5,6-Epoxyeicosatrienoic acid stimulates growth hormone release in rat anterior pituitary cells

The effect of arachidonic acid and some of its metabolites have been examined in rat anterior pituitary cells for their ability to release growth hormone. The cytochrome P-450 metabolite, 5,6-epoxyeicosatrienoic acid is a much more effective growth-hormone releasing agent than 15-hydroxyeicosatetraenoic acid, 15-hydroxyeicosatetraenoic acid methyl ester, 5-hydroxyeicosatetraenoic acid or arachidonic acid. The release of growth hormone is rapid, dose-dependent and reaches an apparent saturation after eight minutes. These studies described herein provide evidence that lipoxygenase and cyclooxygenase products of arachidonic acid are less potent while cytochrome P-450 products are more potent in the release of growth hormone from anterior pituitary cells.     

Effect of porcine gastrin releasing peptide on anterior pituitary hormone release

The effect of porcine gastrin releasing peptide (GRP), a heptacosapeptide with potent gastrin releasing activity which has recently been isolated from porcine non-antral gastric tissue, on pituitary function was investigated in the rat. Graded doses of synthetic porcine GRP were injected intravenously and the animals were killed at various intervals after injection. Growth hormones, LH, FSH, and TSH were measured in serum by specific radioimmunoassays. GRP had no significant effect on growth hormone or FSH serum concentrations at any dose or sampling time studied. In contrast, the heptacosapeptide significantly stimulated LH and suppressed TSH secretion in a dose-related fashion. Since there are striking structural similarities between GRP and bombesin, a tetradecapeptide from amphibian skin which shows amino acid homology with the C-terminal region of GRP, GRP may be the mammalian counterpart of bombesin.     

Effects of alloxan-induced diabetes on dopaminergic receptors in rat striatum and anterior pituitary

The binding of [3H]-spiroperidol after 4 weeks of hyperglycemia was determined in the rat striatum and anterior pituitary. Alloxan-induced diabetes increased the number of dopaminergic binding sites in the striatum but not in the anterior pituitary. The interaction of metoclopramide with striatal dopaminergic receptors was slightly modified, while that of dopamine, bromocriptine and haloperidol was unaffected. These results suggest that chronic hyperglycemia exerts selective effects on nigrostriatal dopaminergic system in the rat.     

Direct effect of estradiol on the number of dopaminergic receptors in the anterior pituitary, in ovariectomized rats

The effect of estradiol on anterior pituitary dopaminergic receptor content was studied in vivo and in vitro, in relation with the serum PRL secretion. A progressive and significant decrease in the number of these receptors was observed, a few hours before the serum release of PRL induced in ovariectomized females by a sequential treatment with different doses of estradiol benzoate. This decrease in the number of dopaminergic membrane receptors can be obtained as well in vitro, when anterior pituitaries, from ovariectomized rats, are incubated with 17 beta-estradiol. These results suggest that the stimulatory effect of estradiol on PRL secretion may be due, at least in part, to the direct "desensitization" to DA of anterior pituitary cells, which is produced by the decrease of dopaminergic receptor level.     

Studies on a possible pituitary effect of monoamines on luteinizing hormone release in ovariectomized rats

Incubation of anterior pituitaries from ovariectomized rats with LHRH and various concentrations of dopamine, norepinephrine or serotonin indicated that none of these neurotransmitters could decrease pituitary LH secretion in response to the releasing hormone. This indicated that the inhibitions of pulsatile LH release previously observed in our laboratory in ovariectomized rats in response to intraventricularly administered catecholamines or stimulation of brain serotoninergic neurons are due to central rather than pituitary effects of these transmitters.     

Inhibition of release of a novel pituitary polypeptide, 7B2, follicle-stimulating hormone, and luteinizing hormone from rat anterior pituitary cells in vitro by human beta-inhibin

We have recently purified a novel pituitary polypeptide designated 7B2. By raising polyclonal antibodies to a synthetic 7B2 fragment in rabbits, we have developed a sensitive and specific radioimmunoassay for this novel polypeptide, and it has been used for the study of the release of immunoreactive 7B2 from rat anterior pituitary cells in vitro. In addition, immunocytochemical study shows that 7B2 is present in the gonadotropin cells of rat anterior pituitary. The aim of the present studies is to investigate the effect of human beta-inhibin, testosterone, and combined testosterone plus human beta-inhibin on the induced release of immunoreactive 7B2, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) in rat anterior pituitary cell culture in vitro. Our results show that both human beta-inhibin and testosterone effectively suppress the stimulatory effect of luteinizing hormone-releasing hormone (LHRH) on immunoreactive 7B2, FSH, and LH release. The present data indicate that the regulation of secretion of 7B2 and pituitary gonadotropins may be under a similar type of feedback mechanism.     

Hemorrhage-induced regional brain thyrotropin-releasing hormone release in conscious rats measured by microdialysis

The septum, nucleus accumbens and preoptic area in the brains of conscious, freely moving rats were perfused using microdialysis probes. The TRH concentration significantly increased in the septum after withdrawal of 30% of the total blood volume but remained at constant levels in the other brain areas. Also, high potassium dose-dependently stimulated TRH release in vivo. These results suggest that blood loss stimulates septal TRH release, probably by membrane depolarization of TRH-containing nerve terminals.     

Effects of anterior pituitary hormones on hepatic corticosterone metabolism in rats

Experiments were conducted to determine the effects of anterior pituitary hormones on hepatic corticosterone metabolism in rats. Hypophysectomy lowered A-ring reduction but did not affect sidechain metabolism. Administration of prolactin, FSH, LH or FSH + LH to hypophysectomized rats affected neither process. Similarly, ACTH or growth hormone, when given alone, did not affect corticosterone metabolism. However, combined treatment with ACTH and growth hormone significantly reduced the rate of ring A metabolism, suggesting that hormonal interactions may be important in the control of hepatic steroid metabolism.     

Effects of corticotrophin-releasing hormone on corticotrophs in anterior pituitary gland allografts in hypophysectomized,orchidectomized hamsters

Summary We investigated the effects of corticotrophin-releasing hormone (CRH) on the percentage of anterior pituitary gland (APG) cells which are corticotrophs as well as the size and shape of corticotrophs. Pituitary glands were removed from 7-week-old male hamsters and placed beneath the renal capsules of hamsters that had been hypophysectomized and orchidectomized 3 weeks previously. Beginning 6 days after each host had received a single allograft, each was injected subcutaneously twice daily with 4 g CRH or vehicle for 16 days. Six hosts in each group were decapitated 16 h after the last injection. Sections of anterior pituitary tissue were stained for ACTH and with hematoxylin. The percentage of corticotrophs among APG cells was greater in allografts exposed to exogenous CRH (20%) than in allografts exposed to vehicle (15%). Exposure to exogenous CRH increased the cross-sectional area of corticotroph cells in allografts to values greater than those measured for corticotrophs in allografts exposed to vehicle, without altering the shape of cells. Results of subsequent studies suggested that hamsters with allografts injected with vehicle do not release ACTH and that exogenous CRH causes an abrupt release of ACTH from allografts. These results indicate that CRH releases ACTH from ectopic corticotrophs and that administration of CRH can increase corticotroph size and the percentage of APG cells that are corticotrophs.     

Effects of growth hormone-releasing hormone on somatotrophs in anterior pituitary gland allografts in hypophysectomized,orchidectomized hamsters

Summary We investigated the influences of growth hormone-releasing hormone (GHRH) on the percentage, size, and shape of somatotrophs in ectopic anterior pituitary tissue. Entire pituitary glands removed from 7-week-old male hamsters were placed beneath the renal capsules of 12-week-old hamsters that had been hypophysectomized and castrated 3 weeks previously. Beginning 6 days after each host had received a single allograft, each was injected subcutaneously twice daily with 4 g GHRH in 100 l of vehicle or 100 l of vehicle for 16 days. Six hosts in each group were killed by decapitation on day 17, 16 h after the last injection. Nine normal male hamsters were also decapitated and their pituitary glands were removed. Sections of anterior pituitary tissue were stained for GH and with hematoxylin. The percentage of anterior pituitary cells that stained for growth hormone was similar in the 3 groups. In contrast, somatotrophs in grafts had a smaller mean cross-sectional area than those observed in glands in situ. This effect was reversed by GHRH. Analysis of the shape of somatotrophs in both groups of grafts disclosed that they were less circular in cross-section than those in glands in situ. The results suggest that GHRH may not play a role in maintaining the percentage of somatotrophs among anterior pituitary cells, but that it does play a role in maintaining their size.     

Specificity of the stimulatory effect of prostaglandins on hormone release in rat anterior pituitary cells in culture

Specificity of the effect of prostaglandins (PGs) on hormone release by the anterior pituitary gland was studied using cells in primary culture. Growth hormone (GH) release is stimulated by all eight PGs studied, PGE₁ and E₂ being 1000-fold more potent than the corresponding PGFs. The release of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin (PRL) remains unchanged upon addition of PGEs. While the basal release of thyrotropin (TSH) is only slightly stimulated by concentrations of PGEs above 10⁻⁶M, an important potentiation of the stimulatory effect of thyrotropin-releasing hormone on TSH release is observed. The release of GH, TSH and LH is stimulated equally well by PGAs and PGBs at concentrations higher than 10⁻⁶M, 3 × 10⁻⁶M, and 10⁻⁵M, respectively. PGFs do not affect the release of any of the measured pituitary hormones at concentrations below 10⁻⁴M. The stimulation of GH release by PGE₂ can be inhibited by the PG antagonist 7-oxa-13-prostynoic acid, a half-maximal inhibition being found at a concentration of 4 × 10⁻⁵M of the antagonist in the presence of 10⁻⁶M PGE₂. In the presence of somatostatin (10⁻⁸M), the inhibition of GH release cannot be reversed by PGE₂ at concentrations up to 10⁻⁴M. 8-bromo-cyclic AMP-induced GH release is additive with that produced by PGE₂.The present data show that 1) of the five pituitary hormones measured, only GH release is stimulated by prostaglandins at relatively low concentrations, 2) the PGE-induced GH release can be competitively inhibited by 7-oxa-13-prostynoic acid, 3) the inhibition of GH release by somatostatin cannot be reversed by PGE₂ and 4) the PGEs increase the responsiveness of the thyrotrophs to TRH.     

Various proteinase inhibitors decrease prolactin and growth hormone release by anterior pituitary cells

Proteinase inhibitors were tested for their ability to inhibit prolactin (PRL) and growth hormone (GH) release by cultured anterior pituitary cells of the rat. Inhibitors of microbial origin (chymostatin, elastatinal, leupeptin) had either no or a moderate effect on hormone release while some tripeptide aldehydes, especially those with lysine at their C terminus, inhibited markedly PRL and to a lesser extent GH release. Boc-DPhe-Phe-lysinal was the most effective on lactotrophs inhibiting PRL release more than 50% at 10(-4) M. The site(s) of action of tripeptide aldehydes remain to be elucidated.     

Influence of cell cycle phases on the electrical activity and hormone release in a transformed line of anterior pituitary cells

Electrophysiological experiments have shown that about 50% of cultured GH3 cells (tumoral cell line, from the anterior pituitary gland) are inexcitable i.e. they do not display action potentials either spontaneously or when depolarized by a current pulse. We report here this inexcitability may be related to cellular kinetics. Thus we have studied the relationship between the various phases of the cell cycle, the electrophysiological properties of GH3/B6 cells and spontaneous or induced Prolactin and Growth Hormone (GH) release rates. Asynchronous populations of viable cells were stained with Hoechst 33 342 DNA fluorescent dye, and sorted using a flow cytometer into G1 and S phases. After selection intracellular potentials were recorded using a single glass micro-electrode; the basal or TRH stimulated rates of PRL and GH secretions were determined by RIA. Electrical properties of the cells i.e. resting potentials, input membrane resistance and excitability, reached a maximum for cells in G2+M phases. Only cells in G2+M displayed action potentials and TRH increased their secretion by 5 times for GH and by 6 times for PRL. In G1 and S phases the cells were electrically inactive and secretion rates remained at their basal levels. These findings demonstrate that the mechanism of stimulus secretion coupling is dependent upon the phases of the cell cycle.     

Effects of acute ovariectomy on anterior pituitary gland follicle-stimulating hormone and luteinizing hormone secretion in the metestrous rat

Changes at the anterior pituitary gland level which result in follicle-stimulating hormone (FSH) release after ovariectomy in metestrous rats were investigated. Experimental rats were ovariectomized at 0900 h of metestrus and decapitated at 1000, 1100, 1300, 1500, 1700 or 1900 h of metestrus. Controls consisted of untreated rats killed at 0900 or 1700 h and rats sham ovariectomized at 0900 h and killed at 1700 h. Trunk blood was collected and the serum assayed for FSH and luteinizing hormone (LH) concentrations. The anterior pituitary gland was bisected. One-half was used to assay for FSH concentration. The other half was placed in culture medium for a 30-min preincubation and then placed in fresh medium for a 2-h incubation (basal FSH and LH release rates). The basal FSH release rate and the serum FSH concentration rose significantly by 4 h postovariectomy and remained high for an additional 6 h. The basal FSH release rate and the serum FSH concentration correlated positively (r=0.71 with 72 degrees of freedom) and did not change between 0900 and 1700 h in untreated or sham-ovariectomized rats. In contrast, the serum LH concentration and the basal LH release rate did not increase after ovariectomy. Ovariectomy had no significant effect on anterior pituitary gland FSH concentration. The results suggest that the postovariectomy rise in serum FSH concentration is the result, at least in part, of changes which cause an increase in the basal FSH secretion rate (secretion independent of the immediate presence of any hormones of nonanterior pituitary gland origin). The similarities between the selective rises in the basal FSH release rate and the serum FSH concentration in the ovariectomized metestrous rat and in the cyclic rat during late proestrus and estrus raise the possibility that an increase in the basal FSH release rate may be involved in many or all situations in which serum FSH concentration rises independently of LH.     

Influence of yohimbine on release of anterior pituitary hormones

We used a double-blind crossover design to study the effects of alpha 2 adrenoreceptor blockade with yohimbine on levels of anterior pituitary hormones. A dose of yohimbine was used which raised plasma norepinephrine from 379 +/- 74 (S.E.) to 730 +/- 143 pg/ml and mean arterial pressure from 83 +/- 4 to 92 +/- 5 torr (p less than 0.025). This dose (125 micrograms/kg, then 1 microgram/kg/min) also altered mood when compared to saline infusion. In spite of these changes, when prolactin, cortisol, ACTH, beta-endorphin, TSH and growth hormone were measured after 45 minutes of yohimbine infusion, no changes from baseline were noted. These data suggest that in normal man, at rest, alpha 2 adrenoreceptors in the hypothalamus, adenohypophysis or other brain areas do not tonically modulate release of these hormones into the blood.