Sex-determined variation in serum insulin and growth hormone response to amino acid stimulation

Regulation of basal, pulsatile, and entropic (patterned) modes of GH secretion in a putatively low-somatostatin milieu in women

Somatostatin (SS) released by hypothalamic neurons inhibits GH exocytosis noncompetitively. Therefore, we postulated that attenuation of GH feedback-induced SS outflow would help to unmask covariates of endogenous secretagogue drive. To this end, 42 healthy pre- and postmenopausal women were randomly assigned to receive leuprolide plus estradiol (E(2)) or leuprolide plus placebo. A putatively low-SS milieu was imposed by L-arginine infusion. Deconvolution and regularity analyses were applied to 6-h GH concentration-time profiles. By two-way ANOVA, age negatively (P < 0.001) and E(2) positively (P = 0.001) determined pulsatile GH secretion in the presumptively SS-deficient milieu (P < 0.001). Comparable effects were exerted on the mass of GH secreted per burst per unit distribution volume (age P = 0.001, E(2) P < 0.001, overall P < 0.001). E(2) alone predicted basal (nonpulsatile) GH secretion (P = 0.004). Stepwise forward-selection multivariate regression demonstrated that age (P = 0.0017) and E(2) (P = 0.0002) together explained 46% of intersubject variability in pulsatile GH secretion (P < 0.001) and fully replaced the negative univariate effect of abdominal visceral fat (r(2) = 0.32, P < 0.001). Moreover, age and E(2) (but not AVF) interacted to supervise GH regularity (P = 0.007). We conclude that age and E(2) availability individually and together constitute primary predictors of basal, pulsatile, and patterned GH secretion in an inferentially feedback-silenced context in healthy women. Therefore, both factors must be considered in framing hypotheses of endogenous GH drive.

Model-projected mechanistic bases for sex differences in growth hormone regulation in humans

Models of physiological systems facilitate rational experimental design, inference, and prediction. A recent construct of regulated growth hormone (GH) secretion interlinks the actions of GH-releasing hormone (GHRH), somatostatin (SRIF), and GH secretagogues (GHS) with GH feedback in the rat (Farhy LS, Veldhuis JD. Am J Physiol Regul Integr Comp Physiol 288: R1649–R1663, 2005). In contrast, no comparable formalism exists to explicate GH dynamics in any other species. The present analyses explore whether a unifying model structure can represent species- and sex-defined distinctions in the human and rodent. The consensus principle that GHRH and GHS synergize in vivo but not in vitro was explicable by assuming that GHS 1) evokes GHRH release from the brain, 2) opposes inhibition by SRIF both in the hypothalamus and on the pituitary gland, and 3) stimulates pituitary GH release directly and additively with GHRH. The gender-selective principle that GH pulses are larger and more irregular in women than men was conferrable by way of 4) higher GHRH potency and 5) greater GHS efficacy. The overall construct predicts GHRH/GHS synergy in the human only in the presence of SRIF when the brain-pituitary nexus is intact, larger and more irregular GH pulses in women, and observed gender differences in feedback by GH and the single and paired actions of GHRH, GHS, and SRIF. The proposed model platform should enhance the framing and interpretation of novel clinical hypotheses and create a basis for interspecies generalization of GH-axis regulation.

Complementary secretagogue pairs unmask prominent gender-related contrasts in mechanisms of growth hormone pulse renewal in young adults

The present study examines the thesis that pulsatile GH secretion is controlled simultaneously by three principal signals; viz., GHRH, GH-releasing peptide (GHRP, ghrelin), and somatostatin (SS). According to this ensemble notion, no single regulatory peptide acts alone or can be interpreted in isolation. Therefore, to investigate gender-specific control of pulsatile GH secretion, we designed dual-effector stimulation paradigms in eight young men and six women as follows: 1) L-arginine/GHRH (to clamp low SS and high GHRH input); 2) L-arginine/GHRP-2 (to clamp low SS and high GHRP drive); 3) GHRH/GHRP-2 (to clamp high GHRH and high GHRP feedforward); vs. 4) saline (unclamped). Statistical comparisons revealed that: 1) fasting pulsatile GH secretion was 7.6-fold higher in women than men (P < 0.001); 2) L-arginine/GHRH and L-arginine/GHRP-2 evoked, respectively, 4.6- and 2.2-fold greater burst-like GH release in women than men (P < 0.001 and P = 0.015); and 3) GHRH/GHRP-2 elicited comparable GH secretion by gender. In the combined cohorts, estradiol concentrations positively predicted responses to L-arginine/GHRP-2 (r2= 0.49, P = 0.005), whereas testosterone negatively predicted those to L-arginine/GHRH (r2= 0.56, P = 0.002). Based upon a simplified biomathematical model of three-peptide control, the current outcomes suggest that women maintain greater GHRH potency, GHRP efficacy, and opposing SS outflow than men. This inference upholds recent clinical precedence and yields valid predictions of sex differences in self-renewable GH pulsatility.

Influence of gonadal steroids on rat pituitary growth hormone secretion

Growth hormone (GH) secretion increases at the time of puberty, and both androgens and estrogens increase the GH response to provocative agents. Little is known, however, with regard to the mechanism or site of action of gonadal steroid-stimulated GH secretion. Using monolayer primary cell culture and radioimmunoassay techniques, insulin-like growth factor I (IGF-I), purified and biosynthetic human GH, 17 beta-estradiol (E2), and testosterone (T) effects on GH release from rat anterior pituitary cells were investigated. Media content of rat GH was measured, both basally and after growth hormone releasing factor (GRF) stimulation. IGF-I at 50 nM concentration inhibited basal rGH release but not in GRF-stimulated cells. E2 (10 pM) stimulated basal secretion of rGH, although higher concentrations did not. High concentrations of T (100,000 pM) caused an increase in GRF-stimulated rGH secretion. Neither purified nor biosynthetic hGH pretreatment influenced subsequent rGH release. These results suggest that IGF-I inhibits and E2 (low dose) and T (high dose) augment rGH release in part at the level of the anterior pituitary.

Effect of phase of menstrual cycle on insulin sensitivity, peripheral blood flow and cardiovascular responses to hyperinsulinaemia in young women with type 1 diabetes

Disturbances of blood glucose control around the time of menstruation are often reported by women with Type 1 diabetes. To investigate the possibility that such changes may be due to alterations in insulin sensitivity or peripheral blood flow we have studied 9 women with Type 1 diabetes during the follicular and luteal phases of the menstrual cycle. Insulin sensitivity was assessed by the glucose clamp technique with insulin doses of 40 (low dose) and 100 (high dose) mU m-2 min-1. Glucose disposal rates were 2.5 +/- 0.3 (mean +/- SE) mg kg-1 min-1 during the follicular phase and 3.2 +/- 0.3 mg kg-1 min-1 in the luteal phase with low dose insulin, and 5.9 +/- 0.4 and 6.4 +/- 0.6 mg kg-1 min-1, respectively, with high dose insulin. These differences were not statistically significant. Forearm blood flow, heart rate, and blood pressure were similar during both phases.

Desipramine increases circulating growth hormone in elderly depressed patients: a pilot study

Serial blood samples were collected from 15 elderly depressed inpatients, ages 62 to 95 years, following random assignment to a 50 mg oral test dose of desmethylimipramine (DMI) or amitriptyline (AMI). Nine female and six male subjects began the 210-min study at 0800h. Serum growth hormone (hGH), cortisol, and prolactin (hPRL) were determined by radioimmunoassay. Baseline hormone concentrations were related to self and observer ratings of anxiety and depression. There was a trend for the hGH, cortisol, and hPRL concentrations to decline during the period of study. This trend for all three hormones reversed in those subjects receiving DMI, beginning approximately 90 min after drug ingestion. The DMI-induced increase of hGH reached statistical significance at the very end of the sampling period. There was an apparent latency in the DMI-induced effect for all three hormones. There was no stimulatory effect of AMI on hGH, cortisol, or hPRL. The female subjects had higher baseline hGH levels than the men. In addition, a significant negative correlation was found between baseline hPRL levels and self ratings of anxiety.

Stilboestrol pretreatment of children with short stature does not affect the growth hormone response to growth hormone-releasing hormone

Oestrogens are known to enhance both basal and stimulated GH secretion. To examine whether this effect is mediated through the hypothalamus or the pituitary we performed insulin tolerance and GH-releasing hormone (GHRH) tests with and without oestrogen priming in a group of 14 short children. Pretreatment with stilboestrol increased basal levels of GH and both peak and incremental levels during insulin hypoglycaemia. In contrast, there was no effect of stilboestrol priming on the GH response to either an i.v. bolus of 100 micrograms or 0.1 microgram/kg (range 2-6 microgram) of GHRH. The children had significantly higher GH responses to an unprimed GHRH than unprimed insulin tolerance test. We conclude stilboestrol priming acts through the hypothalamus presumably by increasing endogenous GHRH release, and that short children with a subnormal GH response to insulin hypoglycaemia show a greater response to GHRH; this suggests the presence of a hypothalamic cause for their decreased GH secretion.

Premarin priming does not alter growth hormone release following exercise

We evaluated the usefulness of premarin priming on exercise induced growth hormone release and the value of combining several growth hormone screening agents in a large population of prepubertal children. Two hundred five short healthy prepubertal children growing below the 5th percentile in height were studied. One hundred forty-four were screened with exercise following glucose ingestion, while 61 were primed with estrogen prior to glucose and exercise testing. Premarin priming did not significantly increase the number of our patients who responded to exercise nor to glucose; 86% and 88.5% of non-primed and primed patients, respectively, responded with a growth hormone increase greater than or equal to 8 ng/ml following exercise and glucose. Glucose loading alone was not associated with a high enough growth hormone rise to rule out growth hormone deficiency in most of our children. Age (less than or equal to 5 yr) was associated with lower post-exercise growth hormone levels and a higher failure rate to testing in both primed and non primed children. Premarin priming does not seem to alter the growth hormone releasing capacity to exercise of prepubertal children. The combined use of exercise, glucose loading and premarin priming in a single screening test does not improve on the results obtained by growth hormone exercise screening alone.

Treatment of growth hormone deficiency

According to the results reported in the literature and from our own experience, the following recommendations for the treatment of children with GHD can be given: In order to start GH replacement therapy in early childhood the diagnosis of GHD should be made as early as possible. The growth hormone dose during prepubertal age should not fall short of 12 IU/m2 per week. During spontaneous or induced puberty, the dose needs to be increased, possibly by a factor of two. Daily subcutaneous injections appear most suitable. Treatment with growth hormone releasing factors in cases with hypothalamic GHD, although a promising alternative to the treatment with hGH (Thorner et al, 1985), must be considered experimental at this point. Thyroxine replacement at a daily dose of 75-100 micrograms/m2 should be given in cases of secondary hypothyroidism. Glucocorticoid replacement, if required, should be given at low doses (e.g. hydrocortisone 10 (to 15) mg/m2 per day in divided doses). In cases with additional gonadotropin deficiency, sex steroids (or anabolic steroids) should be given with frequent monitoring of bone maturity not before the age of 13 in girls or 15 years in boys. In boys depot testosterone starting at low doses (e.g. 50-100 mg/month i.m.) will induce a puberty-like increment in height velocity. Since the effect of oestrogens--even in low doses--on growth is uncertain, their administration before achievement of near-normal adult height should be avoided. With the advancement of diagnostic techniques and with the experience in treatment accumulated over the past 25 years, patients with GHD need no longer become dwarfs.

ACTH 1-17 stimulates GH pituitary secretion in humans

Nineteen normal subjects (12 men and 7 women) were injected with 100 micrograms of ACTH 1-17. Additionally 6 male subjects were studied twice at 3-day intervals with random infusions of ACTH 1-17 and saline. A clear GH response to ACTH 1-17 infusions (GH peak higher than 5 ng/ml) was documented in 10 out of 12 males and in 6 out of 7 women. In the 6 male subjects studied twice, clear-cut GH increments were observed only after peptide administration. PRL levels decreased throughout the study period both in male and female subjects; however, when the PRL percentage decline was evaluated in the same group of subjects after saline and ACTH 1-17, the more obvious decrease of PRL levels after the peptide infusion was not statistically significant. No variation of LH, FSH and TSH levels was documented. With the exception of the specific increase of cortisol levels, no significant change in peripheral steroid pattern (Te, E2, DHEA-S) was observed. In this experiment the effect on GH secretion was quite evident in both sexes. This effect was obtained using the lowest dosage of ACTH preparation documented in the literature.

Postsynaptic serotonergic activity in depressive patients: evaluation of the neuroendocrine strategy

The possible occurrence of hypersensitive postsynaptic serotonin (5-hydroxytryptamine; 5-HT) receptors in depressive patients was investigated using a neuroendocrine strategy: determination of neuroendocrine parameters after activation of the serotonergic system. Activation was achieved by oral administration of l-tryptophan and l-5-hydroxytryptophan (5-HTP). The neuroendocrine parameters measured were the plasma concentrations of prolactin, growth hormone, thyroid stimulating hormone, and cortisol. Since administration of 5-HT precursors caused no significant change in the hormone concentrations, evaluation of neuroendocrine function after stimulation with 5-HT precursors does not appear to provide a reliable index of human postsynaptic serotonergic activity.

A simplified procedure for the estimation of arginine in plasma and urine using arginase

To ensure reliable arginine estimation, other compounds giving positive results with Chinard's ninhydrin reaction were removed by simple ion-exchange chromatography. Eluted arginine was converted to ornithine by arginase and estimated using Chinard's ninhydrin reaction. Plasma arginine level in adults was in the range 56--98 mumol/l, and urine arginine in the range 0.58--1.77 mg/day.

Cardiopulmonary adjustment and metabolic response to maximal and submaximal physical exercise of boys and girls at different stages of maturity

Cardiopulmonary and metabolic variables were investigated at maximal and submaximal bicycle ergometer exercises in 41 swimmers of both sexes, 8--18 years old. VO2 max and VO2 max . HR-1 were higher in boys than in girls and increased with maturity, while VO2 max. kg-1 and HVE were not influenced by this. The HV increased clearly during this growth period, the pubertal and postpubertal subjects showing 16 and 17% higher values for HV and HV . kg-1 than those reported in normal schoolchildren populations. During the submaximal exercise at 70% VO2 max the highest HR values were found in the prepubertal group, whilst the lowest were observed in the postpubertal subjects. These findings suggest that a given percentage of VO2 max as a reference unit, is more reliable than a certain HR to obtain comparable results in subjects with different ages. Blood samples were collected before, during, and after the submaximal exercise. Blood glucose and FFA did not differ in relation to the stages of maturity. During exercise, insulin decreased in prepubertal children, did not alter in pubertal adolescents, and increased in postpubertal subjects. The lactate concentration, during exercise, increased in relation to maturity. The same results were found for HGH, but no differences were found with regard to sex. Since the pattern of HGH secretion during exercise is similar to that found after arginine and insulin administration it is assumed that the same mechanism (i.e., sex hormones) triggers the HGH release.

Prostaglandin E2 (PGE2)-induced growth hormone (GH) release: effect of intrahypothalamic and intrapituitary implants

Ovariectomized rats were unilaterally implanted with a 23-gauge stainless steel cannula in different hypothalamic areas or in the pituitary gland and subsequently were treated with estrogen (sc, 10 micron g estradiol benzoate, Eb). Two days after the estrogen injection, an inner cannula containing PGE2 or PGF2alpha at its tip was inserted into the cannula. Other animals were implanted with an empty inner cannula. Plasma GH concentrations were measured by RIA in blood samples drawn from the jugular vein while the animals were lightly etherized before (-2) and at 20, 40, 60 and 120 min following the implantation. Plasma GH levels in control animals bearing an empty cannula in the body of the arcuate nucleus-median eminence region (BARH-ME) were significantly depressed by the ether stress. The implantation of PGF2alpha in this area was completely ineffective in preventing ether stress-induced decline in plasma GH. By contrast, PGE2 implanted in BARH-ME or the post-chiasmatic region of the hypothalamus (HARH-ME) elevated plasma GH 20 min following its implantation and partially prevented the subsequent decrease in GH levels induced by ether stress. PGE2 implants located in several other hypothalamic areas failed to induce GH release or to prevent the decline in GH levels induced by ether stress. However, PGE2 implanted in the pituitary gland elicited a marked increase in plasma GH at 20 min and completely prevented the subsequent ether stress-induced decline in GH levels. The results suggest that PGE2 can act at both hypothalamic (ARH-ME) and pituitary levels to stimulate GH release. At the hypothalamus, PGE2 may inhibit GH-inhibiting factor (GIF) release or induce release of GH releasing factor (GHRF).

Mean 24-hour growth hormone and testosterone concentrations in relation to pubertal growth spurt in boys with normal or delayed puberty

The mean growth hormone concentration during 24-hour period in 7 boys of short familial stature and a growth rate of 3.2-5.4 cm/year was between 1.0 and 4.6 ng/ml serum. In 7 boys with pubertal growth spurt and familial tallness (growth rate 7.2-11.0 cm/year) it varied from 0.97 to 4.4 ng/ml and in 6 boys with constitutional delay of puberty (a growth rate of 4.2-5.2 cm/year prior to puberty) from 1.3 to 4.3 ng/ml. No correlation was found between the 24-hour mean growth hormone concentration and the mean 24-hour testosterone concentration in serum or the growth rate, but a correlation was found between testosterone and the growth rate. It is concluded that the growth spurt in puberty is not due to a change in growth hormone concentration but rather to the increase of androgen production in puberty.

Provocative tests for growth hormone release

Subnormal growth hormone responses to both insulin-induced hypoglycemia and arginine infusion (peak response less than or equal to 5 ng/ml) were found in five male subjects (aged 10 to 14 years) with short stature but with normal interval growth rates and normal bone ages (in 4 cases). They demonstrated one or more normal GH responses to subsequent provocation by glucagon stimulation, tolbutamide-induced hypoglycemia, and repeat insulin and arginine testing following pretreatment with sex steroids. Two subjects had received exogensou GH therapy for six months prior to the subsequent assessment; each one failed to demonstrate the growth response characteristic of GH deficiency. These studies indicate the need for multiple provocative human GH testing to exclude children whose growth would not be enhanced by GH therapy despite a subnormal response to provocative tests with insulin and arginine.

Urinary metabolites characteristic of urea-cycle amino acid deficiency

Experiments with 45-75-g male rats gave conclusive evidence that the simultaneous absence from the diet of arginine, ornithine, or citrulline caused an immediate and persistent elevation of orotic and citric acids in the urine. The experiments also demonstrated that a deficiency of no other individual amino acid increased urinary citrate and orotate. Elevated urinary excretion of orotic and citric acid occurred independently of the form of nonessential nitrogen. Replacement of arginine isonitrogenously with ornithine or citrulline prevented the rise in urinary orotic acid, but had different effects on growth, urinary citrate, and urinary urea. These differences were probably due to differential uptake of arginine, ornithine, and citrulline by tissues; In the reported experiments employing L-amino acids as sources of dietary nitrogen, a deficiency of any amino acid indispensable for growth and nitrogen balance or a deficiency of arginine, ornithine, or citrulline retarded growth, increased urinary urea, and decreased urinary ammonia. It is concluded that the severe loss of orotic acid during urea-cycle amino acid deficiency arises from a decreased capacity of the urea cycle to detoxify ammonia, thereby causing increased shunting of intramitochondrial carbamyl phosphate into pyrimidine synthesis. The similarities in metabolism during arginine deficiency and ammonia intoxication are discussed. The evidence shows that urinary orotic acid may be a valuable measure of arginine nutrition in mammals.

Growth hormone: metabolic clearance rates, integrated concentrations, and production rates in normal adults and the effect of prednisone

A constant withdrawal pump was used to determine the integrated concentration of growth hormone (ICGH) which was used in conjunction with the metabolic clearance rate (MCR) of growth hormone (GH) to calculate the GH production rates (GHPR) in normal adults, acromegalics, and normal controls receiving prednisone. The mean ICGH for 22 premenopausal females on no medication was 3.0+/-1.6 ng/ml (sd) which is significantly lower (P < 0.005) than the mean of 6.6+/-2.9 for 10 women receiving oral contraceptives and significantly higher than the means of 1.5+/-0.75 for 5 postmenopausal females (P < 0.05) and 1.8+/-1.0 for 16 adult males (P < 0.01) which are comparable. The mean GHPR's in mg/24 hr per m(2) for the four groups are: normal females = 0.52+/-0.24 (sd), females receiving contraceptive pills = 1.65+/-0.58 (P < 0.005), postmenopausal females = 0.26+/-0.12 (P < 0.025), and adult males 0.35+/-0.23 (P < 0.025).Three untreated acromegalic patients had ICGH's of 59, 82, and 93 ng/ml and GHPR's ranging from 14.5 to 17.9 mg/24 hr. Prednisone in a dose of 20 mg t.i.d. for 8 days significantly decreased both the ICGH and GHPR. Alternate day prednisone (60 mg in a single q.o.d. dose) resulted in less consistent inhibition of GH release which may play a role in the more normal growth seen in children receiving q.o.d. prednisone.