GH Decrease Research Articles

Growth hormone gene expression and secretion in aging rats is age dependent and not age-associated weight increase related.

GH secretion declines with age in rats and humans and a reduction in GH gene expression has been demonstrated in aging rats. GH secretion also diminishes in obesity; thus, the aim of this study was to determine whether GH decrease in aging rats is due to body weight gain or to aging. Three groups of male Wistar rats of different ages were studied (young, 3 months; middle-aged, 11 months; old, 27 months). The middle-aged group was established on a statistical analysis and corresponded to the youngest age at which body weight was not significantly different from the old (27 month) group. Thus, by using this group as control for comparison with animals with the same weight and an older age, the effects due to aging itself could be determined. Body weight (g, mean +/- sD) 3 months: 361 +/- 5.6; 11 months: 713 +/- 39; 27 months: 635 +/- 38. In comparison with 3-month-old rats, the 11-month-old animals showed no difference in pituitary GH messenger RNA (mRNA) accumulation and pituitary and serum IR-GH levels. Similarly IGF-I.a, IGF-I.b mRNA transcripts and IG-FBP-3 mRNA accumulation in the liver showed no significant differences between the two groups. On the contrary, when the 27-month-old rats were compared with the 11-month-old animals, lower levels of pituitary GH mRNA and serum and pituitary IR-GH were found. Pituitary GH mRNA decreased 37.5 +/- 7.7% P < 0.001, pituitary IR-GH content diminished (5.2 +/- 3.4 vs. 55 +/- 10.7 ng/mg of protein, P < 0.001) and serum IR-GH decreased (3.5 +/- 1.8 vs. 12.5 +/- 4.2 ng/ml, P < 0.01). Liver IGF-I.a and IGF-I.b mRNA transcripts accumulation and serum IGF-I were significantly diminished. IGF-I.b mRNA accumulation decreased 35.8 +/- 1.2% P < 0.05 and IGF-I.a 36 +/- 5.6% P < 0.05; serum IR-IGF-I levels diminished (759 +/- 152 vs. 1327 +/- 67 ng/ml, P < 0.05). Liver IGFBP-3 mRNA accumulation decreased 79 +/- 4.2% P < 0.001. These results indicate that the decrease in GH gene expression and secretion, as well as the expression of genes induced by GH such as IGF-I and IGFBP-3, is due to aging and not to the increase in body weight that takes place with aging.

Feeding colostrum increases circulating insulin-like growth factor I in newborn pigs independent of endogenous growth hormone secretion.

Our objective was to examine the influence of feeding and endogenous GH secretion on circulating IGF-I in colostrum-deprived newborn pigs fed colostrum (n = 4), formula (control, n = 4), or water (n = 4). In another four formula-fed pigs, GH was ablated (GRF-A) with two intravenous injections of a GH releasing-factor antagonist (N-Ac-Tyr1,D-Arg2)-GRF(1-29)-NH2. Blood was serially sampled in all pigs to measure plasma IGF-I and GH profiles. Feeding increased plasma IGF-I concentration two- to fourfold and decreased GH secretion. Despite a more than 80% decrease in the plasma GH in GRF-A pigs, the circulating IGF-I concentration was similar to that in control pigs. In colostrum-fed pigs, plasma IGF-I was higher than that in control pigs, despite equal nutrient intake and lower circulating GH. There were no differences in plasma IGF binding protein (IGFBP)-3 levels among the treatment groups. However, the relative abundance of plasma IGFBP-4 was lower, and that of IGFBP-1 higher, in unfed pigs than in any of the three fed groups. The plasma insulin concentration was not different among fed pigs, but it was lower in unfed pigs. Our results indicate that the circulating IGF-I concentration is more dependent on nutrient intake than on GH in newborn pigs, despite relatively high GH concentrations. However, because the nutrient content in the formula was designed to match that of colostrum, a factor other than nutrient intake and GH was responsible for the maximal increase in circulating IGF-I concentration observed in colostrum-fed pigs.

Effects of oral glucose administration on plasma growth hormone levels in women with polycystic ovary syndrome

Objective: To evaluate the sensitivity of GH secretion to the suppressive effect of oral glucose administration in women with polycystic ovary syndrome (PCOS). Design: Comparison of the GH response to an oral glucose load in women with PCOS and in weight-matched normally menstruating women (controls). Setting: Reproductive endocrinology unit. Patient(s): Eighteen obese and 11 nonobese patients and 10 obese and 10 nonobese controls. Intervention(s): After an overnight fast, each woman underwent a 75-g, 3-hour oral glucose tolerance test (OGTT). Mean Outcome Measure(s): Growth hormone, glucose, and insulin responses to OGTT. Result(s): No significant differences in the glycemic and insulinemic responses were found between the patients and the weight-matched controls. No decrease in plasma GH was observed in both obese and nonobese patients and in obese controls during the OGTT, whereas a significant GH decrease occurred in nonobese controls 60 and 120 minutes after glucose intake. Conclusion(s): Oral glucose administration was unable to suppress GH levels in nonobese as well as in obese women with PCOS and in obese control women. These data suggest that both PCOS and obesity are associated with a reduced sensitivity of GH secretion to glucose suppression.

Prediction of efficacy of octreotide therapy in patients with acromegaly.

Octreotide (OCT) administration provides a biochemical cure in most acromegalic patients. This drug, however, causes several side effects and is very expensive. Acute testing has been reported to predict chronic responsiveness to OCT administration. The aim of this retrospective study was to evaluate which test, if any, among acute testing, short-term (1 month) administration, and 111In-pentetreotide (111In-DTPA-Phe-D-OCT) scintigraphy, is best in predicting response to long-term OCT treatment. Sixty-eight patients with active acromegaly were studied. An acute test (100 micrograms sc OCT) was performed as usual: a GH decrease greater than or equal to 50% of baseline was considered a positive response. GH and insulin-like growth factor I (IGF-I) were then assayed after 1 month (300 micrograms daily) and 3 months (150-600 micrograms daily) of OCT administration. GH was considered normalized when decreased less than or equal to 5 micrograms/L. Twenty-six of 68 patients were subjected to 111In-pentetreotide scintigraphy. Linear correlation analysis of the results was performed. Sensitivity, specificity, and positive and negative predictive values of the three tests were also calculated. Thirty-eight of 68 patients (56%) responded to the acute test. Among these 38 patients, 20 experienced normalization of GH and IGF-I levels during long-term therapy, as did 8 patients who did not respond to the acute test. No significant correlation was found between GH percent decrease during acute testing and long-term therapy (r = 0.11). Seven patients who responded to the acute test and 2 who did not respond had adenoma shrinkage during therapy. Conversely, GH and IGF-I decrease after short-term treatment significantly correlated with long-term treatment (r = 0.76 and 0.64, P < 0.01). Of the 26 patients subjected to 111In-pentetreotide scintigraphy, 13 had significant tracer uptake: normalization of GH and IGF-I was obtained in 8 patients. A significant correlation was found between tracer uptake and GH/IGF-I inhibition after 3 months of therapy (r = 0.6; P < 0.05). In the whole population, the positive predictive value of acute testing, short-term OCT administration, and 111In-penetreotide scintigraphy was 53%, 70%, and 73%, respectively, when the GH normalization (< 5 micrograms/L) after 3 months of therapy was considered. Moreover, 111-In-pentetreotide scintigraphy had the highest specificity (100% in patients with baseline GH values below 50 micrograms/L) compared with that of acute testing and short-term OCT administration. The acute test cannot be considered as a valuable index to identify patients' responsiveness to long-term OCT therapy, but it can be useful to test tolerability. By contrast, 1 month of OCT administration or the in vivo imaging of somatostatin receptors by 111-In-pentetreotide might better indicate the patients who might effectively benefit from this treatment.

Photosynthetic CO 2-Assimilation, Chlorophyll Fluorescence and Zeaxanthin Accumulation in Field Grown Maple Trees in the Course of a Sunny and a Cloudy Day

Changes in photosynthetic CO 2 fixation, variable chlorophyll fluorescence ratios, carotenoid composition, zeaxanthin accumulation and the de-epoxidation state DEPS of the zeaxanthin cycle were simultaneously determined in leaves of field-grown maple trees (Acer platanoides L.) on a sunny and a cloudy day in August. In the course of the sunny day the photosynthetic net CO 2 assimilation rates P N rose with the PPFD in the morning and were then predominantly determined by the stomata opening (partial closure at noon and further closure in the afternoon as seen from the decrease in gH 2 O-values). When the rate of Co 2 -fixation began to decrease at a high sun irradiance (high PPFD) at about 10 :00 am (partial closure of stomata), large zeaxanthin amounts were accumulated in a biphasic process : 1) by a fast phototransformation of existing violaxanthin into zeaxanthin and 2) by a slow but continuous increase of the zeaxanthin level of 50 to 60% via de novo biosynthesis. A strong zeaxanthin accumulation depends on a high proton gradient and only proceeded when P N was light-saturated and was reduced by a partial closure of stomata. The zeaxanthin accumulation in the morning followed the rise in PPFD with some delay as well as the zeaxanthin retransformation to violaxanthin at the PPFD decrease in the late afternoon and evening. On the sunny day the photochemical and non-photochemical quenching coefficients of the chlorophyll fluorescence, qP and qN, showed an inverse relationship. With increasing PPFD qP declined to very low values (below 0.1) and qN increased to high values >0.9. The variable chlorophyll fluorescence ratios Fv'/Fm' and ΔF/Fm', indicators of the quantum efficiency of photosynthetic electron transport of photosystem II, declined parallel to the high PPFD-induced decline of qP and the rise in qN. There was, however, no clear correlation between the changes in chlorophyll fluorescence parameters and the rise and fall of zeaxanthin in the course of the sunny day. In fact, in the afternoon the zeaxanthin level decreased much earlier than the decline in qN and the rise and regeneration of the initial values of qP, Fv'/Fm' or ΔF/Fm'. Despite the strong increase in qN and the large decrease in qP, Fv'/Fm' and ΔF/Fm', the net CO 2 assimilation rates P N of maple leaves only declined by 22 % as compared to the maximum P N values at 10 :15 am indicating that the photosynthetic apparatus was only marginally photoinhibited. It is assumed that only a small fraction of chloroplasts at the upper leaf side became photoinhibited and functioned as a light filter for the other chloroplasts in the lower mesophyll parts which remained photosynthetically active. The results demonstrate that the changes of the chlorophyll fluorescence parameters qP and qN as well as of the ratios Fv'/Fm' or ΔF/Fm', measured with the PAM fluorometer at the upper leaf side, are not representative of the total pool of leaf chloroplasts. For this reason it is suggested to measure the chlorophyll fluorescence parameters not only at the upper but also at the lower leaf side in order to obtain more representative information on the chloroplasts of the whole leaf. In addition, chlorophyll fluorescence measurements should always be complemented by net CO 2 fixation measurements in order to investigate to which degree a presumed photoinhibition really exists at the whole leaf level. On the cloudy day up to 3 pm the PPFD was very low, yet net photosynthesis (P N ) was performed at good rates.

Characterization of the paradoxical growth hormone inhibitory effect of galanin in acromegaly.

Galanin is a 29-amino acid straight-chain biologically active peptide which has been found to decrease circulating GH levels in some acromegalic patients, whereas is able to increase GH secretion in normal subjects. The aim of our study was to ascertain the incidence, entity and mechanism of the paradoxical GH inhibitory effect of galanin in acromegaly also looking at possible correlations between the GH responses to galanin and the main clinical and biochemical features of the patients. Finally, the effects of either successful or unsuccessful neurosurgical intervention on the GH inhibitory effect of galanin in acromegaly were investigated. A series of 23 consecutive patients with active acromegaly seen at the Endocrine Section of the Department of Internal Medicine of the University of Brescia (Italy) between 1991 and 1994 was examined. The acromegalic patients were subdivided in group 1 (i.e. patients who were 1) untreated, 2) evaluated before surgery, and 3) not cured after surgery and radiotherapy) and group 2 (i.e. surgically cured). All patients were submitted at least once to the following biochemical and radiological evaluations: 1) baseline serum insulin-like growth factor-I and PRL samples, 2) iv infusion of synthetic porcine galanin (500 micrograms in 100 mL saline) from -10 to 30 min, 3) iv bolus injection of TRH (200 micrograms) at time zero, 4) oral glucose tolerance test (75 g glucose, orally) at time zero, and 5) magnetic resonance of the pituitary sella. Adenomatous tissue obtained during neurosurgery in four patients was cultured in vitro, and the effect of the addition of galanin in the culture medium on GH secretion was tested. During galanin infusion in 19 of 21 group 1 patients, serum GH levels were lower with respect to baseline (range of GH decrease, -6.2 to -85.4% with respect to basal levels). During galanin infusion, no reductions in GH levels were observed in the acromegalic patients cured after neurosurgery (group 2); on the contrary, 6 of 7 patients displayed a normal stimulatory response to galanin (range of GH increase, +120-1533.3% of the basal level). A significant correlation between the percent decrease in GH levels after galanin treatment and the percent increase after TRH was found in group 1 patients (r = -0.783; P < 0.05). In three of the four adenomas examined, galanin determined a clear decrease in GH secretion (mean nadir, 63.3 +/- 12% of the baseline secretion rate). In conclusion, we demonstrated that the large majority of numerous patients with active acromegaly show a decrease in serum GH levels after galanin administration.(ABSTRACT TRUNCATED AT 400 WORDS)

Clinical significance of the growth hormone response to vasoactive intestinal peptide and gonadotropin-releasing hormone in acromegaly

GH-secreting pituitary adenomas causing acromegaly can be classified into at least two types, i.e. the lactotroph-like adenoma and somatotroph-like adenoma. From a functional point of view, the lactotroph-like adenoma is characterized by positive GH responses to TRH and bromocriptine (Br) with a GH increase or decrease, respectively, whereas the somatotroph-like adenoma is characterized by a high GH response to GHRH and a low GH response to TRH and Br. In this study, we examined whether the loading of vasoactive intestinal peptide (VIP) and GnRH, another hypothalamic hormone capable of stimulating GH secretion in acromegaly, have a pathophysiological significance as TRH, GHRH, and Br tests. In 52 patients with active acromegaly, we performed iv bolus injections of TRH (500 μg), GHRH (100 μg), VIP (100 μg), and GnRH (100 μg), and a peroral administration of Br (2.5 mg), in order to compare the GH responses to these loads. There was a significant correlation that the higher was the GH response after TRH the greater was the GH decrease after Br. Although statistically insignificant, there was a trend (0.05 < p < 0.1) that the higher was the GH response after GHRH the smaller was the GH decrease after Br. In addition, as novel findings, we observed that the GH responses to GHRH, VIP, and GnRH were in significant positive correlations to each other, and that the higher were the GH responses after VIP and GnRH the smaller was the GH decrease after Br. In agreement with this, we also found that a simultaneous GH responsivity to VIP and/or GnRH in TRH-responsive acromegalics significantly enhanced the GH response to GHRH and lowered the Br responsiveness compared to the data of pure TRH-responders. From these results, we hypothesize that the positive GH responsiveness to VIP and GnRH, like that to GHRH, may be a feature of the somatotroph-like pituitary adenoma causing acromegaly. The present results appear to be of some help in understanding the basis of the great variabilities in the GH responses to various dynamic testings in acromegaly.

Long-term treatment of acromegaly with the slow-release somatostatin analogue lanreotide

Thirteen patients with active acromegaly despite previous surgery were treated with 30 mg lanreotide im twice a month for 9 months. In 10 subjects the treatment continued to 19 months. GH serum levels of all patients decreased significantly from an initial value of 32.0 (29.4) micrograms/l [median (standard error of median)] to 10.0 (3.6) and 19.1 (5.7) after 3 and 9 months of treatment, respectively. In the 10 patients with the treatment longer than one year the decrease in GH was from 46.8 (29.4) micrograms/l to 12.5 (5.0) and 16.1 (5.3) after 13 and 19 months, respectively. IGF-I serum levels decreased significantly from 1193 (73) micrograms/l to 782 (99) and 621 (103) after 3 and 9 months, respectively, and were normalized in 3 patients. In the 10 patients treated for longer than one year, levels decreased significantly from 1318 (74) micrograms/l to 653 (170) and 742 (180) after 13 and 19 months, respectively. IGF BP-3 levels were reduced to the normal range in 6 patients and decreased from 8.7 (1.5) mg/l to 6.4 (0.8) and to 5.4 (1.0) after 3 and 9 months, respectively. In the patients with the 19 months treatment the decrease was from 9.3 (1.6 mg/l to 3.9 (0.9) and 4.8 (0.9) after 13 and 19 months, respectively. The IGF BP-3 to IFG I ratio increased in 7 patients. This elevation significantly correlated with the decrease in bioassayable somatomedin. Prolactin serum levels fell in all patients with increased prolactin secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in the secretion of growth hormone in vivo and in vitro in infantile and prepubertal Holstein bull calves

The average concentration of GH in blood is high at birth and declines during the period of sexual maturation in bulls. The objectives of these studies were (1) to define age-related changes in vivo in the pulsatile secretion of GH from birth to puberty, (2) to determine whether pituitary cell content of GH and characteristics of the secretion of GH in vitro reflect age-related changes in vivo, and (3) to examine whether responsiveness to GH-releasing hormone (GHRH) and somatostatin (SRIF) in vitro changed with age in Holstein bull calves. In experiment 1, calves were bled every 15 min for 12 h at < 1, 12 and 42 weeks of age (n = 5/group), these being representative of infantile, juvenile and pubertal stages of development. Calves were killed 3 to 5 days later and the pars distalis of the anterior pituitary gland was enzymatically dispersed into a suspension of single cells. Aliquots of cells were extracted with 0.01 mol NaHCO3/l to determine the content of GH and cultured for 18 and 72 h. As expected, the average concentration of GH in plasma decreased with age (P < 0.001). The initial decrease in GH was caused by a reduction in the baseline concentration between birth and 12 weeks of age. There was a marked decrease in GH pulse amplitude between 12 and 42 weeks of age and a further reduction in the baseline concentration. In contrast, the pulse frequency of GH increased (P < 0.05) from < 1 week to 12-weeks of age and remained constant thereafter.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of somatostatin in the acute immobilization stress-induced GH decrease in rat

In the present work we have investigated to what extent somatostatin (SRIF) release from median eminence (ME) is affected by stress immobilization (IMO) in unanesthetized rats stereotaxically implanted with a push-pull cannula (PPC). One week after implantation, the ME was perfused with artificial cerebrospinal fluid for 1 hour in basal, stress and recovery conditions respectively. Samples were collected every 15 min and SRIF was measured by RIA. In another group of animals, a jugular cannula was inserted the day before and plasma samples were taken off simultaneously with the ME perfusate for GH and SRIF analysis respectively. SRIF release from the ME is rapidly (15 min) and significantly increased (58 ± 11 vs 28 ± 5 pg/15 min; n=7; P < 0.01) in rats bearing only PPC. Intriguingly, animals bearing a jugular catheter plus a PPC showed no increase in SRIF release during the first 15 min of IMO in spite of a striking decrease of plasma GH (27.2 ± 3.8 vs 3.6 ± 1.3 ng/ml; n=6; P < 0.001) observed at this time. However, in spite that the animals responded with a significant increase in SRIF, the response was later and more reduced than in animals without jugular cannula. Since our two rat groups -as result of jugular cannula surgery 24 hours before- showed differences such as a food intake, body weight gain, plasma GH levels and basal SRIF release, we think that these differences could explain the modifications in the regulatory mechanisms involved in GH control under acute stress.

Effects of a chronic treatment with octreotide in patients with functionless pituitary adenomas.

The effects of a chronic treatment with octreotide were evaluated in 19 patients affected with functionless pituitary adenomas. Octreotide caused a significant decrease of GH and IGF-I levels in all the patients and no significant change in thyroid, adrenal and gonadal function. In contrast, during the therapy, the glucose response to an oral glucose tolerance test considerably increased and was delayed, while the insulin response decreased and was delayed. Serum alpha-subunit (alpha-SU) was above normal in 10 of 16 patients in which this evaluation was performed: octreotide caused a significant decrease (p < 0.01) of alpha-SU levels in 6 of these 10 patients. Octreotide did not induce any significant change in visual fields except in 1 patient, who had a great improvement of visual perimetry and a decrease of alpha-SU levels but unmodified CT scan features. In our series of patients, octreotide was ineffective in reducing tumor mass. The efficacy of octreotide might rely on the presence of somatostatin receptors on adenoma-cell membranes. Therefore patients with functionless adenomas to be treated with octreotide might be identified with pituitary scintiscan using the recently available labeled 111In-octreotide.

Further defects in counterregulatory responses induced by recurrent hypoglycemia in IDDM.

We evaluated the effect of previous experimental hypoglycemia on counterregulatory responses to hypoglycemia in 13 IDDM patients. These patients had defects in counterregulatory responses to hypoglycemia compared with 7 nondiabetic control subjects. Plasma EPI and glucagon responses to hypoglycemia in IDDM patients were approximately 60% of levels in nondiabetic subjects (P less than 0.02 and P less than 0.001, respectively). Hepatic glucose output ([3-3H]glucose) was reduced by approximately 60% of normal (P less than 0.005), and the glucose infusion rate required to maintain plasma glucose was correspondingly greater in people with IDDM (P less than 0.001). With a modified glucose clamp (plasma insulin approximately 330 pM), the diabetic subjects underwent two sequential 120-min periods of hypoglycemia (approximately 3.0 mM) with an intervening 60-min euglycemic recovery period. In the IDDM patients, there were 30-50% decreases in plasma GH (P less than 0.005) and cortisol (P less than 0.001) responses during the second hypoglycemic period compared with the first. In addition, glucose output, already defective compared with that in nondiabetic subjects, was further reduced by 33% (P = 0.03) during the second period of experimental hypoglycemia. There was no effect of repeated hypoglycemia on the responses of plasma glucagon, EPI, or NE, though plasma EPI was correlated directly with glucose output (P less than 0.001) and inversely with glucose uptake (P less than 0.05). There was no correlation between the rise in glucose output during hypoglycemia and antecedent glycemic control as measured by HbA1.(ABSTRACT TRUNCATED AT 250 WORDS)

Genotypic and Drought‐Induced Differences in Carbon Isotope Discrimination and Gas Exchange of Cowpea

Carbon isotope composition may be useful for selection in plant breeding. Theory predicts that 13C discrimination (Δ) by leaves can be associated with the ratio of photosynthesis (as indicated by CO2 assimilation rate, A) to leaf conductance (g) to diffusion of water vapor or CO2. In earlier studies with a cowpea (Vigna unguiculata [L.] Walp.) mutant and parent, drought‐induced differences in Δ were associated with expected differences in A/g based on theory, whereas genotypic differences in Δ were not. Studies were conducted for 2 yr with a broader range of genotypes to determine whether these conclusions are generally valid. Eighteen cowpea accessions were grown under weekly irrigation or stored soil moisture in four randomized split blocks at Riverside, CA, in 1987. Twelve of these cowpea accessions were studied under similar conditions in 1988. Gas exchange measurements were made of A and leaf conductance to water vapor (gh), and leaves were sampled for Δ. Significant drought‐induced reduction in Δ of 1.6 × 10−3 was observed and an increase in A/g of 42% which is similar to the value (36%) predicted based upon the change in Δ. The drought‐induced increase in A/g was due to substantial decreases in gh, which more than compensated for a 59% reduction in A. Genotypic differences in Δ of up to 2 × 10−3 were observed which were predicted to be associated with 43% higher A/g; however, genotypic differences in A/g were small, and genotypic means of A/g and Δ were not associated as expected based on theory. Genotypic differences in Δ were more consistent than differences in A/g, A, orgh, and should be easier to select in breeding, but the physiological basis for the genotypic differences in Δ has not been elucidated for cowpea.

Acute changes in growth hormone-releasing hormone secretion after injection of BIM 23014, a long acting somatostatin analog, in rams

BIM 23014 is a somatostatin analog displaying an increased biological half life due to resistance to enzymatic degradation. This peptide inhibits GH release directly at the level of pituitary somatotrophs. In addition, an action of BIM 23014 at the level of the hypothalamus is possible since somatostatinergic fibers and receptors have been identified on GH-RH neurons. To evaluate the effect of BIM 23014 on GH-RH secretion, hypophysial portal blood (HPB) was continuously collected in conscious sheep. Twelve rams (40–45 kg, 9-month-old) with chronically implanted perihypophysical cannulae were i.v. injected with BIM 23014 (1 mg) or saline. HPB and jugular blood were collected for 3–5 hours before and after the injection for the determinations of GH-RH and GH concentrations respectively. The acute injection of BIM 23014 induced a rapid decrease of plasma GH within the first two hours. Simultaneously, GH-RH in HPB decreased significantly. After reaching a nadir, GH concentrations increased to values greater than baseline. A similar rebound in GH-RH levels in HPB was also observed. These data indicate that BIM 23014 acts at the level of GH-RH hypothalamic neurons, in addition to its well-know effect on the pituitary gland.

The Effect of Long-Acting Analog of Luteinizing Hormone-Releasing Hormone on Growth Hormone Secretory Dynamics in Children with Precocious Puberty*

Long-acting preparations of LHRH (LHRHa), such as leuprolide acetate, have been shown to selectively and reversibly suppress the clinical and biochemical features of central precocious puberty (CPP). The withdrawal of gonadal sex steroids results in a decline of growth velocity and a decrease in the rate of bone age maturation, with resultant improvement in predicted adult stature. The purpose of this study was to define GH secretory dynamics in children treated with leuprolide acetate. Twelve-hour nocturnal GH studies were performed in five children (four girls and one boy) with CPP before and after 6 months of treatment with leuprolide acetate. Mean GH levels, GH secretory rate, and number of GH secretory episodes were determined. Secretory profiles were analyzed using the Cluster program. Growth velocity, somatomedin-C, and dehydroepiandrosterone sulfate were measured before and after 6 months of therapy. By 6 months of therapy, there was a significant decrease in mean growth velocity from 10.5 +/- 3.3 to 6.7 +/- 1.6 cm/yr. Somatomedin-C levels remained the same at 46.90 +/- 9.51 and 52.5 +/- 12.40 nmol/L. Levels of dehydroepiandrosterone sulfate remained unchanged at 118.6 +/- 71.4 and 139.0 +/- 61.3 mumol/L at 0 and 6 months of the study. By 6 months, there was a significant decrease in mean GH levels from 13.6 +/- 5.3 to 6.4 +/- 3.4 micrograms/L (P less than 0.05). Total GH levels decreased from 1367.9 +/- 687.3 to 447.0 +/- 186.5 ng/12 h. The number of GH secretory episodes remained the same at 5.4 +/- 1.5 and 4.8 +/- 1.0/12 h at 0 and 6 months of study. Therefore, the decrease in GH that occurs during the withdrawal of gonadal sex steroids with LHRHa in children with CPP is an amplitude-modulated phenomenon, as the number of secretory peaks remains unchanged.

Estrogen Induction of Growth Hormone in the Thyroidectomized Rat*

GH production in the rat is almost completely dependent upon T3. Estrogens also stimulate GH in some rat models, and androgens have well documented stimulatory effects. This study examined estrogen and androgen effects on pituitary GH in rats with differing thyroid status. Diethylstilbesterol (DES; a potent synthetic estrogen, 5 mg Silastic implant), estradiol benzoate (50 micrograms/kg.48 h), or testosterone propionate (10 mg/kg.48 h) were administered for 3 weeks to ovariectomized rats that were either thyroid-intact or thyroid-ectomized. In intact rats, DES produced a 40% decrease in pituitary GH, whereas estradiol (at a lower relative dose) had no effect; testosterone produced a 65% increase in pituitary GH. Thyroidectomy decreased pituitary GH to less than 0.5% of intact values. DES and estradiol produced 50- to 70-fold increases in pituitary GH in thyroidectomized rats--reaching 23-36% of intact levels. In contrast, testosterone had no effect in thyroidectomized rats. Tamoxifen (an antiestrogen; 1 mg/kg.24 h) increased GH by 15-fold in thyroidectomized rats and completely blocked further GH induction by estradiol. T3 (20 micrograms/kg.24 h) increased pituitary GH levels by 200-fold in thyroidectomized rats--totally reversing the decrease produced by thyroidectomy; tamoxifen inhibited GH induction by T3 by 63%. The results indicate that estrogens powerfully induce pituitary GH in thyroidectomized but not intact rats through an estrogen receptor-mediated process. The DNA-binding domains of estrogen and T3 receptors, as well as their hormone response elements, share structural similarities. The present results are consistent with the hypothesis that estrogens and estrogen receptors may induce GH through unoccupied T3 response elements of the GH gene in thyroidectomized rats.

Neuroendocrine regulation of growth hormone secretion in sheep III. Serotoninergic systems

The role of serotoninergic pathways in the regulation of growth hormone secretion in the sheep has been investigated. Both peripheral and central routes of administration of serotonin agonists and antagonists have been used. Intravenous administration of the serotonin agonist, buspirone, at 1.2 mg/kg/h lowered plasma GH levels (P<0.001) but at 0.21 mg/kg/h there was no significant decrease. Intracerebroventricular ( icv) administration of serotonin itself also depressed GH levels (P<0.01). The serotonin antagonist, cyproheptadine, failed to affect GH concentrations when given either intravenously (0.25 mg/kg/h) or intracerebroventricularly (4 mg). Neither serotonin nor cyproheptadine had any significant effect on plasma glucose or cortisol levels when administered icv. The possible role of somatostatin in mediating the serotonin associated decrease in GH was investigated by concurrent administration of serotonin and a specific, potent anti-somatostatin serum into a cerebral ventricle. This treatment also resulted in a marked, sustained depression in GH (P<0.001). These data suggest that serotonin can inhibit release of GH from the pituitary in sheep and that this is independent of hypothalamic somatostatin.

Is the Mammosomatotrope a Transitional Cell for the Functional Interconversion of Growth Hormone- and Prolactin- Secreting Cells? Suggestive Evidence from Virgin, Gestating, and Lactating Rats*

Serum concentrations of PRL and GH increase and decrease, respectively, during the progression from nonpregnancy through lactation. However, it is unknown whether the secretory capacities and/or relative abundance of cells that release PRL or GH are altered during these physiological states. In the present study anterior pituitaries from adult virgin, gestating, and early or late lactating female rats were dispersed with trypsin and subsequently assayed for PRL and GH release using reverse hemolytic plaque assays. We found that the relative abundance of PRL-secreting cells was greater and that of GH cells lower in pituitaries from lactating females than in those from virgins. Moreover, the relative amounts of both PRL and GH released per cell were diminished in gestating and lactating females. For PRL, this decrease could be accounted for by an increase in the number of cells that released small quantities of hormone. We then performed simultaneous plaque assays to determine whether the shifts in the relative proportions of PRL and GH secretors were due to changes in the percentages of cells that secrete each hormone alone or in the fraction that releases both PRL and GH concurrently. Variations in both single and dual hormone-secreting cells appear to contribute to the overall fluctuations in the relative abundance of PRL and GH cells during these physiological transitions. We conclude that the additional PRL secretors present during lactation may arise from cells that previously released only GH, and that this functional interconversion of GH and PRL secretors might involve an intermediate cell type, the mammosomatotrope.

Cysteamine-Induced Enhancement of Growth Hormone- Releasing Factor (GRF) Immunoreactivity in Arcuate Neurons: Morphological Evidence for Putative Somatostatin/GRF Interactions within Hypothalamus*

The episodic secretion of GH is regulated through the reciprocal release into the hypophyseal portal circulation of two hypothalamic peptides, SRIF and GRF. Recent physiological evidence suggests that, in addition to exerting their well documented opposite actions at the level of the anterior pituitary, SRIF and GRF may interact within the central nervous system to modulate GH secretion. The aim of the present study was to provide morphological evidence to support the concept that SRIF exerts an influence on the GRF-containing neuronal system within the hypothalamus. To accomplish this, we used the thiol agent cysteamine (CSH; 300 mg/kg sc) to manipulate endogenous hypothalamic SRIF and examined its effects on the immunoreactivity and mRNA content of GRF-containing arcuate (ARC) neurons, using immunohistochemistry and semi-quantitative in situ hybridization, respectively. Hypothalamic SRIF content and pulsatile GH secretion were also monitored. CSH treatment reduced hypothalamic immunoreactive SRIF concentrations (to 60% of vehicle-injected control values) and severely suppressed the spontaneous surges of GH release (mean GH peak amplitude: 24.2 +/- 2.7 vs. 168.0 +/- 27.8 ng/ml in H2O-injected controls; P less than 0.001). CSH treatment also resulted in a striking increase in the number and labeling density of GRF-immunoreactive cells detected in the ARC nucleus. Cell counts revealed an overall 126% increase, over controls, in the mean number of detectable GRF-positive ARC neurons. This increase (P less than 0.01) was apparent over all rostrocaudal levels of the ARC nucleus, but was most pronounced within the caudal tier. Administration of CSH also produced a massive qualitative increase in the intensity and extent of staining of GRF-positive terminal elements in the external zone of the median eminence. Conversely, CSH treatment resulted in a decline in GRF mRNA-associated signal in cells of the ARC nucleus, with this decline being most evident in the caudal third of the nucleus. The finding of an increase in GRF immunoreactivity in ARC neurons in association with a decrease in plasma GH, as well as with a decline in relative GRF mRNA content, suggests that CSH leads to an overall decrease in GRF release from the median eminence. These results provide morphological evidence to support the concept of a SRIF-mediated central influence on the GRF-containing ARC neuronal system. Such central regulation of GRF by SRIF may be an important mechanism in the physiological control of pulsatile GH secretion.

Effects of dietary T 3 on growth parameters and hormone levels in normal and sex-linked dwarf chickens

Tri-iodothyronine (T 3) has been administered in the diet, from day of hatch until 8 weeks of age, to sex-linked dwarf and normal chicks of both sexes from a brown-egg slow-growing strain. Feed was supplemented with either 0.1 ppm or 0.5 ppm T 3. A significant genotype by treatment interaction was observed on body weight: the effect of T 3 in males was significantly positive for dwarfs and null for normals, the effect in females was null for dwarfs and significantly negative for normals. Feed efficiency was rather decreased by the treatment in both genotypes. Abdominal fatness was decreased in a dose-dependent manner in both genotypes, while rectal temperature was raised in dwarf chicks only. Plasma T 3 was raised to normal levels in dwarfs receiving 0.1 ppm exogenous T 3, while the 0.5 ppm dose yielded hyperthyroid levels. Plasma GH levels were decreased in a dose-dependent manner by the T 3 treatment, the effect being larger in dwarfs. Surprisingly, plasma IGF-I was unchanged in spite of the GH decrease, whatever the genotype or the sex. It was concluded that exogenous T 3 alone can have a stimulatory effect on growth in dwarfs but can not fully restore a normal growth rate. Both T 3 and IGF-I are important for a normal growth and the relationships between T 3 and IGF-I production should be further investigated in order to better understand the physiological modifications due to the sex-linked dwarf gene.