Androgens modulate growth hormone-releasing factor-induced GH release from bovine anterior pituitary cells in static culture

Effects of testosterone on hormonal content and calcium-dependent basal secretion in female rat pituitary cells

In vivo and in vitro effects of elevated androgens on agonist-induced gonadotropin secretion have been addressed previously. Here we investigated the effects of testosterone on hormonal content and basal (in the absence of agonists) hormone release in pituitary lactotrophs, somatotrophs and gonadotrophs from female rats. Furthermore we tested the hypothesis that testosterone action is dependent on the pattern of spontaneous and Bay K 8644 (a L-type calcium channel agonist) -induced calcium signalling. Mixed anterior pituitary cells were cultured in steroid containing or depleted media, and testosterone (1pM to 10nM) was added for 48h. Cells were studied for their spontaneous and Bay K 8644-induced calcium signalling pattern and total hormone levels (release and hormonal content). In lactotrophs, somatotrophs and gonadotrophs testosterone did not affect the pattern of spontaneous calcium signalling. Bay K 8644-induced calcium signalling and hormone release were not affected by testosterone. In both steroid-depleted and -containing medium, testosterone inhibited prolactin (PRL), luteinizing hormone (LH) and growth hormone (GH) cellular content and release in a dose-dependent manner, with IC(50)s in a sub-nanomolar concentration range. These results indicate that testosterone inhibits basal hormone release from lactotrophs, somatotrophs and gonadotrophs without affecting intracellular calcium signalling. This action of testosterone is not dependent on the presence of other steroid hormones.

Short-term testosterone supplementation does not activate GH and IGF-I production in postmenopausal women

BACKGROUND

Testosterone (Te), oestradiol, GH and IGF-I concentrations fall in postmenopausal women.

OBJECTIVE

To test the effect of increased Te availability on impoverished GH/IGF-I secretion in this context.

DESIGN/PATIENTS

Placebo (Pl) and a low vs. high dose of Te were administered transdermally for 7 days to each of 15 healthy older women (ages 48-81 years) in a randomized, double-blind crossover design.

MEASUREMENTS

Frequent blood sampling, GHRP-2 (Growth hormone releasing peptide-2) infusion, and deconvolution analysis were used to assess GH secretion.

RESULTS

Graded Te supplementation increased serum Te concentrations (nmol/l) from 0.87 +/- 0.06 [Pl] to 5 +/- 1 [low] and 7.3 +/- 1.6 [high] (P < 0.001), but did not affect: (i) pulsatile GH secretion (microg/l/12 h; conversion factor: 1 microg/l = 0.33 mU/l), 29 +/- 7.2, 32 +/- 5.6 and 27 +/- 4.9; (ii) GH regularity (approximate entropy), 0.52 +/- 0.05, 0.57 +/- 0.05 and 0.56 +/- 0.05; (iii) IGF-I concentrations (microg/l; conversion factor: 1 microg/l = 0.131 nmol/l), 126 +/- 13, 135 +/- 15 and 141 +/- 13; (iv) IGFBP-3 (mg/l), 3.7 +/- 0.2, 3.6 +/- 0.1 and 3.7 +/- 0.2; (v) IGFBP-1 (microg/l), 41 +/- 2.3, 44 +/- 3.8 and 44 +/- 3.2; and (vi) the mass of GH secreted (microg/l/3 h) after GHRP-2 injection, 123 +/- 27, 146 +/- 32 and 147 +/- 38.

CONCLUSION

Up to 8-fold elevation of Te concentrations for 1 week in postmenopausal women does not detectably amplify GH secretion or action, thus indicating that low androgen availability is not a proximate acute mediator of hyposomatotropism in postmenopausal individuals.

Estrogen and androgen elicit growth hormone release via dissimilar patterns of hypothalamic neuropeptide secretion

The dimorphic pattern of growth hormone (GH) secretion and somatic growth in male and female mammals is attributable to the gonadal steroids. Whether these hormones mediate their effects solely on hypothalamic neurons, on somatotropes or on both to evoke the gender-specific GH secretory patterns has not been fully elucidated. The purpose of this study was to determine the effects of 17beta-estradiol, testosterone and its metabolites on release of GH, GH-releasing hormone (GHRH) and somatostatin (SRIF) from bovine anterior pituitary cells and hypothalamic slices in an in vitro perifusion system. Physiological concentrations of testosterone and estradiol perifused directly to anterior pituitary cells did not affect GH releases; whereas, dihydrotestosterone and 5alpha-androstane-3alpha, 17beta-diol increased GH. Perifusion of testosterone at a pulsatile rate, and its metabolites and estradiol at a constant rate to hypothalamic slices in series with anterior pituitary cells increased GH release. The androgenic hormones increased GHRH and SRIF release from hypothalamus; whereas, estradiol increased GHRH but decreased SRIF release. Our data show that estradiol and the androgens generated distinctly different patterns of GHRH and SRIF release, which in turn established gender-specific GH patterns.

Effect of exogenous estradiol on plasma concentrations of somatotropin, insulin-like growth factor-I, insulin-like growth factor binding protein activity, and metabolites in ovariectomized Angus and Brahman cows

To determine the effect of breed and estradiol-17 beta on selected hormones and metabolites, ovariectomized (> or = 3 mo) Angus (n = 14) and Brahman (n = 12) cows were paired by age and body weight and randomly assigned as either nonimplanted controls (CON) or implanted with estradiol (E2) for 45 d. After Day 7 and through Day 42, plasma concentration of somatotropin was greater for E2 than CON cows (treatment x day, P < 0.05). During an intensive blood sampling on Day 36, E2 cows tended (P < 0.10) to have greater somatotropin pulse amplitudes than CON cows, but other parameters of somatotropin release were not affected (P > 0.10) by E2 treatment. The effect of breed was apparent on Day 36 as Brahman cows had greater (P < 0.05) somatotropin pulse amplitude, basal secretion, and mean concentration than Angus cows. Overall, plasma concentration of IGF-I was greater (P < 0.01) for E2 than CON cows (158.3 vs. 104.2 ng/ml) and was greater for Brahman than Angus cows (164.1 vs. 98.4 ng/ml). However, there was a trend (P < 0.10) for a treatment x breed x day interaction for IGF-I (i.e., the magnitude of increase in IGF-I concentration was greater in E2-Angus than E2-Brahman cows). After Day 7 and through Day 42, total plasma IGF binding protein (IGFBP) activity was greater (P < 0.01) for E2 than CON cows. Ligand blotting revealed at least five forms of IGFBP activity, and E2 cows had greater (P < 0.05) binding activity of IGFBP-3 and the 30- and 32-kDa IGFBP than CON cows. Brahman cows had greater (P < 0.05) IGFBP-3 and the 32-kDa IGFBP than Angus cows. After Day 14 and through Day 42, concentration of urea nitrogen (PUN) was greater (P < 0.001) for CON than E2 cows (treatment x day, P < 0.001). Brahman had greater (P < 0.01) PUN than Angus cows (16.6 vs. 14.2 mg/dl). Plasma concentration of glucose was greater (P < 0.01) for E2 than CON cows (78.9 vs. 76.4 mg/dl) but was not affected (P > 0.10) by breed. In summary, these data suggest that some, but not all, of the positive effects of estradiol on peripheral concentration of IGF-I and IGFBP activity can be attributed to increased somatotropin. Moreover, breed influenced basal and E2-induced secretion of somatotropin and IGF-I such that differences between Brahman and Angus cows in plasma IGF-I concentrations were abated within 3 wk of estradiol implantation. Thus, breed influences the metabolite and hormonal response of cattle to estrogenic implants.

Perifusion model system to culture bovine hypothalamic slices in series with dispersed anterior pituitary cells

Dispersed bovine anterior pituitary cells were incubated either in static or perifusion cultures to assess basal growth hormone release as well as stimulatory and inhibitory effects of growth hormone-releasing hormone and somatostatin, respectively, on growth hormone release. Total concentrations of growth hormones over a 12-hour incubation period were fivefold greater in perifused than in static cultures (2034 +/- 160 vs. 387 +/- 33 ng/12 h). A dose-dependent increase in growth hormone secretion in response to challenge with growth hormone-releasing hormone (10(-12) to 10(-8) M) for 1 h was observed in both static and perifusion cultures; however, perifused cells were more responsive to the same concentration of neuropeptide than those in static culture. Concentrations of somatostatin (10(-12) to 10(-8) M) for 1 h did not inhibit basal growth hormone secretion in either static or perifusion cultures. To establish the hypothalamic-pituitary in series-perifusion model, slices of the hypothalamus, immediately adjacent to the sagittal midline, were perifused in series with anterior pituitary cells, and media effluent was assayed for growth hormone concentrations. Release of growth hormone was pulsatile and seemed to mimic the episodic pattern of bovine secretion. Hypothalamic slices were placed in one chamber of the perifusion system, and basal secretion of growth hormone-releasing hormone and somatostatin was pulsatile in media effluent. Tissue viability of hypothalamic slices and anterior pituitary cells was evaluated by KCl depolarization. Tissues were viable for at least 120 h. Thus, this hypothalamo-pituitary dual chamber perifusion system is a valid in vitro model to study regulation of growth hormone secretion.

Interaction between dietary intake and ovariectomy on concentrations of insulin-like growth factor-I, GH and LH in plasma of heifers

The objective of this study was to determine if alterations in dietary intake and(or) ovariectomy influence plasma concentrations of IGF-I, GH and LH in heifers. Cyclic heifers (n = 23) were individually fed for 10 wk either 1) 1.8% of body weight in dry matter per day (GAIN; n = 7) 2) 1.1% of body weight in dry matter per day (MAINT; n = 8); or 3) 0.7% of body weight in dry matter per day (LOSE; n = 8). After 10 wk of dietary treatment, heifers were ovariectomized 36 to 40 h following the second injection of prostaglandin F2alpha analog (2 injections 11 d apart). Heifers weighed 444 +/- 13, 387 +/- 8, and 349 +/- 9 kg in the GAIN, MAINT and LOSE groups, respectively, at the time of ovariectomy; the average daily weight gains during the 10-wk period were 0.96, 0.17 and -0.31 kg, respectively (P < 0.001), for the 3 groups. Blood plasma was collected for 6 h at 15-min intervals 1 d before and 2 wk after ovariectomy. The MAINT group of heifers had greater IGF-I concentrations than either the LOSE or GAIN groups; IGF-I decreased (P < 0.05) by 23 and 35% after ovariectomy in the MAINT and GAIN groups, respectively, but did not change (P > 0.10) in the LOSE groups. Dietary restriction tended to increase (P < 0.10) GH pulse frequency and mean GH. Ovariectomy had no effect (P > 0.10) on mean GH or GH pulse frequency but increased (P < 0.05) GH pulse amplitude in the GAIN groups. Dietary treatment had no effect (P > 0.10) on mean LH, or LH pulse amplitude and frequency. However, across dietary treatments, ovariectomy increased mean LH and LH pulse frequency but did not affect (P > 0.10) LH pulse amplitude. In summary, dietary restriction increased GH secretion while ovariectomy increased LH secretion. There appears to be a dichotomy of response between GH and IGF-I in the way heifers respond to dietary treatment and(or) ovariectomy.