Administration of human pancreatic growth hormone-releasing factor (GRF) analogs enhances responsiveness of cultured rat pituitary cells to GRF

Effects of long-term growth hormone-releasing factor treatment on growth, feed conversion efficiency and dry matter intake in growing female buffaloes (Bubalus bubalis)

Effects of long-term growth hormone-releasing factor (GRF) on growth performance, feed conversion efficiency (FCE) and dry matter intake (DMI) were studied in growing buffaloes. Twelve female Murrah buffaloes of 6-8 months of age were divided into two groups of six each on the basis of their body weights so that the average body weights of the groups did not differ (p > 0.05). Animals of each group were administered intravenously with either sterile distilled water (control group) or equal volume of GRF solution containing 10 mug GRF (1-44)-NH2/100 kg body weight (treatment group) at fortnight interval from week 6 (5-week pre-treatment period) for 36 weeks (weeks 6-42 treatment period). Thereafter a 10-week post-treatment period was added. All the animals were weighed consecutively 2 days in a week and the average body weight of the two observations in the week was thus considered for further calculation. Dry matter intake was recorded daily. Average daily gain, FCE and DMI/100 kg body weight were also calculated. Plasma progesterone was estimated in the samples collected twice a week at 3-4-day intervals to assess whether either group had begun ovarian cycles. It was found that ADG and FCE were higher (p < 0.01) in GRF-treated animals during treatment and even 10-week post-treatment period. Interestingly, total DMI was not different (p > 0.05) between the groups during treatment period but found to be lower in GRF treated animals during post-treatment period. The DMI/100 kg body weight was lower (p < 0.01) in GRF-treated animals during treatment and even after cessation of treatment for 10 weeks. The GRF administration for long-term increased (p < 0.05) plasma progesterone. Plasma progesterone concentrations suggest that no animal from either group reached puberty till the end of the experiment. In conclusion, repeated GRF administration for longer term decreased (p < 0.01) DMI/100 kg body weight and increased (p < 0.01) FCE and enabled the animals to grow faster.

Human GHRH reduces voltage-gated K+ currents via a non-cAMP-dependent but PKC-mediated pathway in human GH adenoma cells

1. Whole-cell voltage-gated K+ currents and the K+ current response to growth hormone-releasing hormone (GHRH) were characterised in primary cultures of human acromegalic somatotropes. 2. Both delayed rectifier (IK) and transient (IA) K+ currents were recorded from human somatotropes held at -80 mV and bathed in a solution containing Cd2+ (1 mM), TTX (1 microM) and a low concentration of Ca2+ (0.5 mM). Only IK was recorded, however, when a holding potential of -40 mV was used. 3. GHRH (10 nM) immediately and significantly reduced the amplitude of both IA and IK. While the reduction in the amplitude of IA was fully reversed following the removal of GHRH, the amplitude of IK had only partially recovered 10 min after GHRH removal. In addition, GHRH shifted the voltage-dependent inactivation curve of IA by 13.5 mV in the negative direction. 4. In a low Ca2+ and Cd2+-containing solution, the Ca2+-activated K+ channel blockers apamin (100 nM and 1 microM) and charybdotoxin (1 microM) did not alter K+ currents or the effect of GHRH on the recorded K+ currents. 5. The whole-cell K+ currents and their responses to GHRH were unaffected by the application of 8-bromo-cAMP (100 microM), Rp-cAMP (100 microM) or the protein kinase A (PKA) inhibitor H89 (1 microM). In addition, intracellular dialysis of the PKA inhibitory peptide PKI (10 microM) had no effect on the K+ current response to GHRH. 6. While the application of protein kinase C (PKC) inhibitors calphostin C (100 nM) or chelerythrine (1 microM) did not affect the amplitude of the K+ currents, the K+ current response to GHRH was significantly attenuated. Downregulation of PKC with phorbol 12,13-dibutyrate (PDBu, 0.5 microM for 16 h) also abolished the K+ current response to GHRH. In addition, intracellular dialysis of somatotropes with the PKC inhibitory peptide PKC19-36 (1 microM) prevented the GHRH-induced decrease in the amplitude of the voltage-gated K+ currents. Local application of PDBu (1 microM) significantly reduced the amplitude of the voltage-gated K+ currents in a similar manner to that induced by GHRH, but without clear recovery upon removal. 7. This study demonstrates that GHRH decreases voltage-gated K+ currents via a PKC-mediated pathway in human adenoma somatotropes, rather than by the cAMP-PKA pathway that is usually implicated in the actions of GHRH.

Human pancreatic growth hormone-releasing factor (1-44) stimulates GH cells in an anuran amphibian (Rana perezi)

Subcellular responses of amphibian growth hormone (GH)-producing cells to in vivo administration of human pancreatic growth hormone-releasing factor (1-44) (hpGRF) were investigated. The volume density (Vv) of secretory granules (SG), rough endoplasmic reticulum (RER), and Golgi complex (GC) and the numerical density (Nv) of the granules were estimated by ultrastructural morphometry. Immunogold staining was applied to ultrathin sections using an antiserum to ovine GH to identify GH-producing cells. In vivo treatment with hpGRF significantly decreased the Vv of the SG and the Nv of medium SG after 6 hr. The peptide also stimulated development of the cellular biosynthetic machinery and increased the number of small SG 24 hr after stimulation. Four-day-stimulated GH cells did not recover control morphological appearances. These morphological results suggest that: (1) In vivo administered hpGRF stimulates GH cells in Rana perezi by inducing hormone release and enhances biosynthetic activity; (2) after four injections, the cellular response is more intense, indicating that GH cells remain hyperactive, probably because the exogenous hpGRF overcomes the endogenous inhibitory control of GH secretion.

GRF treatment of late pregnant ewes alters maternal and fetal somatotropic axis activity

To examine the effects of anabolic agents given during late gestation on the maternal and fetal somatotropic axes, we injected pregnant ewes twice daily with 0.15 mg somatocrinin (GRF)-(1-29) for 10 days beginning on day 130 of gestation. Maternal and fetal endocrine changes were compared with control animals using both in vivo and in vitro approaches. Treatment with GRF increased maternal plasma levels of growth hormone (GH) and insulin-like growth factor I (IGF-I;P less than 0.05) but not IGF-II. Under in vitro test conditions, maternal pituitary cells showed a greater maximal response (P less than 0.001) to GRF. In the fetuses of treated ewes, cord plasma GH levels were not significantly increased compared with controls. These animals had similar IGF-I but higher IGF-II (P less than 0.05) plasma levels. The maximal response of fetal pituitary cells to GRF was increased (P less than 0.001). GRF treatment had no influence on maternal and fetal pituitary cell responses to somatostatin under either basal or GRF-stimulated conditions. In addition, these treatments did not affect plasma levels of placental lactogen, glucose, or free fatty acids in the maternal and fetal sheep. These data are compatible with the hypothesis that treatment of pregnant ewes in the last days of gestation with GRF could support accelerated fetal growth.

Receptor-associated resistance to growth hormone-releasing factor in dwarf "little" mice

Anterior pituitaries from the dwarf mouse strain "little" did not release growth hormone or accumulate adenosine 3',5'-monophosphate (cyclic AMP) in response to human and rat growth hormone-releasing factor (GRF). Dibutyryl cyclic AMP, as well as the adenylate cyclase stimulators forskolin and cholera toxin, markedly stimulated growth hormone (GH) release. The basis of the GH deficiency in the little mouse may therefore be a defect in an early stage of GRF-stimulated GH release related either to receptor binding or to the function of the hormone-receptor complex.

Octapeptide analogs of somatostatin exhibiting greatly enhanced in vivo and in vitro inhibition of growth hormone secretion in the rat

Analogs of a superactive somatostatin (SRIF) octapeptide (code named SMS 201-995 (1)) were synthesized using solid-phase synthetic methodology and assayed for their ability to inhibit growth hormone release from cultured rat anterior pituitary cells and in sodium pentobarbital-anesthetized rats. One analog: (Formula: see text) exhibited greatly enhanced in vitro inhibitory activity (greater than 1,000x) relative to both the parent octapeptide molecule and to the 14 amino acid SRIF molecule. This analog which was also very potent in vivo contains a tyrosine residue and, given its high in vitro activity, may be of investigative importance as a radioiodinated ligand in receptor assays. An octapeptide retro-inverso analog also exhibited significant SRIF-like activity. Several very low activity octapeptide analogs were synthesized and were found to be devoid of SRIF-antagonist activity. A dodecapeptide analog previously shown to be superactive in vivo also demonstrated high in vitro activity.

Adenohypophyseal response to hypophysiotropic hormones in male obese Zucker rats

Description of the recessive, homozygote obese Zucker rat (fafa) includes disorders of growth and reproduction. The aim of this study was to compare responsiveness of adenohypophyseal cells, obtained from male fafa rats and from their lean siblings, to growth hormone-releasing factor (GRF) and to luteinizing hormone-releasing hormone (LHRH). Pituitary cells were cultured for 4 days and were then challenged with either GRF-29 (the NH2-terminal 29 amino acid GRF peptide that expresses full biological activity of its parent 44 amino acid molecule) or [D-Trp6]LHRH (LHRH-A, an LHRH agonist). Medium was assayed for growth hormone (GH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) by radioimmunoassay. Dose-response curves were compared using the computer program ALLFIT. The median effective GRF-29 concentration (EC50) computed for hypophyseal cells cultured from lean animals (0.30 +/- 0.01 fM; means +/- SE of 4 experiments) was less (P less than 0.01) than that calculated for cells obtained from fafa rats (15.8 +/- 6.7 fM). In contrast, cells derived from lean littermates required a larger (EC50) concentration of LHRH-A than did gonadotrophs cultured from obese rats [58.2 +/- 1.2 vs. 10.7 +/- 1.2 pM (P less than 0.01) and 59.4 +/- 10.4 vs. 15.7 +/- 7.6 pM (P less than 0.05)] to secrete LH and FSH, respectively. Our data describe an attenuated pituitary response to GRF-29 and an enhanced response to LHRH-A in the fafa.