ABSTRACT

It has been reported that GH stimulates fibroblast growth and wound healing. In the present study we measured the effect of locally applied GH on insulin-like growth factor (IGF-I) mRNA concentrations and granulation tissue formation in wound cylinders, implanted s.c.

Four stainless-steel wiremesh cylinders were implanted s.c. in the back of male rats (280 g). Each cylinder was then injected every day with either 0·014 or 0·14 U human GH, or vehicle only. Ingrown granulation tissue and wound fluid was obtained on day 17 after implantation. The wet weight of granulation tissue was determined and concentrations of IGF-I mRNA in the tissue were measured by solution hybridization/RNAase protection assay. Similar assays were used to measure the levels of IGF-I receptor mRNA and GH receptor mRNA, while the IGF-I concentration in wound fluid and serum was determined by radioimmunoassay (RIA) after acid–ethanol extraction.

The concentrations of IGF-I mRNA in ingrown granulation tissue as well as the wet weight of this tissue were significantly higher in the GH-treated cylinders. There was no significant effect of GH on IGF-I receptor mRNA and GH receptor mRNA levels. Consistent with the results of previous studies, wound fluid IGF-I levels were lower than serum IGF-I levels, but no significant difference was found between the GH-treated cylinders and the control cylinders.

The results of the present study show that GH stimulates granulation tissue formation and increases the concentration of IGF-I mRNA in the ingrown granulation tissue.

Journal of Endocrinology (1992) 132, 293–298

ABSTRACT

The influence of endogenous gonadal steroids in male and female rats on basal and growth hormone-releasing factor (GRF)-stimulated GH secretion from perifused anterior pituitaries was studied. After 75 min of perifusion with basal medium, freshly dissected pituitaries were exposed to human GRF(1–44) (10 nmol/l) for 15 min. Neonatal (day 1–2) or prepubertal (day 25) gonadectomy of male rats suppressed baseline GH release (ng/min per mg dry weight) as well as GRF-stimulated GH release by 40–70%. This effect was slightly more pronounced in neonatally gonadectomized animals. In prepubertally gonadectomized male rats, the suppression of GH release was completely reversed by testosterone replacement therapy. In female rats, prepubertal gonadectomy did not affect GH secretion from perifused pituitaries. However, treatment of ovariectomized female rats with oestradiol reduced baseline and GRF-induced GH release to levels lower than those observed in sham-operated or vehicle-treated ovariectomized animals. The data suggest that testicular androgen secretion in adult male rats increases the pituitary GH release in response to GRF in vitro, whereas ovarian oestrogen secretion is of less importance for the GRF responsiveness of female rat pituitaries.

J. Endocr. (1987) 113,249–253

ABSTRACT

The plasma GH levels of female rats during late pregnancy were determined using an automatic method for repetitive blood sampling from conscious animals. The plasma GH patterns were analysed by a pulse analysis computer program (PULSAR). The mean plasma GH levels were about twofold higher in pregnant females on days 15, 18 and 22 of gestation than in age-matched non-pregnant females. The basal plasma GH levels were also increased, while there was no change in GH pulse amplitude or frequency. The augmentation of GH release was even more pronounced on day 20 of gestation, with a fourfold increase in mean plasma GH levels compared with those in non-pregnant females. This increase reflected an increase in both basal plasma GH levels and GH pulse amplitude, but there was no increase in pulse frequency. In female rats that delivered on day 22 of gestation, the basal and mean plasma GH levels increased during parturition.

Pregnant females consistently responded to multiple i.v. infusions of 1 μg human GH-releasing factor analogue (hGRF(1–29)-NH₂) given at 45-min intervals on day 18 of gestation. Both basal and GRF analogue-stimulated plasma GH levels were undetectable after hypophysectomy of pregnant rats.

The present study demonstrates an increase in basal plasma GH levels during late pregnancy and a marked increase in both basal plasma GH levels and GH pulse amplitude on day 20 of gestation. Furthermore, hypophysectomy of pregnant rats results in undetectable GH levels, indicating that the high levels of GH during pregnancy are derived from the pituitary.

Journal of Endocrinology (1990) 124, 191–198

ABSTRACT

The serum concentration of corticosteroid-binding globulin (CBG) is higher in female rats than in males. Combined hypophysectomy and gonadectomy of female rats reduced the serum concentration of CBG as measured by steady-state polyacrylamide gel electrophoresis, whereas hypophysectomy of male rats increased serum CBG. These effects were seen despite replacement therapy with thyroxine and glucocorticoids. Moreover, neither androgen nor oestrogen treatment affected the serum concentrations of CBG in hypophysectomized rats. Continuous infusions of human or bovine GH (1·4 U/kg per day), by means of osmotic minipumps for 1 week, increased serum concentrations of CBG in both hypophysectomized male and female rats. In contrast, intermittent GH replacement therapy by s.c. injections at 12-h intervals either had no effect or suppressed serum CBG levels. In male rats, neonatal (days 1–2) gonadectomy increased CBG levels more than did prepubertal (day 25) gonadectomy, and testosterone replacement therapy reversed these effects.

It is concluded that GH increases the serum CBG levels of hypophysectomized rats when it is given in a continuous manner, but not when given intermittently. The sex difference in serum CBG levels of normal rats may, therefore, be attributed to the more continuous secretory pattern of GH previously observed in female rats.

Journal of Endocrinology (1989) 122, 725–732

ABSTRACT

GH accelerates hepatic regeneration in the rat. Hepatocyte growth factor (HGF), a potent hepatocyte mitogen in vitro, is considered to be a major regulator of hepatic regeneration. In the present study, the effects of GH and insulin-like growth factor-I (IGF-I) on HGF gene expression in regenerating rat liver was investigated. In hypophysectomized rats treated with GH, hepatic HGF mRNA levels were increased 3 h after partial hepatectomy and reached peak levels after 5 h. In rats with intact pituitaries and in hypophysectomized rats not given GH treatment, HGF mRNA levels in liver were unchanged during the first 5 h following hepatectomy and reached peak levels after 10-18 h. DNA synthesis in the liver of GH-treated rats increased from low levels 10 h after hepatectomy to peak levels after 18 h. In rats without GH treatment the synthesis of DNA was still low 18 h after hepatectomy and was increased after 26 h. Treatment of hypophysectomized rats with IGF-I promoted increases in hepatic HGF mRNA levels and DNA synthesis 3·5 h and 15 h after hepatectomy respectively. HGF mRNA levels were constantly lower after sham-hepatectomy than after partial hepatectomy. In summary, in hypophysectomized rats the responses of hepatic HGF gene expression and DNA synthesis to partial hepatectomy were both accelerated by treatment with GH or IGF-I.

Journal of Endocrinology (1992) 135, 59–67

ABSTRACT

Young hypophysectomized rats were maintained with chronic indwelling i.v. cannulae attached via swivels to a multichannel pumping system programmed to deliver GH in a continuous or pulsatile pattern for several days. Continuous i.v. infusions of human GH for 5 days produced dose-dependent increases in body weight and tail length, without increasing food intake. A comparison of GH infusions by the s.c. or i.v. route showed that the direct i.v. route was threefold more effective. Pulsatile i.v. infusions of human or bovine GH at two doses (12 or 36 mu./day, eight pulses/day, 5-min duration, every 3 h) produced greater increases in body weight than continuous i.v. infusions of GH at the same daily dose. Continuous infusions of bovine GH produced a lower growth rate in the second of two consecutive 5-day treatment periods, whereas the responses to pulsatile GH did not diminish with time. Both body weight gain and long-bone growth were affected by the frequency of GH pulses; nine pulses per day were more effective than three pulses per day which in turn produced larger growth responses than one pulse per day. Keeping GH pulse frequency constant and varying pulse duration (4, 16 or 64 min) did not affect growth rates. In conclusion, long-term pulsatile i.v. infusions of GH mimic the endogenous secretory pattern, and are most effective when given at the physiologically appropriate pulse frequency.

J. Endocr. (1985) 104, 53–61

Growth hormone (GH) is of importance for normal bone remodelling. A recent clinical study demonstrated that MK-677, a member of a class of GH secretagogues (GHSs), increases serum concentrations of biochemical markers of bone formation and bone resorption. The aim of the present study was to investigate whether the GHSs, ipamorelin (IPA) and GH-releasing peptide-6 (GHRP-6), increase bone mineral content (BMC) in young adult female rats. Thirteen-week-old female Sprague-Dawley rats were given IPA (0.5 mg/kg per day; n=7), GHRP-6 (0.5 mg/kg per day; n=8), GH (3.5 mg/kg per day; n=7), or vehicle administered continuously s.c. via osmotic minipumps for 12 weeks. The animals were followed in vivo by dual X-ray absorptiometry (DXA) measurements every 4th week. After the animals were killed, femurs were analysed in vitro by mid-diaphyseal peripheral quantitative computed tomography (pQCT) scans. After this, excised femurs and vertebrae L6 were analysed by the use of Archimedes' principle and by determinations of ash weights. All treatments increased body weight and total tibial and vertebral BMC measured by DXA in vivo compared with vehicle-treated controls. However, total BMC corrected for the increase in body weight (total BMC:body weight ratio) was unaffected. Tibial area bone mineral density (BMD, BMC/area) was increased, but total and vertebral area BMDs were unchanged. The pQCT measurements in vitro revealed that the increase in the cortical BMC was due to an increased cross-sectional bone area, whereas the cortical volumetric BMD was unchanged. Femur and vertebra L6 volumes were increased but no effect was seen on the volumetric BMDs as measured by Archimedes' principle. Ash weight was increased by all treatments, but the mineral concentration was unchanged. We conclude that treatment of adult female rats with the GHSs ipamorelin and GHRP-6 increases BMC as measured by DXA in vivo. The results of in vitro measurements using pQCT and Archimedes' principle, in addition to ash weight determinations, show that the increases in cortical and total BMC were due to an increased growth of the bones with increased bone dimensions, whereas the volumetric BMD was unchanged.

IGF-I is a neuroprotective hormone, and neurodegenerative disorders, including Alzheimer’s disease, have been associated with decreased serum IGF-I concentration. In this study, IGF-I production was inactivated in the liver of adult mice (LI-IGF-I^−/−), resulting in an approximately 80–85% reduction of circulating IGF-I concentrations. In young (6-month-old) mice there was no difference between the LI-IGF-I^−/− and the control mice in spatial learning and memory as measured using the Morris water maze test. In old (aged 15 and 18 months) LI-IGF-I^−/− mice, however, the acquisition of the spatial task was slower than in the controls. Furthermore, impaired spatial working as well as reference memory was observed in the old LI-IGF^−/− mice. Histochemical analyses revealed an increase in dynorphin and enkephalin immunoreactivities but decreased mRNA levels in the hippocampus of old LI-IGF-I^−/− mice. These mice also displayed astrocytosis and increased metabotropic glutamate receptor 7a-immunoreactivity. These neurochemical disturbances suggest synaptic dysfunction and early neurodegeneration in old LI-IGF-I^−/− mice. The decline in serum IGF-I with increasing age may therefore be important for the age-related decline in memory function.