Receptor-mediated internalization is critical for the inhibition of the expression of growth hormone by somatostatin in the pituitary cell line AtT-20.
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The inhibitory effect of the neuropeptide somatostatin on the expression of growth hormone was measured by quantitative polymerase chain reaction in the pituitary cell line AtT-20. We demonstrate that this effect is dependent on the internalization of somatostatin-receptor complexes and that it is totally independent from the peptide-induced inhibition of adenylate cyclase. Indeed, the inhibitory effect of the peptide on growth hormone mRNA levels was totally insensitive to pertussis toxin treatment but was totally abolished under conditions which block somatostatin receptor internalization. Comparative confocal microscopic imaging of fluorescent somatostatin sequestration and fluorescence immunolabeling of sst1, sst2A, and sst5 receptors suggests that sst2A is most probably responsible of the inhibitory effect of somatostatin on growth hormone expression. (+info)
Mimicry in primary rat hepatocyte cultures of the in vivo perivenous induction by phenobarbital of cytochrome P-450 2B1 mRNA: role of epidermal growth factor and perivenous oxygen tension.
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Treatment of male rats with phenobarbital (PB) results in a perivenous and mid-zonal pattern of cytochrome P-450 (CYP)2B1 mRNA expression within the liver acinus. The mechanism of this zonated induction is still poorly understood. In this study sinusoidal gradients of oxygen and epidermal growth factor (EGF) besides those of the pituitary-dependent hormones growth hormone (GH), thyroxine (T4), and triiodothyronine (T3) were considered to be possible determinants for the zonated induction of the CYP2B1 gene in liver. Moreover, heme proteins seem to play a key role in oxygen sensing. Therefore, the influence of arterial (16% O2) and venous (8% O2) oxygen tension (pO2), and of the heme synthesis inhibitors CoCl2 and desferrioxamine (DSF) on PB-dependent CYP2B1 mRNA induction as well as the repression by EGF and, for comparison, by GH, T4, and T3, of the induction under arterial and venous pO2 were investigated in primary rat hepatocytes. Within 3 days, phenobarbital induced CYP2B1 mRNA to maximal levels under arterial pO2 and to about 40% of maximal levels under venous pO2. CoCl2 annihilated induction by PB under both oxygen tensions, whereas desferrioxamine and heme abolished the positive modulation by O2, suggesting that heme is a necessary component for O2 sensing. EGF suppressed CYP2B1 mRNA induction by PB only under arterial but not under venous pO2, whereas GH, T4, and T3 inhibited induction under both arterial and venous pO2. Thus, in hepatocyte cultures, an O2 gradient in conjunction with EGF mimicked the perivenous induction by PB of the CYP2B1 gene observed in the liver in vivo. (+info)
The role of growth hormone and glucocorticoid in glucose handling in vivo.
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Growth hormone (GH) can oppose the catabolic effects of glucocorticoids. However, both hormones have adverse effects on carbohydrate metabolism. Here we examined the interactive effects of GH and the glucocorticoid methylprednisolone (MP) on glucose tolerance, insulin resistance and [3H]2,6-deoxyglucose uptake of peripheral tissues in rats. Female Wistar rats received either saline, GH (2.7 mg/kg), MP (5.0 mg/kg) or GH+MP. After 7 days treatment, animals were subjected to an i.v. glucose tolerance test. In a second experiment, animals treated as above were anesthetized and injected with human insulin (0.5 U/kg), [3H]2,6-deoxyglucose (500 microCi/kg), and [14C]mannitol (25 microCi/kg), to estimate insulin resistance and [3H]2,6-deoxyglucose uptake in fat and muscle. Weight gain in controls was 7.6+/-1.7 g, while GH treatment increased the mean body weight by 18.7+/-2.2 g (P<0.0002) and MP inhibited weight gain down to 0.0+/-1.0 g (P<0.004). This drop in weight gain was reversed back to normal when GH was given in combination with MP. After a glucose tolerance test no significant differences in glucose area under the curve were detected when comparing individual groups with the control group, but samples taken just before this test revealed that basal insulin was significantly elevated in the group treated with GH (174+/-27 pM, P<0.008), or GH+MP (209+/-21 pM, P<0.004), when compared with controls (107+/-17 pM). MP alone had no effect (122+/-19, P<0.3). After an i.v. bolus of insulin the group receiving GH+MP had a significantly (P<0.007) higher level of circulating glucose compared with controls (6.5+/-0.3 mM vs 4.4+/-0.7 mM). Despite this, there were no differences in peripheral glucose uptake between the two groups. In conclusion this study shows that a combined administration of GH and MP decreases the potency by which insulin decreases circulating glucose levels, but that peripheral tissues are not primarily involved in this insulin resistance. (+info)
Leptin regulates GH secretion in the rat by acting on GHRH and somatostatinergic functions.
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Leptin is a hormonal product of adipose tissue whose expression reflects the body state of nutritional reserves. Previous experiments have demonstrated that leptin is one of the metabolic signals capable of regulating GH secretion. The aim of the present study was to evaluate whether CNS-mediated mechanisms underlie the GH-releasing activity of leptin. Freely moving mature male rats were injected i.c.v with leptin or isovolumetric amounts of diluent once daily for 3 days and were killed 2 h after the last administration. Central injection of leptin increased pituitary GH mRNA levels by 53. 2% and hypothalamic GHRH mRNA by 61.8%, and reduced somatostatin mRNA levels by 41.5%. To evaluate the direct effect of leptin on the pituitary, it was added alone or in combination with GHRH to primary cultures of anterior pituitary cells. Addition of leptin (10(-11)-10(-7) M) did not alter basal GH release nor the GH-releasing activity of GHRH. These results demonstrate that leptin is a metabolic signal that regulates GH secretion in the rat by acting on hypothalamic GH-regulatory hormones. (+info)
Endocrine control of Na+,K+-ATPase and chloride cell development in brown trout (Salmo trutta): interaction of insulin-like growth factor-I with prolactin and growth hormone.
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A 2-factorial (3x3) injection experiment was used to investigate the effect and interaction between different hormones on the initial phase of seawater (SW) acclimation in brown trout (Salmo trutta). Each fish was given 4 injections on alternate days in freshwater (FW). Factor 1 was either saline, 2 micrograms ovine prolactin (oPRL)/g, or 2 micrograms ovine growth hormone (oGH)/g. Factor 2 was either 0, 0. 01, or 0.1 mirograms recombinant human insulin-like growth factor-I (rhIGF-I)/g. In each of the 9 treatment groups, half of the fish were subjected to a 48-h SW-challenge test, and the remaining fish were sham-transferred to FW one day after the last injection. Hypo-osmoregulatory performance was increased by GH and impaired by PRL treatment as judged by changes in plasma osmolality, [Na+], [Cl-], total [Mg] and muscle water content (MWC) after SW transfer. IGF-I reduced plasma osmolality after transfer to SW but had no effect on plasma total [Mg] or MWC. The effects of the two factors on plasma osmolality, [Na+], [Cl-], and MWC were additive. In sham-transferred fish, GH and IGF-I, alone and in combination, stimulated Na+,K+-ATPase alpha-subunit mRNA (alpha-mRNA) content in the gill. This was paralleled by an overall increase in gill Na+, K+-ATPase activity in fish treated with 0.01 micrograms IGF-I/g. Simultaneous administration of PRL completely inhibited the increase in gill alpha-mRNA observed in the IGF-I-injected groups. Combination of GH and IGF-I did not further affect the alpha-mRNA level relative to the single hormone-injected groups. There was an overall decrease in Na+,K+-ATPase activity in pyloric caeca and middle intestine by the low dose and both doses of IGF-I respectively. No effect was observed in the posterior intestine. PRL and GH treatments did not affect enzyme activity in any intestinal segment. Both doses of IGF-I increased Na+,K+-ATPase-immunoreactive (NKIR) cell density in gill primary filaments. PRL and GH had no effect on primary filament NKIR cell density. GH and both doses of IGF-I reduced secondary lamellar NKIR cell density, whereas PRL had no effect. The main conclusion is that IGF-I and GH induce an overall redistribution of NKIR cells away from the secondary lamella onto the primary filament of FWacclimated trout. This is associated with an overall increased alpha-mRNA level in the gill, which may reflect an increased expression within individual NKIR cells in the primary filament. PRL completely abolished the IGF-I stimulation of alpha-mRNA levels, suggesting a desensitisation of the gill tissue to IGF-I, which may explain the overall anti-SW adaptive effect of PRL. (+info)
Multi-label image analysis of secretory cell juxtaposition in the sheep pituitary gland.
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An image analysis system, assigned different pseudocolors to different types of immunolabeled cells, allowed us to make a montage from two images of the respective types of cells. This system was therefore used for simultaneous identification of two or more types of immunolabeled cells in the sheep anterior pituitary. Morphometry--including a neighboring proportion defined as the probability of a cell type adjoining other cell types--was performed. We also conducted a simulation of an artificial cell mass with an image analyzer to evaluate the effects of cell populations on the neighboring proportion. Simulation analysis showed that the predominant cell type tended to have a higher neighboring proportion, while rarer cell types had lower proportions according to their small population density. In the sheep pituitary gland, the neighboring proportions against PRL-, GH-immunolabeled cells were high (about 65% and 55%, respectively), according to their large populations. The neighboring proportion of LH beta-immunolabeled cells to the same type of cells was lower (11%) than that against other types of cells. It was thus suggested that LH cells were scattered throughout the anterior lobe. The neighboring proportion of ACTH-immunolabeled cells to the same type of cells was somewhat higher, but that of ACTH cells to PRL cells was low (52%). Accordingly, this cell type was often distributed in clusters. These quantitative results confirmed the topographical characteristics of secretory cells deduced from visual observation. In addition, a low topographical affinity between PRL and ACTH cells was indicated. (+info)
N-glycans are not a universal signal for apical sorting of secretory proteins.
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In MDCK cells, N-glycans have been shown to determine the sorting of secretory proteins and membrane proteins to the apical domain in the absence of a dominant basolateral targeting signal. We have examined the sorting of endogenous proteins in ECV304 cells in the presence and absence of tunicamycin, an inhibitor of N-linked glycosylation. A prominent apically secreted protein of 71 kDa was not N-glycosylated and continued to be secreted apically in the presence of tunicamycin. In contrast, other endogenous proteins that were N-glycosylated were secreted preferentially into the basolateral medium or without polarity. When rat growth hormone was expressed in MDCK and ECV304 cells, we observed 65 and 94% of the secretion to the basolateral medium, respectively. Introduction of a single N-glycan caused 83% of the growth hormone to be secreted at the apical surface in MDCK cells but had no significant effect on the polarity of secretion of growth hormone in ECV304 cells. These results indicate that not all cell lines recognise N-glycans as a signal for apical sorting and raises the possibility of using ECV304 cells as a model system for analysis of apical sorting molecules. (+info)
The secretory patterns of growth hormone in pregnant and hysterectomized ewes.
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This work was undertaken to determine the secretory patterns of GH during pregnancy, and to evaluate the effect, if any, of hysterectomy during early pregnancy on subsequent secretion of GH in ewes. The concentrations of GH were determined in the plasma of jugular blood samples collected at 15-min intervals during a 6-h period on days 20, 40, 60, 80, 100 and 120 post-mating, and three times per week between days 29 and 120 post-mating from 5 pregnant ewes and from 5 ewes from which the gravid uterus was removed on day 30 post-mating. A pulse analysis program (Pulsar) was used to analyse the secretory patterns of GH in individual profiles of the serial sampling period. In the two groups of ewes, peripheral concentrations of GH fluctuated in an episodic manner during the frequent blood sampling of any stage of the post-mating period examined. The overall GH concentrations, the basal GH concentrations, the frequency and the amplitude of GH pulses remained fairly stable between days 20 and 120 post-mating in the two groups of ewes. The parameters of GH secretion were not different between the two groups of ewes. The secretory patterns of GH, as determined in plasma of blood collected three times per week between days 29 and 120 post-mating were also not different between the two groups of ewes. In conclusion, results of this study show that (i) the pulsatile secretion of GH does not change as pregnancy advances, and (ii) hysterectomy performed during early pregnancy does not subsequently affect the secretory patterns of GH. These findings suggest that the gravid uterus and/or the feto-placental unit secretory products are unlikely to be involved in the control of GH secretion during pregnancy in the ewe. (+info)