Leptin is a potent stimulator of spontaneous pulsatile growth hormone (GH) secretion and the GH response to GH-releasing hormone.

Pulsatile GH secretion is exquisitely sensitive to perturbations in nutritional status, but the underlying mechanisms are largely unknown. Leptin, a recently discovered adipose cell hormone, is thought to be a sensor of energy stores and to regulate body mass, appetite, and metabolism at the level of the brain. Receptors for leptin are abundantly expressed in hypothalamic nuclei known to be involved in GH regulation, suggesting that leptin may serve as an important hormonal signal to the GH neuroendocrine axis in normal animals. To test this hypothesis, we examined the effects of intracerebroventricular infusion of recombinant murine leptin, at a dose of 1.2 microg/day for 7 days, on both spontaneous and GH-releasing hormone (GHRH)-stimulated GH secretion in free-moving adult male rats. Concomitant with suppressive effects on food intake, body weight, and basal plasma insulin-like growth factor I, insulin, and glucose concentrations, central infusion of leptin resulted in a 2- to 3-fold augmentation of GH pulse amplitude, 5-fold higher GH nadir levels, and a 2- to 3-fold increase in the integrated area under the 6-h GH response curve compared with those in vehicle-infused controls (P < 0.001). The intracerebroventricular infusion of leptin also produced a 3- to 4-fold increase in GHRH-induced GH release at GH trough times (P < 0.01). These studies demonstrate a potent stimulatory action of leptin on both spontaneous pulsatile GH secretion and the GH response to GHRH. The results suggest that the GH-releasing activity of leptin is mediated, at least in part, by an inhibition of hypothalamic somatostatin release. Thus, leptin may be a critical hormonal signal of nutritional status in the neuroendocrine regulation of pulsatile GH secretion.

Mechanisms of impaired growth hormone secretion in genetically obese Zucker rats: roles of growth hormone-releasing factor and somatostatin.

GH secretion is markedly blunted in obesity; however, the mechanism(s) mediating this response remains to be elucidated. In the present study we examined the involvement of the two hypothalamic GH-regulatory hormones, GH-releasing factor (GRF) and somatostatin (SRIF), using the genetically obese male Zucker rat. Spontaneous GH, insulin, and glucose secretory profiles obtained from free moving, chronically cannulated rats revealed a marked suppression in amplitude and duration of GH pulses in obese Zucker rats compared to their lean littermates (mean 6-h plasma GH level, 3.9 +/- 0.4 vs. 21.5 +/- 3.8 ng/ml; P less than 0.001). Obese rats also exhibited significant hyperinsulinemia in the presence of normoglycemia. The plasma GH response to an iv bolus of 1 microgram rat GRF-(1-29)NH2, administered during peak and trough periods of the GH rhythm, was significantly attenuated in obese rats at peak (137.4 +/- 26.1 vs. 266.9 +/- 40.7 ng/ml; P less than 0.02), although not at trough, times. Passive immunization of obese rats with a specific antiserum to SRIF failed to restore the amplitude of GH pulses to normal values; the mean 6-h plasma GH level of obese rats given SRIF antiserum was not significantly different from that of obese rats administered normal sheep serum. Both pituitary wet weight and pituitary GH content and concentration were reduced in the obese group. Measurement of hypothalamic GRF immunoreactivity revealed a significant (P less than 0.05) reduction in the mediobasal hypothalamic GRF content in obese rats (503.2 +/- 60.1 pg/fragment) compared to that in lean controls (678.1 +/- 50.2 pg/fragment), although no significant difference was observed in hypothalamic SRIF concentration. Peripheral SRIF immunoreactive levels were significantly (P less than 0.01) elevated in both the pancreas and stomach of obese rats. These results demonstrate that the genetically obese Zucker rat exhibits 1) marked impairment in both spontaneous and GRF-induced GH release, which cannot be reversed by SRIF immunoneutralization, 2) significant reduction in pituitary GH concentration, 3) depressed hypothalamic GRF content, and 4) elevated gastric and pancreatic, but not hypothalamic, SRIF levels. The findings suggest that the defect in pituitary GH secretion observed in the genetically obese Zucker rat is due, at least partially, to insufficient stimulation by hypothalamic GRF, and that SRIF does not play a significant role.

Tamoxifen attenuates pulsatile growth hormone secretion: mediation in part by somatostatin.

Tamoxifen, a partial competitive antagonist to the estrogen receptor, is a potent inhibitor of the proliferation of experimental mammary carcinoma in the rat and is widely used clinically in the treatment of breast cancer. Blockade of estrogen receptors present on neoplastic cells represents the classic mechanism of action of tamoxifen, but the drug has a variety of other actions that may contribute to its antiproliferative properties. While it is recognized that estrogens play an important role in modulating pulsatile GH release, the effect of antagonists to sex steroid receptors on GH secretory dynamics has not previously been described. In the present study we examined the effect of tamoxifen on pulsatile GH secretion in free-moving adult male and female rats. The drug, when administered in a manner previously shown to be associated with antineoplastic activity, caused a marked suppression of the amplitude of spontaneous GH secretory bursts and significantly reduced mean 6-h plasma GH levels in both sexes compared to those in their respective peanut oil-injected controls. Inhibition of spontaneous GH pulses persisted for up to 7 weeks after tamoxifen administration in both sexes. Immunoneutralization of endogenous somatostatin in tamoxifen-treated male rats completely restored both GH pulse amplitude (121.6 +/- 9.5 vs. 62.5 +/- 13.5 ng/ml in tamoxifen-treated rats given normal sheep serum; P less than 0.02) and mean 6-h plasma GH levels (53.3 +/- 6.6 vs. 17.9 +/- 3.6 ng/ml in normal sheep serum-treated rats; P less than 0.01) to levels observed in our peanut oil-injected controls. These results demonstrate that 1) tamoxifen has potent inhibitory effects on pulsatile GH secretion; and 2) the blunting of GH pulse amplitude by tamoxifen is mediated at least in part by increased release of endogenous somatostatin. These findings motivate further investigation of the clinical significance of tamoxifen-induced suppression of GH secretion in relation to the antineoplastic activity of this commonly used drug.

Temporal relationship between the sexually dimorphic spontaneous GH secretory profiles and hepatic STAT5 activity.

STAT5 transduces transcriptional responses to GH in liver and other tissues and is proposed to mediate the sexually dimorphic effects of plasma GH secretory profiles on rodent liver gene expression. Previous studies have suggested that STAT5 undergoes repeated activation in direct response to successive GH pulses in adult male rats, with STAT5 activation being desensitized in females by their more persistent pattern of GH exposure. These findings, however, were based on in vitro studies or single blood samples analyzed for GH in vivo. In view of the highly pulsatile nature of rat GH secretion, we presently examined these hypotheses by concurrent monitoring of spontaneous GH secretory profiles and hepatic STAT5 activity in conscious, free-moving adult male and female rats. Rats were killed at times associated with spontaneous peaks or troughs of the GH rhythm; livers were removed and analyzed for STAT5 DNA-binding activity. In males, liver STAT5 activity was highest during the initial phase (15-60 min) of a GH secretory episode (mean +/- SE relative STAT5 activity = 86.5 +/- 11.4; plasma GH = 146.7 +/- 22.4 ng/ml) and was significantly lower (P < 0.01) during the downswing of a pulse, 45-75 min after the GH peak (STAT5 = 26.1 +/- 1.7; GH = 33.3 +/- 13.1 ng/ml), consistent with a time-dependent down-regulation of GH signaling to STAT5. The lowest STAT5 activity was observed during the subsequent GH trough period (STAT5 = 3.6 +/- 1.1; GH = 2.6 +/- 0.1 ng/ml). In females, liver STAT5 activity was significantly lower (P < 0.05) than peak male levels during the initial phase of a GH secretory burst (STAT5 = 35.1 +/- 15.9; GH = 68.1 +/- 31.6 ng/ml) although similar to that of males during a plasma GH nadir (STAT5 = 11.0 +/- 2.6; GH = 8.4 +/- 2.2 ng/ml). We conclude that: 1) liver STAT5 is repeatedly activated by successive, spontaneous GH secretory episodes in intact adult male rats at approximately 3- to 3.5-h intervals; 2) time-dependent down-regulation of GH signaling to hepatic STAT5 in vivo begins by 45 min after GH peak stimulation; and 3) the lower level of liver STAT5 activation seen in adult female rats, compared with males, is a consequence of the sex-dependent differences in GH secretory patterns that characterize these animals (i.e. lower-amplitude GH pulses and lack of prolonged interpulse nadir of GH in the feminine, compared with masculine profile).

Neuropharmacological regulation of episodic growth hormone and prolactin secretion in the rat.

The effects on GH and PRL secretion of several pharmacological agents known to modify central neurotransmitter action were determined in unanesthetized male rats. Phenoxybenzamine, an alpha-adrenergic blocker (5 mg/kg iv), abolished episodic GH secretion and caused elevation of serum PRL levels. Propranolol, a beta-adrenergic blocker (5 mg/kg iv), had no effect on GH secretion and caused a small but significant depression in PRL levels. Parachlorophenylalanine methyl ester, an inhibitor of tryptophan hydroxylase (300-350 mg/kg ip), resulted in significant inhibition of GH pulsatile secretion and suppressed PRL levels. Methysergide hydrogen maleinate (25 mg/kg iv), a serotonin receptor antagonist, also inhibited GH secretion, but produced a transient stimulation in PRL levels. Atropine sulfate (2 mg/kg iv) caused significant suppression in GH secretion, but had no effect on PRL. Picrotoxin, a gamma-aminobutyric acid antagonist, in a subconvulsive dose of 1-3 mg/kg iv, also depressed GH episodic secretion but did not affect PRL levels. These results indicate that several neurotransmitters, i.e., norepinephrine, serotonin, acetylcholine, and gamma-aminobutyric acid, found in high concentration in the hypothalamus, influence GH and PRL secretion.

Evidence for a primary involvement of somatostatin in clonidine-induced growth hormone release in conscious rats.

The GH-releasing activity of the alpha 2-adrenergic agonist clonidine has been extensively studied in the rat, but the mechanism(s) by which clonidine stimulates GH release remains controversial. In the present study, we examined the effects of various doses of clonidine on spontaneous pulsatile GH secretion in conscious rats, and tested the hypothesis that the GH-releasing activity of clonidine is mediated primarily by an inhibition of hypothalamic somatostatin (SRIF) release. In the first experiment, free-moving adult male rats were given either saline or various doses of clonidine i.v. (30, 50 and 125 micrograms/kg) at times of spontaneous peaks (1100 h) and troughs (1300 h) in the GH rhythm. Clonidine, at all doses tested, failed to stimulate GH release when administered at the time of a spontaneous peak. In contrast, injection of clonidine at trough times (when SRIF tone is high) consistently augmented plasma GH levels (mean +/- S.E.M. integrated GH release; 30 micrograms/kg, 1843.0 +/- 484.0; 50 micrograms/kg, 1469.0 +/- 490.3; 125 micrograms/kg, 1675.6 +/- 513.4 vs 201.3 +/- 100.1 ng/ml per 45 min in saline-injected controls; p < 0.05 or less). No significant regression was observed between increasing doses of clonidine and GH release. In the second experiment, i.v. administration of 30 micrograms clonidine/kg during a GH trough period, 30 min prior to GH-releasing factor (GRF) challenge, significantly potentiated the GH response to GRF compared with rats given saline (7218.7 +/- 806.6 vs 4206.9 +/- 1068.1 ng/ml per 30 min; p < 0.05). Clonidine treatment, at all doses tested, resulted in hyperglycaemia and behavioural effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Pituitary involvement in renal adaptation to phosphate deprivation.

The present study was undertaken to examine 1) the effect of phosphate restriction on growth hormone (GH) secretory dynamics in freely moving, chronically cannulated rats and 2) the effect of hypophysectomy on the renal adaptive responses to phosphate deprivation. Phosphate restriction led to an increase in renal brush-border membrane Na+-dependent phosphate transport (2,511 +/- 283 vs. 1,006 +/- 122 pmol.mg protein-1.15 s-1, P less than 0.001) and in the plasma concentration of 1 alpha,25-dihydroxyvitamin D [1,25(OH)2D] (127 +/- 10 vs. 63 +/- 4 pg/ml, P less than 0.001). In contrast, phosphate deprivation had no effect on either amplitude or frequency of spontaneous GH secretory bursts and did not alter pituitary GH concentration. Hypophysectomy led to a decrease in brush-border membrane Na+-dependent phosphate transport (669 +/- 78 vs. 1,006 +/- 122 pmol.mg protein-1. 15 s-1, P less than 0.003) and to a fall in plasma 1,25(OH)2D (42 +/- 9 vs. 63 +/- 4 pg/ml, P less than 0.02). Phosphate restriction of hypophysectomized rats elicited a twofold increase in Na+-dependent phosphate transport (1,312 +/- 106 vs. 669 +/- 78 pmol.mg protein-1.15 s-1, P less than 0.001) but no rise in plasma 1,25(OH)2D. We conclude that the renal adaptive responses to phosphate deprivation are not mediated by specific alterations in pulsatile GH secretion. Moreover, we demonstrate that the adaptive increase in brush-border membrane phosphate transport occurs after hypophysectomy, is not dependent on increased vitamin D hormone production, and is most likely subject to a different regulatory mechanism.