Rat milk stimulates pituitary growth hormone secretion of neonatal pituitaries in vitro

Aqueous extracts of rat milk stimulated growth hormone (GH) secretion from superfused pituitaries of two-day old rats. The GH stimulatory effect of milk increased with the time elapsed postpartum; growth hormone releasing hormone and thyrotropin releasing hormone seem to be the major milk borne GH releasing factors. These results indicate that milk intake may play a role in maintaining the high plasma GH levels observed in the neonatal period.     

Possible mediation of GABA induced growth hormone secretion by increased calcium-flux in neonatal pituitaries.

Gamma-aminobutyric acid (GABA) stimulates growth hormone (GH) secretion from pituitaries of young (less than 20-day old) rats (1,2). Present work revealed that the GH stimulatory effect of GABA was abolished in the absence of calcium and response was attenuated by Nifedipine. The calcium efflux from 45CaCl2 preloaded neonatal pituitaries was enhanced by GABA or by muscimol, and this effect was antagonized by the GABA antagonist picrotoxin. In pituitaries of 21 day old or adult rats GABA stimulated neither GH secretion nor calcium efflux. These results indicate that in neonatal pituitaries GABA influences calcium transport and its GH releasing effect is linked to the presence of calcium.     

Rat hypothalamic corticotropin-releasing hormone secretion in vitro is stimulated by interleukin-1 in an eicosanoid-dependent manner

We studied the effect of interleukin-1 alpha (IL-1) on corticotropin-releasing hormone (CRH) secretion by explanted rat hypothalami in vitro. We also assessed possible mediation of arachidonic acid metabolites on IL-1-stimulated CRH secretion, by preincubating hypothalami with the cyclooxygenase inhibitor indomethacin (INDO, 1 microM), the lipoxygenase and cyclooxygenase inhibitor eicosatetraynoic acid (ETYA, 10 microM), or the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA, up to 30 microM). In additional experiments, prostaglandins (PG) E2 and F2 alpha were added to the cultures treated with INDO or ETYA. Finally, we investigated the effect of dexamethasone (DEX) on IL-1-stimulated CRH secretion. IL-1 stimulated immunoreactive CRH (iCRH) secretion by explanted hypothalami in a concentration-dependent fashion. Both INDO and ETYA inhibited IL-1-(10nM)-stimulated iCRH secretion, whereas NDGA did not have any effect. The addition of PGF2 alpha (10 nM) restored the secretion of iCRH inhibited by INDO. DEX treatment significantly inhibited IL-1-stimulated iCRH release. Our results suggest that the stimulatory effect of IL-1 on the hypothalamic CRH neuron is mediated by the cyclooxygenase metabolites of arachidonic acid, and, among others, by PGF2 alpha.     

Growth hormone inhibits growth hormone secretion from the rainbow trout pituitary in vitro

Growth hormone (GH) secretion in salmonids and other fish is under the control of a number of hypothalamic factors, but negative feed-back regulation by circulating hormones can also be of importance for the regulation of GH secretion. Mammalian studies show that GH has a negative feed-back effect on its own secretion. In order to elucidate if GH levels present a direct ultra-short negative feedback loop at the pituitary level GH secretion was studied in intact pituitaries from 50 g fish in an in vitro perifusion system. Following an initial equilibrium period pituitaries were exposed to five increasing concentrations (1-1,000 ng ml(-1)) of ovine GH (oGH) in 20-min steps, before being returned to a GH-free perifusion. Ovine GH caused a significant dose-dependent inhibition of GH secretion and it is concluded that GH can exert a direct negative feedback control on GH secretion at the pituitary level.     

Growth hormone signalling and apoptosis in neonatal rat cardiomyocytes

Growth hormone (GH) has been reported to be useful to treat heart failure. To elucidate whether GH has direct beneficial effects on the heart, we examined effects of GH on oxidative stress-induced apoptosis in cardiac myocytes. TUNEL staining and DNA ladder analysis revealed that hydrogen peroxide (H₂O₂)-induced apoptosis of cardiomyocytes was significantly suppressed by the pretreatment with GH. GH strongly activated extracellular signal-regulated kinases (ERKs) in cardiac myocytes and the cardioprotective effect of GH was abolished by inhibition of ERKs. Overexpression of dominant negative mutant Ras suppressed GH-stimulated ERK activation. Overexpression of Csk that inactivates Src family tyrosine kinases also inhibited ERK activation evoked by GH. A broad-spectrum inhibitor of protein tyrosine kinases (PTKs), genistein, strongly suppressed GH-induced ERK activation and the cardioprotective effect of GH against apoptotic cell death. GH induced tyrosine phosphorylation of EGF receptor and JAK2 in cardiac myocytes, and an EGF receptor inhibitor tyrphostin AG1478 and a JAK2 inhibitor tyrphostin B42 completely inhibited GH-induced ERK activation. Tyrphostin B42 also suppressed the phosphorylation of EGF receptor stimulated by GH. These findings suggest that GH has a direct protective effect on cardiac myocytes against apoptosis and that the effect of GH is attributed at least in part to the activation of ERKs through Ras and PTKs including JAK2, Src, and EGF receptor tyrosine kinase.     

Stimulating effect of gamma-aminobutyric acid on rat hippocampal neurons in vitro

Effects of GABA on the background and electrically stimulated activity of single neurons and population spikes were investigated in isolated hippocampal slices. Application of relatively large GABA concentrations (10(-3) mol/l and more) depressed an antidromic population spike, field EPSP and neuronal background activity. Low concentrations of GABA (less than 10(-3) mol/l) added to the bath increased the population spikes amplitude and the late component of field EPSP, facilitated single neurone responses, their background activity and epileptiform discharges. GABA-evoked depolarization was observed in the majority of the studied neurons. The duality of the GABA action on central neurons are discussed.     

Galanin stimulates rat pituitary growth hormone secretion in vitro

The effect of galanin on growth hormone (GH) secretion was investigated in monolayer cultures of rat anterior pituitary cells. Galanin caused a gradual increase in GH concentrations into the culture medium that was maximal at 90 minutes and sustained after 180 minutes. The ED50 for galanin-stimulated GH secretion was approximately 200 nM compared to an ED50 for rat GH-releasing factor (rGRF)-stimulated GH secretion of 10pM. Galanin and rGRF were additive in increasing GH release into the incubation medium. These data indicate that porcine-derived galanin has a direct effect on pituitary GH secretion in vitro.     

Synthesis and secretion of phosphorylated growth hormone by rat pituitary glands in vitro

Rat anterior pituitary glands were incubated in buffered medium, pH 7.4, containing 32Pi. After incubation the tissue and medium were separated and a post-mitochondrial supernate (PMS) of the tissue homogenate was prepared. Gel filtration of the PMS and medium resulted in a radioactive peak which coincided with the elution volume of authentic rat growth hormone (rGH). Polyacrylamide gel electrophoresis of the radioactive peak under denaturing condition resulted in a protein-staining band having the same mobility as authentic rGH. Autoradiography of the gels revealed radioactivity precisely at the position of growth hormone as well as elsewhere. The specific radioactivity of the PMS [32P]GH was estimated to be 5 to 10 times greater than that of tissue [32P]GH. These results indicate that phosphorylated GH is synthesized and secreted by pituitary glands in vitro.     

Interaction of beta-endorphin, naloxone and dopamine: effects on melanocyte-stimulating hormone secretion of amphibian pituitaries in vitro

Neurointermediate lobes from amphibians (Rana pipiens) were incubated in Medium 199 containing dopamine, beta-endorphin or dopamine plus beta-endorphin. Dopamine inhibited melanocyte-stimulating hormone (MSH) secretion as measured by bioassay in hypophysectomized frogs, an effect which was transiently reversed by beta-endorphin. The effects of endorphin were in turn partially suppressed by the opiate antagonist, naloxone hydrochloride. Cells treated with all three agents exhibited expanded rough endoplasmic reticulum and decreased secretory granule content, indicative of peptide release and new synthesis. Beta-Endorphin alone did not stimulate MSH secretion above control levels, and at one time period was seen to reduce MSH secretion. The findings indicate a complex interaction between beta-endorphin and dopamine directly upon MSH secretion at the level of the neurointermediate lobe.     

Growth hormone secretion in the rat as a function of age.

The first appearance of rat growth hormone (RGH) in serum was in the 19 day-old foetus. The level was high on the 21st day of gestation (129 +/- 7 ng/ml serum), it decreased after birth and descended to 17 +/- 2 ng/ml in the 15 day-old rat. After weanling it again rose to reach a plateau at 80 cays. The half-life and metabolic clearance rate (MCR) of RGH were compared in the 4 and 15 day-old rat and in normal and hypophysectomized adult rats after a single intravenous injection of hormone. As the MCR was essentially the same in all groups, serum RGH levels reflect the rate of hormonal secretion of the pituitary gland.     

Growth hormone secretion pattern is an independent regulator of growth hormone actions in humans

The importance of gender-specific growth hormone (GH) secretion pattern in the regulation of growth and metabolism has been demonstrated clearly in rodents. We recently showed that GH secretion in humans is also sexually dimorphic. Whether GH secretion pattern regulates the metabolic effects of GH in humans is largely unknown. To address this question, we administered the same daily intravenous dose of GH (0.5 mg. m(-2). day(-1)) for 8 days in different patterns to nine GH-deficient adults. Each subject was studied on four occasions: protocol 1 (no treatment), protocol 2 (80% daily dose at 0100 and 10% daily dose at 0900 and 1700), protocol 3 (8 equal boluses every 3 h), and protocol 4 (continuous GH infusion). The effects of GH pattern on serum IGF-I, IGF-binding protein (IGFBP)-3, osteocalcin, and urine deoxypyridinoline were measured. Hepatic CYP1A2 and CYP3A4 activities were assessed by the caffeine and erythromycin breath tests, respectively. Protocols 3 and 4 were the most effective in increasing serum IGF-I and IGFBP-3, whereas protocols administering pulsatile GH had the greatest effects on markers of bone formation and resorption. All GH treatments decreased CYP1A2 activity, and the effect was greatest for pulsatile GH. Pulsatile GH decreased, whereas continuous GH infusion increased, CYP3A4 activity. These data demonstrate that GH pulse pattern is an independent parameter of GH action in humans. Gender differences in drug metabolism and, potentially, gender differences in growth rate may be explained by sex-specific GH secretion patterns.     

Prolactin secretion and intracellular Ca(2+) change in rat lactotroph subpopulations stimulated by thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) may stimulate lactotrophs to increase intracellular Ca(2+) and to secrete prolactin (PRL). In this study, PRL contents in lactotrophs were determined by the sequential cell immunoblot assay (SCIBA) and their changes in intracellular Ca(2+) was analyzed by confocal microscopy. Significant correlations were found in the corresponding parameters between TRH treatments with a recovery interval of 2 h. Measuring the PRL contents after the first TRH treatment and then determining the intracellular Ca(2+) changes after the second TRH treatment revealed four lactotroph subpopulations. Type I cells (51%) showed significant responses of both PRL secretion and intracellular Ca(2+) concentration. Type II cells (22%) increased in PRL secretion, but without changes in intracellular Ca(2+). Type III cells (17%) have increased in intracellular Ca(2+), but without changes in PRL secretion. Type IV cells (10%) did not show changes in PRL secretion and intracellular Ca(2+).     

Effects of gamma-aminobutyric acid on secretagogue-induced exocrine secretion of isolated, perfused rat pancreas

Because GABA and its related enzymes have been determined in beta-cells of pancreas islets, effects of GABA on pancreatic exocrine secretion were investigated in the isolated, perfused rat pancreas. GABA, given intra-arterially at concentrations of 3, 10, 30, and 100 microM, did not exert any influence on spontaneous or secretin (12 pM)-induced pancreatic exocrine secretion. However, GABA further elevated CCK (10 pM)-, gastrin-releasing peptide (100 pM)-, or electrical field stimulation-induced pancreatic secretions of fluid and amylase dose dependently. The GABA (30 microM)-enhanced CCK-induced pancreatic secretions were completely blocked by bicuculline (10 microM), a GABA(A) receptor antagonist, but were not affected by saclofen (10 microM), a GABA(B) receptor antagonist. The enhancing effects of GABA (30 microM) on CCK-induced pancreatic secretions were not changed by tetrodotoxin (1 microM) but were partially reduced by cyclo-(7-aminoheptanonyl-Phe-D-Trp-Lys-Thr[BZL]) (10 nM), a somatostatin antagonist. In conclusion, GABA enhances pancreatic exocrine secretion induced by secretagogues, which predominantly induce enzyme secretion, via GABA(A) receptors in the rat pancreas. The enhancing effect of GABA is partially mediated by inhibition of islet somatostatin release.