Polymers for biodegradable medical devices. II. Hydroxybutyrate-hydroxyvalerate copolymers: hydrolytic degradation studies

The hydrolytic degradation of poly(hydroxybutyrate) together with a series of hydroxybutyrate-hydroxyvalerate copolymers has been studied. The effects of copolymer composition and molecular weight are presented together with the results of varying pH and temperature on the degradation rate. Degradation has been monitored by weight loss and water uptake measurements together with goniophotometric, surface energy and scanning electron microscopic studies. Some comparisons with the more widely used so-called 'biodegradable' polymers, poly(glycolic acid), poly(dioxanone) and the glycolic-lactic acid (90:10) copolymers are presented together with the effect of blood plasma on the degradation process.

Polymers for biodegradable medical devices. VIII. Hydroxybutyrate-hydroxyvalerate copolymers: physical and degradative properties of blends with polycaprolactone

The physical and degradative properties of polyhydroxybutyrate-hydroxyvalerate copolymer blends with polycaprolactone were investigated. Blends containing low levels of polycaprolactone (less than 20%) were found to possess a considerable degree of compatibility, whilst those with higher levels of polycaprolactone were incompatible and showed phase separation behaviour. This incompatibility was most marked in blends containing approximately 50% of each component. In blends containing low levels of polycaprolactone, processing conditions governed the ease of crystallization of polycaprolactone in the polyhydroxybutyrate-hydroxyvalerate matrix and thus the mechanical property of the blend. The degradation rate of these blends was found to be influenced by a complex set of factors, including temperature, pH and polycaprolactone content of the blend. Although crystallinity affected the mechanical properties of the blends, its influence on the hydrolytic degradation rate was masked by the large difference in the molecular weight of the polyhydroxybutyrate-hydroxyvalerate copolymers (MW approximately 300,000) and polycaprolactone. (MW approximately 50,000). The polyhydroxybutyrate-hydroxyvalerate/polycaprolactone blends were found to be much more stable to hydrolytic degradation than polyhydroxybutyrate-hydroxyvalerate/polysaccharide blends previously studied. Here the combined techniques of goniophotometry and surface energy measurements proved extremely valuable in monitoring the early stages of degradation, during which surface, rather than bulk degradation, processes predominate.

Gonadotropin-releasing hormone (GnRH) pulse pattern regulates GnRH receptor gene expression: augmentation by estradiol

GnRH acts via a single cell surface receptor (GnRH-R), and the number of pituitary GnRH-R increases on proestrus, after gonadectomy, or in response to pulsatile GnRH in the rat. Estradiol (E2) is known to exert a transient positive action to increase GnRH-R number, and the rise in plasma E2 contributes to initiation of the midcycle LH surge. The present study was designed to determine the effect of GnRH pulse amplitude and frequency on GnRH-R messenger RNA (mRNA) levels and to assess the relative contributions of GnRH and gonadal steroids to increasing GnRH-R gene expression. These studies were conducted in vivo using previously characterized GnRH-deficient male (castrate testosterone-replaced) and ovariectomized phenoxybenzamine-treated female models. To investigate the effect of GnRH pulse amplitude, adult male and female rats received GnRH iv (5-250 ng/pulse at 30-min intervals; saline pulses to controls) for 12 or 24 h. In males, GnRH-R mRNA was increased by all pulse doses, with maximal effects (3-fold) at 5-25 ng/pulse. In contrast, only lower doses (5-10 ng/pulse) were effective in females (2-fold increase). In a subsequent study, GnRH pulses (25 ng for males; 10 ng for females) were given at 8-, 30-, or 240-min intervals for 12 or 24 h. Some animals received a continuous GnRH infusion (200 ng/h). In males, GnRH-R mRNA levels were stimulated by all GnRH pulse intervals (maximal after 30-min pulses), whereas continuous GnRH was ineffective. In females, only 30- and 240-min pulse intervals increased GnRH-R mRNA levels, with faster (8-min) pulses or continuous GnRH being ineffective. To determine the relative roles of ovarian steroids and GnRH, ovariectomized phenoxybenzamine-treated female animals received GnRH (10 ng/pulse, 30-min interval), E2 (via sc implants; plasma E2 levels, approximately 50 pg/ml), or their combination for 12-24 h (saline pulses to controls). In the absence of E2, GnRH-R concentrations fell by 70% between 12-24 h. E2 alone tended to increase GnRH-R mRNA at 12 h, with a 2-fold rise observed after 24 h. Pulsatile GnRH alone increased GnRH-R mRNA by 50% at 12 h (compared to saline-pulsed controls; P < 0.05) and by 6-fold after 24 h. When GnRH and E2 were combined, the magnitude of the increase (vs. saline controls) was greater than that seen for either GnRH or E2 alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Polymers for biodegradable medical devices. VII. Hydroxybutyrate-hydroxyvalerate copolymers: degradation of copolymers and their blends with polysaccharides under in vitro physiological conditions

The hydrolytic degradation of hydroxybutyrate-hydroxyvalerate copolymers was monitored in vitro at 37 degrees C and pH 7.4. Direct use of bulk properties such as weight loss and tensile strength did not reveal substantial changes in the polymer matrix over degradation periods of several months. Despite this, the polymers were demonstrated to undergo significant modification during this period, in ways that markedly influence their subsequent behaviour. Combined use of goniophotometry and surface energy measurements revealed that surface modification begins at an early stage and is accompanied by diffusion of water into the matrix and a progressive increase in polymer porosity. Relatively little change in the molecular weight and some increase in the crystallinity of the matrix occurred during these early months. As a result, the tensile strength of the polymer varies little in this period. As the porosity of the matrix increases, hydrolytic chain scission within the matrix and diffusion out of degradation products proceeds more effectively. Decrease in matrix molecular weight, increase in matrix erosion, weight loss and loss of tensile strength began at a much more dramatic rate. The apparent resistance of the polymer to degradation in the early months is followed by an accelerated degradation phase around and beyond 1 yr. The use of filters that can dissolve or hydrolytically degrade more rapidly than the hydroxybutyrate matrix accelerates the development of porosity within the matrix and thus enhances the decomposition process.

Polymers for biodegradable medical devices. VI. Hydroxybutyrate-hydroxyvalerate copolymers: accelerated degradation of blends with polysaccharides

The hydrolytic degradation of poly(hydroxybutyrate)-poly(hydroxyvalerate) (PHB-PHV) copolymers in the form of blends with the polysaccharides amylose, dextran, dextrin and sodium alginate, has been studied under a range of conditions (pH 2.3, 7.4 and 10.6 and at 37 degrees C and 70 degrees C). The hydrolytic degradation of the PHB-PHV copolymers was found to be dramatically affected by the presence of polysaccharides. Its progress was characterized by an initial increase in the wet weight, with concurrent decrease in the dry weight as the polysaccharides eroded from the matrix. Surface energy measurements and goniophotometry proved to be particularly useful in monitoring this stage of the degradation process. The concurrent increase in internal porosity leads to the eventual collapse of the matrix, a process which occurs, but less rapidly, in the degradation of the unblended PHB-PHV copolymers. Information obtained from molecular weight and crystallinity studies enabled a comprehensive profile of the overall degradation process to be built up.

Gonadotropin-releasing hormone pulse frequency regulates expression of pituitary follistatin messenger ribonucleic acid: a mechanism for differential gonadotrope function

Follistatin (FS) is a monomeric glycoprotein that selectively inhibits both secretion of FSH and expression of FSH beta messenger RNA (mRNA), presumably via its ability to bind activin. FS mRNA and protein are present in the gonadotrope, suggesting a local action in regulating FSH beta. Pituitary FS mRNA increases after gonadectomy and at the midcycle gonadotropin surge of the estrous cycle, times of increased GnRH secretion. Thus, the purpose of the present studies was to assess the role of GnRH secretion on the regulation of pituitary FS. To confirm GnRH regulation of FS and to study the role of gonadal steroids, adult male rats were gonadectomized (2-36 h), with some animals receiving either testosterone (T) replacement, LRF-147 (a GnRH antagonist, AC-DTrp1-pCl-DPhe2-DTrp3-Ser4-Tyr5-DArg6-L eu7-Arg8-Pro9-DAla10), or both for 36 h (from the time of castration). Pituitary FS mRNA increased rapidly after castration, with levels rising 3-fold by 12 h and 4-fold by 36 h when compared to intact animals (P < 0.05). This rise was completely abolished by administration of LRF-147 and prevented by T replacement. Because GnRH pulse frequency can selectively regulate FSH beta mRNA expression, we next examined the effect of GnRH pulse interval (8-480 min) on FS mRNA expression. Fast frequency GnRH pulses (8 min), which did not increase FSH beta mRNA, were associated with an increase in FS mRNA (2.5-fold). The 30-min interval increased FS and gonadotropin subunit mRNAs. Slower pulse frequencies (> or = 120 min), which selectively stimulated a rise in FSH beta mRNA, did not increase FS mRNA. These results indicate that pituitary FS mRNA is regulated by GnRH. In addition, GnRH frequency modulation of pituitary FS provides a mechanism whereby a single hypothalamic GnRH can differentially regulate the gonadotropins, LH and FSH.

Polymers for biodegradable medical devices. V. Hydroxybutyrate-hydroxyvalerate copolymers: effects of polymer processing on hydrolytic degradation

The effects of melt processing on those properties of polyhydroxybutyrate-hydroxyvalerate copolymers that control hydrolytic stability were studied. Initial experiments using a non-thermal preparation technique enabled the relationship between initial molecular weight and rate of hydrolytic degradation to be established. This rate was conveniently expressed in terms of the time for 10% by weight of the sample to be eroded. By varying melt processing conditions, the range of effects of processing on sample molecular weight and molecular weight distribution were investigated. These effects were found to be quite dramatic, with a reduction to 50% of the initial molecular weight being readily achieved. Temperature, shear rate and dwell time interrelated to control the extent of thermal degradation achieved during processing. The principal effect of copolymer composition resulted from the fact that an increase in hydroxyvalerate content led to a reduction in melting point and enabled milder processing conditions to be used. Enhanced crystallinity, resulting from more favourable processing conditions reduced hydrolytic degradation rates. As a result, melt processing produced a complex interrelation of effects, all of which influenced the hydrolytic stability of the fabricated specimen. In general, molecular weight effects predominated.

Gonadotropin-releasing hormone regulation of gonadotropin subunit gene expression in female rats: actions on follicle-stimulating hormone beta messenger ribonucleic acid (mRNA) involve differential expression of pituitary activin (beta-B) and follistatin mRNAs

GnRH is the primary stimulus in the regulation of gonadotropin subunit mRNA expression. Additionally, local (pituitary) production of activin and follistatin appear to modulate the expression of FSH beta mRNA. The current studies aimed to determine whether GnRH regulation of pituitary activin (beta-B) and follistatin mRNAs could play a role in the differential actions of GnRH pulse pattern on gonadotropin mRNA expression in female rats. In response to altered GnRH pulse amplitude, the expression of FSH beta and follistatin mRNAs followed an inverse pattern. Only high dose GnRH increased expression of follistatin whereas, in contrast, beta-B and FSH beta expression were increased following lower doses of GnRH. To determine whether increased follistatin mRNA expression was correlated with FSH beta mRNA responses, we examined their temporal relationship following high dose GnRH. Both FSH beta and follistatin mRNAs were increased within 2 h and remained increased through 6 h. However, by 12 h FSH beta mRNA levels returned to values seen in controls, suggesting that increased follistatin requires 6-12 h to reduce FSH beta mRNA. In response to altered GnRH pulse frequency, FSH beta expression was increased at all pulse intervals (8-240 min) examined. Rapid GnRH pulse frequencies (8-min intervals) increased follistatin expression, whereas beta-B mRNA was only increased after 30-min pulse intervals, which also resulted in maximal FSH beta mRNA concentrations. These results suggest that changes in pituitary activin (beta-B) and follistatin mRNA expression may be important components of gonadotrope responses to pulsatile GnRH, and potentially imply that GnRH stimulation of activin and follistatin peptide production provides regulatory control over the production of FSH.

GnRH regulates steroidogenic factor-1 (SF-1) gene expression in the rat pituitary

Recent studies have demonstrated that the nuclear receptor, steroidogenic factor 1 (SF-1) plays a role in the regulation of pituitary gonadotropin gene expression. As GnRH is critical to stimulating LH and FSH gene expression, the present study was conducted to determine whether GnRH also regulates pituitary SF-1 mRNA. Pituitary SF-1 mRNA levels were measured in individual animals by RNase protection assay. In the first study, adult male and female rats were gonadectomized (GDX) for 7 days and some received testosterone (T) to prevent the post-GDX rise in GnRH, and compared to intact animals. Pituitary SF-1 mRNA levels increased significantly (3 fold in males, 2 fold in females; p < 0.05 vs intacts) after gonadectomy, which was blocked by exogenous T. Similar changes were observed in serum LH. To directly test whether GnRH stimulates SF-1 mRNA, we used a GnRH-deficient rat model (phenoxybenzamine-treated, ovariectomized females) and administered GnRH pulses for 6h (5ng at 30 min intervals; saline pulses to controls). Pulsatile GnRH stimulated a 51-64% increase in SF-1 mRNA levels (p < 0.05 vs controls). These results show that GnRH stimulates SF-1 gene expression, which may be a critical component in GnRH stimulation of gonadotropin subunit transcription.

Involvement of toll-like receptor 4 in alveolar bone loss and glucose homeostasis in experimental periodontitis

BACKGROUND AND OBJECTIVE

There is general agreement that certain fatty acids and lipopolysaccharides (LPS) promote inflammation through toll-like receptor 4 (TLR4), and that inflammation promotes insulin resistance. We therefore hypothesized that mice with periodontitis and a TLR4 loss-of-function (LOF) mutation fed a high-fat (HF) diet would develop improved glucose homeostasis compared with wild-type (WT) animals with periodontitis fed a HF diet.

MATERIAL AND METHODS

Wild-type and TLR4 mutant mice fed a HF diet were divided into four groups (n = 6/group): WT; WT with periodontitis (WT/P); mutant (Mut); and mutant with periodontitis (Mut/P). Periodontitis was induced by placing LPS soaked ligatures around maxillary second molars. Fasting insulin and glucose levels were measured weekly for 10 wk. Glucose tolerance was evaluated at baseline (week 1) and at 9 wk. Insulin signaling (phosphorylation of Akt) and tumor necrosis factor-α (TNF-α) mRNA levels in liver were determined when the mice were killed at week 10.

RESULTS

Mut/P mice developed less alveolar bone loss compared with WT/P mice (p < 0.05). Fasting glucose levels were improved after 8 wk of feeding a HF diet (weeks 9 and 10) in Mut/P mice compared with Mut, WT and WT/P mice (p < 0.05). Glucose tolerance was impaired in all groups compared with baseline (p < 0.05), except for the Mut/P group. Insulin signaling was improved (p < 0.05), and expression of TNF-α was decreased (p < 0.05) in the liver of Mut/P mice compared with the liver of WT/P mice.

CONCLUSION

The TLR4 LOF mutation partially protects against alveolar bone loss and improves glucose homeostasis in mice with periodontitis fed a HF diet.

Are immediate early genes involved in gonadotropin-releasing hormone receptor gene regulation? Characterization of changes in GnRH receptor (GnRH-R), c-fos, and c-jun messenger ribonucleic acids during the ovine estrous cycle

GnRH regulates the secretion and synthesis of gonadotropins by binding to specific receptors located in the plasma membrane of the pituitary gonadotroph. Like the concentration of the signaling ligand GnRH, the number of GnRH receptors (GnRH-R) varies dynamically with the changing endocrine milieu during the ovine estrous cycle. With the recent success in cloning of the mammalian GnRH-R gene, it is becoming increasingly evident that some of the changes in GnRH-R numbers may be mediated at least in part via changes in GnRH-R gene transcription. However, the regulatory steps involved in the GnRH-R transcription are unknown. The present studies were conducted to 1) characterize in detail the changes in GnRH-R gene expression during the 16-day ovine estrous cycle, 2) determine whether or not changes in GnRH-R gene expression during the estrous cycle are paralleled by alterations in the expression of c-fos and c-jun mRNAs, and 3) determine whether GnRH can induce expression of c-fos and c-jun mRNAs. Results revealed that concentrations of GnRH-R mRNA were highest on the day before estrus, when circulating LH concentrations were still low. GnRH-R mRNA concentrations declined steadily starting at 5 h postestrus, the time of the preovulatory LH surge, reaching their lowest levels by 24 h after estrus. Changes in c-jun mRNA levels, in general, paralleled changes in GnRH-R mRNA concentrations, being highest on the day before estrus and declining thereafter. c-Fos mRNA followed a different time course than c-jun mRNA, remaining elevated from Day 8 prior to estrus until the onset of estrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Gonadotropin subunit and gonadotropin-releasing hormone receptor gene expression are regulated by alterations in the frequency of calcium pulsatile signals

Previously, we have shown that intermittent calcium (Ca2+) stimuli increase alpha, LHbeta, and FSHbeta messenger RNAs (mRNAs), and only LHbeta mRNA was increased by continuous Ca2+. As gonadotropin subunit and GnRH receptor (GnRH-R) mRNAs are differentially regulated by alterations in GnRH pulse interval, we aimed to determine whether changes in the frequency of Ca2+ signals play a role in this effect. Cultured adult female rat pituitary cells in perifusion were given pulses of the Ca2+ channel activator BayK 8644 (10 microM; with 10 mM KCl in the injectate), at intervals of 16, 60, or 180 min for 24 h (vehicle pulses or 100 pM GnRH to controls). Pulsatile Ca2+ influx stimulated a rise in all mRNAs examined (P < 0.05 vs. vehicle controls); however, optimal pulse intervals differed. Alpha and LHbeta mRNAs were maximally stimulated by 16- or 60-min pulses (57% and 74% increases, respectively), with 180-min pulses being less effective. In contrast, FSHbeta and GnRH-R mRNAs were selectively stimulated by 180-min pulses (51% and 41% increases, respectively). Pulsatile GnRH produced similar increases in GnRH-R and subunit mRNAs (53-78% vs. controls). These results reveal that alterations in the frequency of Ca2+ signals can regulate gonadotrope gene expression in a differential manner, producing effects similar to previous findings for GnRH. Thus, intermittent increases in intracellular Ca2+ may be an important step in the transmission of GnRH pulse signals from the plasma membrane to the gene.

Detection of glycemic abnormalities in adolescents with beta thalassemia using continuous glucose monitoring and oral glucose tolerance in adolescents and young adults with β-thalassemia major: Pilot study

BACKGROUND

Both insulin deficiency and resistance are reported in patients with β-thalassemia major (BTM). The use of continuous blood glucose monitoring (CGM), among the different methods for early detection of glycemic abnormalities, has not been studied thoroughly in these adolescents.

MATERIALS AND METHODS

To assess the oralglucose tolerance (OGT) and 72-h continuous glucose concentration by the continuous glucose monitoring system (CGMS) and calculate homeostatic model assessment (HOMA), and the quantitative insulin sensitivity check index (QUICKI) was conducted in 16 adolescents with BTM who were receiving regular blood transfusions every 2-4 weeks and iron-chelation therapy since early childhood.

RESULTS

SIXTEEN ADOLESCENTS WITH BTM (AGE: 19.75 ± 3 years) were investigated. Using OGTT, (25%) had impaired fasting blood (plasma) glucose concentration (BG) (>5.6 mmol/L). 2-h after the glucose load, one of them had BG = 16.2 mmol/L (diabetic) and two had impaired glucose tolerance (IGT) (BG > 7.8 and <11.1 mmol/L). Monitoring the maximum (postprandial) BG using CGMS,4 adolescents were diagnosed with diabetes (25%) (BG >11.1 mmol/L) and 9 with IGT (56%). HOMA and QUICKI revealed levels <2.6 (1.6 ± 0.8) and >0.33 (0.36 ± 0.03), respectively, ruling out significant insulin resistance in these adolescents. There was a significant negative correlation between the β-cell function (B%) on one hand and the fasting and the 2-h BG (r=-0.6, and - 0.48, P < 0.01, respectively) on the other hand. Neither fasting serum insulin nor c-peptide concentrations were correlated with fasting BG or ferritin levels. The average and maximum blood glucose levels during CGM were significantly correlated with the fasting BG (r = 0.68 and 0.39, respectively, with P < 0.01) and with the BG at 2-hour after oral glucose intake (r = 0.87 and 0.86 respectively, with P < 0.001). Ferritin concentrations were correlated with the fasting BG and the 2-h blood glucose levels in the OGTT (r = 0.52, and r = 0.43, respectively, P < 0.01) as well as with the average BG recorded by CGM (r = 0.75, P < 0.01).

CONCLUSION

CGM has proven to be superior to OGTT for the diagnosis of glycemic abnormalities in adolescents with BTM. Defective β-cell function rather than insulin resistance appeared to be the cause for these abnormalities.

Leptin in pediatrics: A hormone from adipocyte that wheels several functions in children

The protein leptin, a pleiotropic hormone regulates appetite and energy balance of the body and plays important roles in controlling linear growth, pubertal development, cardiovascular function, and immunity. Recent findings in the understanding of the structure, functional roles, and clinical significance of conditions with increased and decreased leptin secretion are summarized. Balance between leptin and other hormones is significantly regulated by nutritional status. This balance influences many organ systems, including the brain, liver, and skeletal muscle, to mediate the essential adaptation process. The aim of this review is to summarize the possible physiological functions of leptin and its signaling pathways during childhood and adolescence including control of food intake, energy regulation, growth and puberty, and immunity. Moreover, its secretion and possible roles in the adaptation process during different disease states (obesity, malnutrition, eating disorders, delayed puberty, congenital heart diseases and hepatic disorders) are discussed. The clinical manifestations and the successful management of patients with genetic leptin deficiency and the application of leptin therapy in other diseases including lipodystrophy, states with severe insulin resistance, and diabetes mellitus are discussed.

Pituitary activin receptor subtypes and follistatin gene expression in female rats: differential regulation by activin and follistatin

The activins, hormones produced in the gonads and extragonadal tissues (including the pituitary), rapidly increase FSH beta messenger RNA (mRNA) and FSH secretion. In the rat, activin acts via a family of activin receptor (ActR) subunits that includes at least one type I (ActRI or ALK-2) and two homologous type II (IIA and IIB) subunits. We have previously reported that ActRIIA mRNA rises after ovariectomy (OVX). Potentially, the OVX-induced increases in ActR mRNAs could result from altered activin or the activin-binding protein follistatin. It was the purpose of the current studies to determine whether activin and/or follistatin regulated activin receptor subunit mRNAs. Adult female rat pituitaries were dissociated and plated for 48 h, transferred to wells containing follistatin or activin for 2 or 24 h, then RNA extracted for measurement of ActRI, IIA, and IIB and follistatin mRNAs. All three ActR mRNAs were easily detectable in pituitary RNA, with the relative abundance of ActRI > IIA >> IIB (18:9:1). Between 2-24 h, levels of all three ActR mRNAs increased 2- to 3-fold in wells containing medium alone, whereas levels of follistatin mRNA were unchanged. Follistatin significantly reduced FSH secretion and follistatin mRNA, but not the ActR mRNAs. Activin increased ActRI (4-fold, at 2 h), ActRIIB (2-fold, at 24 h), and follistatin (2-fold, at 24 h) mRNAs and FSH release (2-fold, at 24 h), but did not alter ActRIIA mRNA levels. We conclude that 1) pituitary ActR mRNA expression is under inhibitory tone in vivo, as suggested by the effect of pituitary removal and cell dispersion and an earlier report after OVX. 2) Pituitary-derived activin stimulates follistatin (but not ActR) mRNA production, and additional increases in follistatin mRNA can be induced by exogenous activin. 3) Higher concentrations of activin differentially regulate pituitary ActR mRNA expression, suggesting that activin exerts a positive feedback effect on its own receptor.

Regulation of pituitary follistatin and inhibin/activin subunit messenger ribonucleic acids (mRNAs) in male and female rats: evidence for inhibin regulation of follistatin mRNA in females

The regulation of FSHbeta messenger RNA (mRNA) expression is complex and involves signals from the hypothalamus and gonads. Additionally, the local (pituitary) production of activin and follistatin appears to serve as an important modulator of endocrine signals for FSHbeta regulation. The purpose of these studies was to identify factors controlling pituitary activin/inhibin subunit and follistatin mRNA production in male and female rats. Both males and females expressed the follistatin, inhibin alpha, and betaB mRNAs, whereas the betaA mRNA was not detected. In males, levels of FSHbeta and follistatin were higher than those in females. After gonadectomy, levels of FSHbeta and follistatin increased in both sexes, whereas betaB rose only in females. In males, blockade of GnRH action from the time of castration prevented the increase in FSHbeta and follistatin, suggesting that GnRH is the primary stimulus for these gene products. In females, treatment with a GnRH antagonist only partially prevented the rise in FSHbeta, follistatin, and betaB expression, suggesting that other factors were also important. Passive immunoneutralization of circulating inhibin increased FSHbeta and follistatin (but not betaB), providing evidence that inhibin is a physiological regulator of follistatin. Replacement of estradiol at the time of ovariectomy prevented the increase in betaB mRNA, suggesting that gonadal steroids may also act via local factors to regulate FSHbeta. In summary, these studies provide evidence that GnRH, gonadal steroids, and gonadal peptides probably regulate FSHbeta expression at least in part via the intrapituitary activin/follistatin system.

Enhanced effectiveness of pulsatile 3',5'-cyclic adenosine monophosphate in stimulating prolactin and alpha-subunit gene expression

cAMP is involved in the regulation of secretory activity in lactotrope, thyrotrope, and gonadotrope cells. The present study examined whether pulsatile or intermittent changes in cAMP are more effective than a continuous stimulation in increasing pituitary hormone gene expression. Pituitaries from adult female rats were dissociated, plated for 48 h (7-8 x 10(6) cells per well) to allow attachment to Matrigel-coated plastic coverslips, then inserted into perifusion chambers (five to eight chambers per group). After 24 h of treatment, the cells were recovered, RNA extracted, and messenger RNAs (mRNAs) determined by dot blot hybridization. Perfused cells were exposed to either hourly pulses of monobutyryl cAMP (Bt cAMP, 0.01, 0.1, or 1 mM; 1 mM butyrate pulses to controls), or continuously to forskolin (10 microM). Bt cAMP pulses increased both PRL and alpha-subunit mRNAs, maximal after the 0.1 mM dose for PRL (51% increase vs. butyrate controls) and after the 1 mM dose for alpha (60% increase). However, forskolin was ineffective in increasing PRL or alpha mRNA concentrations. TSH, LH, and FSH beta-subunit mRNAs were not altered by Bt cAMP pulses or forskolin. To confirm the different effects of pulsatile vs. continuous cAMP on PRL and alpha-subunit mRNAs, the response to pulsatile 8-bromo cAMP (1 mM) or Bt cAMP (0.5 mM) was compared to continuous Bt cAMP (0.5 mM). PRL and alpha-subunit mRNAs were increased by both cAMP analogs given in a pulsatile manner but not by continuous Bt cAMP. PRL and LH secretory responses (determined in perifusate samples after 2 h and 22 h of treatment) revealed that both PRL and LH release was increased by cAMP stimulation, given either in a pulsatile or continuous manner. These results show that PRL and alpha-subunit gene expression were sensitive to changes in cAMP stimulation, whereas that of TSH, LH, and FSH beta were unaltered. Only intermittent cAMP stimuli were effective in increasing PRL and alpha mRNAs. These data suggest that pulsatile fluctuations in intracellular cAMP may be essential for maximal expression of the PRL and alpha genes. Thus, intermittent changes in intracellular second messengers may be a necessary part of the pathway involved in the transduction of signals from the plasma membrane to the nucleus.

Ovarian inhibin subunit gene expression: regulation by gonadotropins and estradiol

Inhibin and FSH maintain a dynamic inverse relationship throughout the rat estrous cycle. In particular, inhibin alpha- and beta A-subunit messenger RNAs (mRNAs) have been shown to be maximally expressed immediately after the midcycle gonadotropin surge, when both circulating estradiol (E2) and inhibin are also elevated. The current study was designed to investigate the regulation of inhibin subunit gene expression and secretion in vivo by recombinant human FSH (rhFSH) and estradiol. Initially, we determined if physiological levels of rhFSH regulated ovarian inhibin subunit gene expression and secretion. Hypophysectomized (HYPOX) adult female rats received hCG (10 IU, sc) and were then treated for 24 h with either rhFSH (0.5-20 IU every 6 h, i.v.) or saline. Hypophysectomy reduced inhibin subunit mRNAs as well as serum inhibin and estradiol. Although 0.5 IU rhFSH was ineffective in increasing inhibin subunit mRNAs, all doses between 2.5-20 IU increased inhibin subunit gene expression and inhibin secretion. Inhibin alpha-, beta A-, and beta B-subunit mRNAs were increased to a similar degree (3- to 5-fold) by all rhFSH doses of 5 IU or more. Similarly, serum E2 and inhibin were increased 2- and 3-fold, respectively, above HYPOX values after all doses of rhFSH of 5 IU or more. To investigate the role of a pure FSH signal in a physiological dose on inhibin subunit gene expression, HYPOX rats were given either rhFSH (5 IU, i.v., every 6 h for 24 or 48 h), hCG (10 IU, sc), or their combination. Neither gonadotropin when given alone altered inhibin subunit gene expression or serum E2 concentrations. Inhibin secretion rose in response to rhFSH alone, but not to hCG. The combination of hCG and rhFSH resulted in increased inhibin subunit mRNAs (3- to 5-fold) as well as circulating E2 and inhibin concentrations. We next studied the effects of E2 replacement in HYPOX rats at both physiological (serum approximately equal to 40 pg/ml) and higher doses (serum approximately equal to 800 pg/ml, to mimic intraovarian concentrations) in the presence or absence of exogenous gonadotropins (for 24 and 48 h). Although not as effective as gonadotropins, both E2 regimens increased inhibin alpha to a similar degree (2-fold), whereas beta-subunit mRNAs were unchanged at 24 h. Serum inhibin concentrations were increased only 48 h after high dose E2 treatments. As the actions of E2 and gonadotropins on alpha-subunit mRNA were not additive, E2 appears to mediate gonadotropin regulation of alpha-subunit gene expression.(ABSTRACT TRUNCATED AT 400 WORDS)

Testosterone is required for gonadotropin-releasing hormone stimulation of luteinizing hormone-beta messenger ribonucleic acid expression in female rats

Pulsatile GnRH stimulates the synthesis and secretion of LH and FSH in both male and female rats. In the male rat, exogenous GnRH pulses increase alpha, LH and FSH beta messenger RNAs (mRNAs) 3-fold within 24 h. In contrast, the results of recent in vivo and in vitro studies have shown that GnRH stimulates an increase in alpha and FSH beta mRNAs, but not LHbeta. However, during the estrous cycle, LHbeta mRNA increases during the GnRH-induced LH surge on proestrus afternoon. This increase in LHbeta mRNA appears to be coincident with a transient rise in serum testosterone (T). Therefore, the present study was conducted to determine whether T has a role in facilitating GnRH stimulation of LHbeta mRNA expression. In the first group of studies, adult female rats were ovariectomized, and T implants were inserted sc 7 days before the study (serum T, 1.86 ng/ml). Animals received iv pulses of GnRH (25 ng; 30-min interval) for 6-24 h (saline pulses to controls). The data showed that in the presence of T, GnRH stimulated a significant increase in LHbeta (as well as alpha and FSH beta) mRNAs within 6 h (P < 0.05 vs. saline-pulsed controls). Other results revealed that T treatment was critical to the stimulatory effect of GnRH on LH beta mRNA. A second group of studies examined the time course and dose effects of T on LH beta mRNA expression. Maximal LH beta mRNA responses to GnRH (3-fold increase vs. saline controls; P < 0.05) were seen after pretreatment with the lowest dose of T examined (serum T, 0.42 ng/ml), which is similar to T concentrations on proestrus. Higher doses of T suppressed LH release, as well as LH mRNA responses to GnRH. The T-induced LHbeta mRNA response to pulsatile GnRH was seen within 24 h of exposure to T and was the result of an androgenic action, as similar results were observed in rats that received dihydrotestosterone. These findings suggest that T is required to facilitate GnRH stimulation of LHbeta mRNA in the female rat. Moreover, in the presence of the concentrations of T seen on proestrus, LHbeta mRNA increases within 6 h, which is similar to the time course seen during the LH surge. Thus, the present results also suggest that the combined effects of the rise in serum T and increased GnRH secretion induce the rapid rise in LHbeta mRNA expression on the afternoon of proestrus.

Differential actions of thyrotropin (TSH)-releasing hormone pulses in the expression of prolactin and TSH subunit messenger ribonucleic acid in rat pituitary cells in vitro

The influence of TRH pulse pattern on PRL and TSH alpha- and beta-subunit gene expression was examined in vitro. Pituitaries from adult female rats were dissociated and plated for 48 h to allow attachment to collagen-coated microcarrier beads. The beads were perifused for 24 h with TRH (pulses or continuous). To examine the effects of TRH pulse amplitude, TRH pulses (0.032-100 nM) were given every 60 min (controls received either medium pulses or continuous 100 nM TRH). PRL mRNA rose progressively to a peak at 4 nM TRH/pulse (93% increase vs. medium-pulsed controls), but higher TRH pulse amplitudes were less effective. alpha-Subunit mRNA also rose with increasing TRH pulse dose, with maximal (137%) elevations after 100-nM pulses. TSH beta mRNA concentrations were increased by TRH pulse doses between 0.8-20 nM, but a clear dose-response pattern was not seen. Continuous TRH (100 nM) resulted in PRL, TSH beta, and alpha mRNAs that were less than the values in medium-pulsed controls. To assess the effects of pulse frequency, 4-nM TRH pulses were given at intervals between 15-240 min (controls received medium pulses or continuous 4 nM TRH). PRL mRNA was increased (55-107% increase) after all pulse intervals, except 240 min, and rose to a similar degree after 4 nM TRH given continuously. alpha-Subunit mRNA concentrations increased by a lesser degree and also did not rise after the slowest (240 min) pulse interval. In contrast, TSH beta mRNA levels increased progressively as pulse intervals were increased, and maximal (85%) elevations were seen after 240-min pulses. Continuous 4 nM TRH did not alter alpha or TSH beta mRNAs. These results show that pulsatile TRH is more effective than continuous TRH in stimulating PRL, TSH beta, and alpha mRNAs. Further, the pattern of TRH pulsatile signals can influence the expression of these pituitary hormone genes in a differential manner.

L-mandelate dehydrogenase from Rhodotorula graminis: comparisons with the L-lactate dehydrogenase (flavocytochrome b2) from Saccharomyces cerevisiae

L-Lactate dehydrogenase (L-LDH) from Saccharomyces cerevisiae and L-mandelate dehydrogenase (L-MDH) from Rhodotorula graminis are both flavocytochromes b2. The kinetic properties of these enzymes have been compared using steady-state kinetic methods. The most striking difference between the two enzymes is found by comparing their substrate specificities. L-LDH and L-MDH have mutually exclusive primary substrates, i.e. the substrate for one enzyme is a potent competitive inhibitor for the other. Molecular-modelling studies on the known three-dimensional structure of S. cerevisiae L-LDH suggest that this enzyme is unable to catalyse the oxidation of L-mandelate because productive binding is impeded by steric interference, particularly between the side chain of Leu-230 and the phenyl ring of mandelate. Another major difference between L-LDH and L-MDH lies in the rate-determining step. For S. cerevisiae L-LDH, the major rate-determining step is proton abstraction at C-2 of lactate, as previously shown by the 2H kinetic-isotope effect. However, in R. graminis L-MDH the kinetic-isotope effect seen with DL-[2-2H]mandelate is only 1.1 +/- 0.1, clearly showing that proton abstraction at C-2 of mandelate is not rate-limiting. The fact that the rate-determining step is different indicates that the transition states in each of these enzymes must also be different.

Ovariectomy and inhibin immunoneutralization acutely increase follicle-stimulating hormone-beta messenger ribonucleic acid concentrations: evidence for a nontranscriptional mechanism

After ovariectomy (ovx), FSH beta mRNA levels and serum FSH increase 2- to 3-fold within 12 h, and this persists in the presence of a GnRH antagonist. As a fall in plasma estradiol and progesterone appears to regulate FSH beta via increased GnRH secretion, it is thought that the acute (by 2 h) changes in FSH beta mRNA after ovx reflect falling levels of plasma inhibin. The current study addressed the following questions. 1) Does a reduction of circulating inhibin (via passive immunoneutralization or gonadectomy) increase FSH beta mRNA levels? 2) If so, are the acute increases in FSH beta mRNA associated with changes in the transcription rate? Adult male and female rats received 0.5 ml antiinhibin antiserum, iv, and were killed 2 or 12 h later. A second group of rats was gonadectomized; some received a GnRH antagonist and were killed at various intervals between 2 h and 7 days later. In adult males, no change in gonadotropin mRNA levels was observed after either addition of inhibin antiserum or removal of the testes. In contrast, in adult female rats, both ovx and inhibin antiserum increased FSH beta mRNA levels (2-fold) within 2 h, and a similar increase occurred in the presence of a GnRH antagonist. To determine if the increase in FSH beta resulted from increased mRNA synthesis, adult female rats were ovx, and half received a GnRH antagonist. Animals were killed 2 or 12 h later, and transcription rates were measured by nuclear run-off assay in pituitaries pooled from three rats. The transcription rate of the alpha-subunit, although not altered by ovx, was decreased in animals receiving the GnRH antagonist. Transcription of the LH beta gene was increased within 2 h after ovx, a change that was abolished by the GnRH antagonist. mRNA concentrations of either alpha or LH beta do not increase acutely after ovx, suggesting that GnRH regulates alpha and LH beta gene transcription and 12 h or more of mRNA synthesis are required to increase cytoplasmic concentrations. The FSH beta gene transcription rate was unchanged in both ovx and GnRH antagonist-treated animals, but serum FSH increased at 12 h. These data indicate that the rapid GnRH-independent increase in FSH beta mRNA levels seen immediately after ovx is not associated with altered mRNA synthesis and suggest that inhibin may also regulate FSH beta gene expression through nontranscriptional mechanisms.