Sonya Conway

ROLE OF INSULIN-LIKE GROWTH FACTOR I IN REGULATING GROWTH HORMONE RELEASE AND FEEDBACK IN THE MALE RAT

In vivo and in vitro (static incubation and perifusion) procedures were used to examine the role of insulin-like growth factors (IGFs) in growth hormone (GH) feedback. An alpha 2-adrenergic agonist, clonidine (CLON; 2 x 10(-8) M in vitro or 30 micrograms/ml/kg body weight i.v. in vivo), which mimics the hypothalamic mechanism triggering GH release, was injected to induce a GH surge. Feedback was initiated by human GH (hGH; 2 x 10(-6) M) in vitro or ovine GH (oGH) (20 micrograms/2 microliters intraventricularly) in vivo. GH-releasing factor (GRF; 1 x 10(-8) M) was added at the end of in vitro experiments to test pituitary responsiveness. The involvement of somatostatin (SRIF), GRF and IGFs in mediating GH feedback was evaluated in hypothalamic-pituitary coperifusion. CLON-induced GH release in this system was associated with increased GRF and decreased SRIF release, and the pattern was reversed by hGH. The influence of hGH was mimicked by IGF-I (1.5 x 10(-8) M), except that the GH release was depressed below baseline levels, suggesting a direct effect of IGF-I on the pituitary. Furthermore, the inhibitory effect of hGH on the CLON-induced GH surge and hypothalamic releasing factors (increased SRIF and decreased GRF) was reversed by antisera to IGF-I (1:100), IGF-II (1:100), or both. To determine whether IGF-I is released from hypothalamus or pituitary in response to GH, tissues were tested separately in static incubation. As compared with basal levels, incubation of hypothalami with hGH increased IGF-I and SRIF and decreased GRF release. Because GH and IGF-I release remained unchanged when pituitaries were incubated alone with hGH, the site of IGF-I release and GH feedback is most likely at the hypothalamic level. To evaluate the role of IGFs on GH feedback in vivo, male rats were prepared with permanently implanted 3rd-ventricular and jugular cannulae. CLON was administered intravenously, and oGH, IGF-I (0.5 microgram/2 microliters), and IGF-I and -II antisera (1:100) were injected intraventricularly. In this as in in vitro studies, IGF-I mimicked the inhibitory feedback effect of GH on the CLON-induced GH surge, and IGF antisera blocked GH feedback. We propose that these studies suggest that endogenous hypothalamic IGF-I mediates the influence of GH in the feedback mechanism by increasing SRIF and depressing GRF release.

The effect of acute ethanol exposure on clonidine-induced growth hormone release in male rats

The influence of acute ethanol (ETOH) on the regulation of GH release has not been clearly elucidated. In this study the effect of ETOH on clonidine (CLON)-induced GH secretion was examined. To test the effect of ETOH on CLON-induced GH release doses of 1, 2, and 3 g/kg ETOH in a 25% ETOH/saline solution or 10 ml/kg saline were injected IP 24 and 1 h before CLON (30 micrograms/kg BW IV). Blood samples were withdrawn through a chronic jugular cannula. ETOH was found to alter the GH surge, which occurred 15 min after CLON administration in controls, in a dose-dependent manner. The 1 g/kg dose reduced the GH surge slightly but not significantly. The 2 g/kg dose suppressed the GH surge which was significantly lower than in controls. The 3 g/kg dose eliminated the GH surge completely. To determine if pituitary GH release is directly influenced by ETOH, animals were injected with GRH (250 ng/kg IV) one hour after the second dose of ETOH (3 g/kg IP). There was no difference in the GH surge in control or ETOH-injected animals. Anti-SRIF administered 30 minutes before ETOH or saline did not alter the response to GRH. These results suggest that ETOH reduces the GH responsiveness to alpha 2-GRH stimulation.

The influence of ethanol exposure on insulin-like growth factor (IGF) type II receptors in fetal rat tissues

The effect of ethanol (ETOH) exposure on the IGF type II receptor concentration was examined in 18 and 20 day fetal rat tissues. Pregnant dams were fed an ETOH (36% of calories derived from ETOH; 6.6% v/v) liquid diet. Control fetuses were offspring of dams either pair-fed a control liquid diet or ad libitum-fed a standard pelleted lab chow. Fetal brain, heart, kidney, liver, lung and skeletal muscle were removed and whole homogenates from individual animals were analyzed. Results of immunoquantification of IGF type II receptors in whole tissue homogenates show that there is a trend towards increased receptor concentration between 18 and 20 days in all tissue and this trend is significant for lung, liver and muscle. There were no significant differences in receptor concentration between treatment groups. These studies suggest that during the later stages of fetal development, there is an increase in total IGF type II receptors and this increase is undisturbed by ETOH exposure.

A novel hypothalamic-dispersed pituitary co-perifusion model for the study of growth hormone secretion

This study presents a novel, in vitro, hypothalamic-dispersed pituitary co-perifusion system (HPPS) developed to examine the influence of the hypothalamus on pituitary growth hormone (GH) secretion in a controlled environment. In this perifusion system, dispersed rat pituitary cells were loaded onto Biogel P-2 (P-2) beads in a 0.5-ml plexiglas chamber and were submerged in a 37 degrees C water bath. After stabilization, two hypothalami were placed into each chamber on a thin layer of P-2 beads and the chamber was re-equilibrated. To test the system, pituitary cells were stimulated either directly with growth hormone-releasing factor (GRF) or indirectly via the hypothalamus, with clonidine, an alpha 2-adrenergic (alpha 2) receptor agonist. Perifusion of HPPS or pituitary cells with GRF (40 ng/ml) induced a substantial endogenous GH surge. Clonidine (2 x 10(-8) M) treatment stimulated a GH surge in HPPS chambers, but not in chambers containing only pituitary cells. Thus, somatotrophs respond to hypothalamic factors released in response to clonidine and not directly to alpha 2 stimulation. To determine if the components involved in GH feedback are present in the perifusion system, HPPS chambers were sequentially perifused with hGH, clonidine, and GRF. hGH pretreatment suppressed the clonidine but not the GRF-induced GH surge(s) observed in chambers perifused with clonidine and GRF only. In chambers only containing pituitary cells, GH was only increased in response to GRF when sequentially perifused with all three substances. This study demonstrates the dynamic interaction between the hypothalamus and pituitary in the regulation of GH secretion in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

The Development of Sexually Dimorphic Sensitivity to Growth Hormone (GH) Feedback of the Clonidine-Induced GH Surge in the Rat

This study investigates the development of sexually dimorphic sensitivity of the GH system to alpha 2-adrenergic stimulation and GH feedback in the rat. Sensitivity to alpha 2-adrenergic stimulation was tested with clonidine (CLN, an alpha 2-adrenergic agonist) which stimulates GH release in the adult male rat. Feedback was examined by testing whether human (h)GH suppressed the CLN-induced GH surge as previously demonstrated in adult male rats. The integrity of the pituitary and its capacity to respond to stimulation was tested at the end of the experiment by perifusing with GRF. Studies were conducted using a hypothalamic-pituitary coperifusion system which allows incubation of these tissues without the confounding influences of peripheral hormonal and extrahypothalamic neural factors. Tissue from prepubertal rats of 10, 20, 25 and 30 days of age, 50-day-old and adult rats (90-100 days) were evaluated. Results indicate that tissue from both male and female rats is sensitive to alpha 2-adrenergic stimulation at 10 days of age. In male tissue, there is an increase in GH release in response to CLN until 30 days of age, after which a slight decline in sensitivity occurs by 50 days of age and is maintained in adulthood. In regard to GH release from female tissue, a GH surge occurs in response to CLN until 30 days of age. At 50 days and in adulthood, this response is substantially diminished. Additionally, there is a profound sexual dimorphism in the capacity of hGH to suppress the CLN induced GH surge. In tissue from male rats, by 20 days of age there is an apparent GH-associated inhibition of the CLN-induced GH surge which is significant by 25 days, is more pronounced by 30 days of age, and is maintained after puberty at 50 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of acute ethanol exposure on growth hormone release in female rats

This study investigates the site (hypothalamic or pituitary) at which ethanol (ETOH) alters GH release in female rats. Both the hypothalamic response to clonidine (CLON), an alpha 2-adrenergic agonist, and the pituitary response to growth-hormone releasing hormone (GRH) were tested. Jugular cannulae were inserted for drug administration and undisturbed blood sampling. ETOH was injected IP 24 and 1 h before experimentation. In animals receiving saline or ETOH (1, 2, or 3 g/kg), there was no response to CLON and no difference in GH levels between groups. On the other hand, there was a significant surge in GH release in response to a high dose of GRH (1000 ng/kg) in both saline controls and in ETOH (3 g/kg) animals. Although there was no difference in the height of the surge between groups, baseline GH levels were higher in animals that received ETOH. In response to a low dose of GRH (250 ng/kg) the GH surge was only significant in the ETOH animals. In animals receiving somatostatin antiserum (anti-SRIF; 0.5 ml) in combination with the low GRH dose, the surge in GH levels was significant in both saline and ETOH animals, however, the surge was higher in saline compared to ETOH animals. The results of this study suggest that: 1) ETOH alters the SRIF system (release of reception) in female rats and that this interaction is evident when GRH concentration is low, and 2) ETOH may also inhibit GH release by interfering with the GRH system, however, the site of this influence most likely does not involve an alpha 2-GRH component.