.89) with their biological potency on GH secretion. Furthermore, postnatal development patterns of [125I]iodo-Tyr1-SRIF binding sites follow the ability of SRIF to inhibit GH release. Thus, [125I]iodo-Tyr1-SRIF binding to adenohypophyseal membranes seems to reflect interaction with SRIF receptors on adenohypophyseal cells. Since biological effects of the peptide have been reported on GH, thyrotropin-stimulating hormone, and PRL secretion, further studies are required to determine the cell types upon which this binding occurs.

Animals↗
PubMed1982

Reciprocal interactions of somatostatin with thyrotropin-releasing hormone and vasoactive intestinal peptide on prolactin and growth hormone secretion in vitro.

Reciprocal interactions of somatostatin (SRIF) and vasoactive intestinal peptide (VIP) or TRH on in vitro PRL and GH release from male rats hemipituitaries were investigated. SRIF did not modify basal PRL release, but TRH- or VIP-induced release was inhibited by SRIF in a dose-dependent manner [effective concentration-fifty (EC50) = 1.7 +/- 0.9 nM for SRIF inhibition of TRH stimulation and EC50 = 0.8 +/- 0.5 nM for SRIF inhibition of VIP stimulation]. VIP and TRH did not affect GH release by themselves, but reduced the inhibition of GH secretion elicited by SRIF (EC50 = 7.6 +/- 3.4 nM for TRH blockade of SRIF inhibition and EC50 = 4.6 +/- 3.1 nM for VIP blockade of SRIF inhibition). Secretin, a partial structural analog of VIP, also blocked SRIF-induced inhibition of GH and stimulated PRL release. Secretin stimulation of PRL release was also prevented by SRIF. [D-Trp8,D-Cys14]SRIF, a potent analog of SRIF, antagonized VIP stimulation of PRL secretion with the same apparent affinity as the native peptide. The maximal stimulation, but not the apparent affinity of VIP action on prolactin release was reduced by SRIF, suggesting that the interaction is of a noncompetitive nature. This conclusion as further substantiated by the observation that neither TRH nor VIP were able to displace specific 125I-labeled [Tyr1] SRIF high affinity binding to pituitary membranes. The three peptides tested thus appear to exhibit reciprocal interactions mediated by independent receptor sites on GH as well as on PRL-producing cells.

Animals↗