Dual inhibition of angiotensin-converting enzyme and neutral endopeptidase in rats with hypertension

Angiotensin converting enzyme (ACE) and neutral endopeptidase (NEP), are two mechanistically similar enzymes involved in the metabolism of several vasoactive peptides. Selective inhibitors of ACE are effective antihypertensive agents in high-renin, renovascular rats and normal-renin, spontaneously hypertensive rats (SHR), but are not effective in the low-renin, deoxycorticosterone acetate (DOCA)-salt hypertensive rats. In contrast, NEP inhibitors are only effective in the low-renin model of hypertension. Treatment with a combination of selective inhibitors or with a dual inhibitor of both enzymes produces an antihypertensive response regardless of basal plasma renin activity. In this study, we compared the activities of MDL 100,173, a novel subnanomolar inhibitor of both ACE and NEP, with those of equimolar doses of captopril, a selective ACE inhibitor, following intravenous administration in these three rat models of hypertension. Treatment with MDL 100,173 significantly lowered blood pressure compared to vehicle treatment in all three models, whereas captopril treatment lowered blood pressure in the renovascular and SHR models only. Administration of MDL 100,173 also significantly elevated diuresis and natriuresis compared to either vehicle or captopril treatment in the SHR and DOCA-salt rats. Urinary excretion of atrial natriuretic peptide (ANP) was increased by MDL 100,173 treatment in all three models of hypertension. Treatment with captopril did not alter urine, sodium, or ANP excretion in any of the models. However, plasma-renin activity was elevated by both MDL 100,173 and captopril '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' ''''''''

Reduction in endotoxin-induced organ dysfunction and cytokine secretion by a cyclic nitrone antioxidant

Multiple organ dysfunction (MOD) is the leading cause of mortality in septic patients with circulatory shock. Recent evidence suggests that the overproduction of the cytokine, tumor necrosis factor-alpha(TNF), and oxygen free radical molecules may mediate the progression of sepsis to MOD and death. In this study, we have examined the ability of MDL 101,002, a free radical scavenger, to reduce organ dysfunction and cytokine secretion induced by lipopolysaccharide (LPS) administration in rats. Treatment with MDL 101,002(10-60 ng/kg, i.p.) 30 min prior to an LPS challenge resulted in a dose-dependent reduction in several markers indicative of organ dysfunction and mortality. MDL 101,002 markedly decreased LPS-induced liver and kidney damage as indicated by serum levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) or urea and creatinine, respectively. MDL 101,002 also prevented LPS-induced pulmonary edema, but did not prevent leukopenia and only partially reduced thrombocytopenia. Associated with these improvements in organ dysfunction and survival was a modest decrease in LPS-stimulated interleukin-1 alpha (IL-1 alpha) and interleukin-1 beta (IL-1 beta) secretion and a marked ( > 90%) inhibition of TNF secretion by MDL 101,002. The data are consistent with a role for oxygen free radicals in the development of endotoxin-induced organ dysfunction and shock and suggest that free radical scavengers could reduce the mortality consequent to sepsis by decreasing organ dysfunction, at least in part, through a reduction in free radical stimulated cytokine secretion.

Growth hormone-releasing hormone immunoreactivity in mouse placenta, maternal blood, and amniotic fluid: molecular characterization and secretion from primary cell cultures in vitro

The GH-releasing hormone (GRH) gene, along with those of many other hypothalamic hormones, is abundantly expressed in mouse and rat placenta. The presence of GRH immunoreactivity (GRH-IR) is described in mouse placenta, maternal blood, and amniotic fluid, and its molecular form has been characterized using HPLC. Two different molecular forms of mouse GRH-IR (mGRH-IR) were detected in the mouse hypothalamus and one in placenta. Twenty-five percent of mGRH-IR in the hypothalamus corresponded to mGRH(1-42)OH, whereas the remainder, and all of the mGRH-IR in placenta, had a retention time consistent with the GRH precursor. High levels of mGRH-IR were detected in both maternal plasma and amniotic fluid. In addition, a mouse placental cell primary culture system was established to study the regulation of mGRH-IR release. Turnover of mGRH in placental cells was rapid, resulting in a 24-h media content of 10 times that present in cells. Both 1-oleoyl-2-acetyl-sn-glycerol and 1,2-dioctanoyl-sn-glycerol significantly stimulated the release of mGRH-IR from cultured placental cells into the incubation media but had no effect on total peptide synthesis. These results suggest that the release of mGRH-IR from placental cells is mediated, at least in part, by the activation of protein kinase C. The HPLC elution profiles of mGRH-IR released from placental cells under basal and 1-oleoyl-2-acetyl-sn-glycerol-stimulated conditions were similar to those in placental tissue. Although the biological function of mGRH-IR in placental, maternal plasma, and amniotic fluid is not yet clear, the presence of mGRH-IR in these tissues and circulating fluids suggests the possibility that mGRH-IR may exert an important role in both fetal and maternal physiology.

Protective effects of a cyclic nitrone antioxidant in animal models of endotoxic shock and chronic bacteremia

Evidence of a role for oxygen-derived free radicals in the pathophysiology of endotoxic shock has been found in animal models. However, the importance of free radicals in chronic models of bacterial infection has not been examined. In this study a novel nitrone radical spin trap is described and its activity in animal models of endotoxic shock and chronic bacteremia were explored. MDL 101,002 is a cyclized variant of alpha-phenyl N-tert-butyl nitrone (PBN), an established spin trap. MDL 101,002 can react with free radicals to form persistent adducts as demonstrated by electron paramagnetic resonance (EPR) spectroscopy. This agent is about 10 times more potent than PBN as an in vitro antioxidant and scavenger of hydroxyl radicals. In a rat endotoxic shock model MDL 101,002 (3-30 mg/kg, i.p.) administered 30 min prior to endotoxin (30 mg/kg, i.p.) treatment reduced mortality in a dose-dependent manner. Peroxide-enhanced chemiluminescence in hepatic homogenates from endotoxin treated rats was elevated indicating that oxidative stress and antioxidant depletion was increased. Importantly, treatment with MDL 101,002 (30 mg/kg, i.p.) 30 min prior to, and 120 min following endotoxin, minimized the increase in chemiluminescence. MDL 101,002 also reduced mortality in a model of chronic bacteremia employing implantation of infected fibrin clots into the peritoneal cavity of gentamicin-treated leukopenic rats. MDL 101,002 (2.5 mg/kg/hr) increased survival from 24% to 52% in these rats. These data are consistent with a role for free radicals in the pathophysiology of endotoxic shock and suggest free radicals are also important mediators in chronic models of sepsis.

Development of neuroepithelial tumors of the adrenal medulla in transgenic mice expressing a mouse hypothalamic growth hormone-releasing hormone promoter-simian virus-40 T-antigen fusion gene

Expression of the mouse GH-releasing hormone (GRH) gene is restricted to neurons within the hypothalamus and to placenta. In an attempt to generate immortalized mouse hypothalamic neurons expressing GRH, the proximal 872-nucleotide segment of the 5'-flanking region of the hypothalamic mouse GRH gene was cloned by polymerase chain reaction and ligated to a 2.7-kilobase DNA sequence encoding the simian virus-40 (SV40) T-antigen, so that regulation of SV40 T-antigen expression was dependent on sequences within the mGRH 5'-flanking region. This region contains both TATA and CAAT boxes. The mouse GRH/SV40 T-antigen fusion gene was injected into 1-cell mouse embryos, and SV40 T-antigen incorporation in the mouse genome was found in 11 of 77 live births (3 males and 8 females). Although no evidence of hypothalamic tumors was found, all mice that expressed the transgene also developed tumors originating in the adrenal medulla. Gene copy number varied from 1-20 and was inversely proportional to survival, which ranged from 7-16 weeks. Corticosterone levels were normal. The male transgenic mice were fertile, and their progeny expressed the transgene and developed similar tumors. Microscopic examination of the tumors revealed a primitive neuroectodermal neoplasm that exhibited hematogenous and lymph node metastases and contained 100 ng norepinephrine, 2.85 ng epinephrine, and 1.1 ng dopamine/mg tumor tissue. Primary culture of dispersed tumor cells released norepinephrine into the medium (180 pg/ml.24 h). Cell lines from 2 tumors were established and exhibited characteristics similar to those of mixed neuroblastoma or primitive neuroectodermal tumors. In conclusion, the proximal 872 nucleotides of the hypothalamic mouse GRH promoter contain elements directing tissue-specific expression limited to early adrenal neuroectodermal cells. Other GRH DNA sequences appear to be required for restricted expression of mouse GRH within the hypothalamus.

Impaired growth hormone-releasing hormone signal transduction in the dwarf (dw) rat is independent of a generalized defect in the stimulatory G-protein, Gs alpha

The homozygous dwarf (dw) rat pituitary is characterized by a 95% reduction in GH content as well as a 75-80% reduction in the number of somatotrophs. The nature of the mutation responsible for this phenotype is unknown. Previous investigations from our laboratory indicate that dw somatotrophs exhibit decreased sensitivity and a reduced GH secretory response to GH-releasing hormone in vitro, accompanied by a decreased generation of cAMP. We hypothesized that dw rats have a defect in the pathway linking the GH-releasing hormone receptor to adenylyl cyclase and focused on the expression and function of the stimulatory G-protein, Gs alpha. When corrected for differences in pituitary size and cell number, GH mRNA content was reduced by 78% compared to that in controls. However, there was no difference in Gs alpha mRNA content or size in dw pituitaries. Similarly, there was no difference in the content or size of mRNA for the pituitary transcription factor pit-1 in dw pituitaries. Immunoblot analysis of pituitary membrane proteins using a Gs alpha-specific antibody revealed no differences in size, quantity, or relative distribution of Gs alpha peptides between control and dw pituitaries. In addition, cholera toxin effectively ribosylated dw Gs alpha, and there were no differences in the size, quantity, or relative distribution of ribosylated membrane proteins between dw and control pituitaries. Finally, to examine for mutations in other regions of the Gs alpha-coding sequence, we cloned the full-length Gs alpha cDNA from dw rat pituitaries by polymerase chain reaction. The sequence of this clone was identical to that of normal rat Gs alpha cDNA. These results indicate that 1) 52-kilodalton Gs alpha appears to predominate in both normal and dw pituitary; 2) the content, function, and sequence of Gs alpha in adult dw rats are normal; and 3) a generalized Gs alpha regulatory or structural mutation as the cause of the observed phenotype can be excluded. The results also demonstrate that there is no decrease in pit-1 expression in the adult dw pituitary.

Obesity-associated decrease in growth hormone-releasing hormone gene expression: a mechanism for reduced growth hormone mRNA levels in genetically obese Zucker rats

The secretion of growth hormone (GH) is impaired in the genetically obese Zucker rat where GH gene expression and plasma GH levels are depressed; however, the underlying mechanism of this abnormality remains unclear. We have evaluated the potential causative role of hypothalamic GH-releasing hormone (GHRH) and somatostatin (SRIH) gene expression in the onset of the decreased GH mRNA levels by studying both GHRH and SRIH mRNA and peptide levels in obese and lean rats at 5 weeks of age when the decrease in GH mRNA is first detected. At that age both GHRH content and GHRH mRNA were significantly reduced in obese rats as compared to lean controls; hypothalamic SRIH content was also decreased in obese rats, but SRIH mRNA levels did not differ. Since GHRH is capable of stimulating GH gene expression, the decreased GHRH mRNA level could be a critical factor in causing the attenuation in GH gene expression and consequent diminution of circulating plasma GH.

Mouse hypothalamic growth hormone-releasing hormone and somatostatin responses to probes of signal transduction systems

Signal transduction mechanisms involved in mouse growth hormone-releasing hormone (GRH) and somatostatin (SRIH) release were investigated using an in vitro perifusion system. Hypothalamic fragments were exposed to depolarizing agents, protein kinase A and C activators, and a calcium ionophore. The depolarizing agents, KCl (60 mM) and veratridine (50 microM), induced similar patterns of GRH and SRIH release. Somatostatin release in response to both agents was twofold greater than that of GRH. Forskolin (10 microM and 100 microM), an adenylate cyclase activator, stimulated both GRH and SRIH release, though with different secretory profiles. The SRIH response was prolonged and persisted beyond removal of the drug from the system, while the GRH response was brief, ending even prior to forskolin removal. Neither GRH nor SRIH were stimulated by 1,9-dideoxy-forskolin (100 microM), a forskolin analog with cAMP-independent actions. A23187 (5 microM), a calcium ionophore, stimulated the release of SRIH to a much greater extent than that of GRH. The GRH and SRIH secretory responses to PMA (1 microM), a protein kinase C activator, were similar, though delayed. The results suggest that 1) GRH and SRIH secretion are regulated by both protein kinase A and C pathways, and 2) depolarizing agents are important for the release of both hormones.

Congenital gigantism due to growth hormone-releasing hormone excess and pituitary hyperplasia with adenomatous transformation

The cause of gigantism in most patients is a GH-secreting pituitary tumor. In this report, a case of congenital gigantism due to probable central hypersection of GH-releasing hormone (GHRH) is described. Normal at birth (4.4 kg; 53 cm), our 7-yr-old male patient grew progressively thereafter to attain a height of 182 cm and a weight of 99.4 kg at the time of our evaluation. The markedly increased baseline plasma levels of GH (730 micrograms/L) did not suppress during a standard 3-h oral glucose tolerance test, but did increase 54% after iv infusion of GHRH. Baseline plasma levels of insulin-like growth factor-I, PRL, and immunoreactive GHRH were also markedly increased. Computed imaging of the head showed a large, partially cystic sellar and suprasellar mass. Extensive imaging studies did not localize a potential source of GHRH. Preoperative treatment with octreotide and bromocriptine for 4 months resulted in a 25% reduction of suprasellar tissue mass. The pituitary tissue removed at transsphenoidal and transfrontal operations showed massive somatotroph, lactotroph, and mammosomatotroph hyperplasia. Areas of GH- and PRL-secreting cell adenomatous transformation were also evident. No histological or immunohistochemical evidence of a pituitary source of GHRH was found. The peripheral plasma immunoreactive GHRH concentration remained unaffected by pharmacological and surgical interventions. We suspect that a congenital hypothalamic regulatory defect may be responsible for the GHRH excess in this case.

Growth hormone-releasing factor analogs with hydrophobic residues at position 19. Effects on growth hormone releasing activity in vitro and in vivo, stability in blood plasma in vitro, and secondary structure

To test the hypothesis that replacing Ala19 in growth hormone-releasing factor (GRF) with more hydrophobic residues will increase growth hormone releasing activity, four GRF analogs were prepared and tested. The molecules were made by substituting Val, Ile, or Leu at position 19 of [Thr2,Ala15,Leu27]bGRF(1-29)NH2. The compounds were evaluated for growth hormone (GH) releasing activity in vitro (rat anterior pituitary cells) and in vivo (steers). Additionally, their half-life in vitro was determined in bovine plasma, and their secondary structure was examined by circular dichroism. In pituitary cells, peptides with substitutions at position 19 had the following potencies: Ala (native), 0.37; Val, 1.16; Ile, 0.37; Leu, 0.043. When assayed in steers as a single iv bolus, over a 2-h period, the compounds gave the following integrated GH response: Ala, 2.75; Val, 2.67; Ile, 2.57; Leu, 1.55. Only the Leu analog was statistically different from the other three (p = 0.05). In bovine plasma, the half-lives (hours) were as follows: Ala, 4.9; Val, 6.6; Ile, 12.3; Leu, 14.7. In phosphate buffer the compounds were calculated to have the following percent helical content: Ala, 26; Val, 21; Ile, 27; Leu, 32. For these analogs, helicity in aqueous buffer is inversely related to their in vitro activity. Using a linear multiple regression model, the plasma half-life of the analogs positively correlated (r2 = 0.999) with both the hydrophobicity of the residue at position 19 and the helicity of the analog. Although the Val analog had both increased inherent activity and increased plasma stability in vitro compared to the Ala analog, in this study we were unable to demonstrate an increase in activity in vivo. The in vivo GH releasing activity of the analogs was not simply related to a combination of their intrinsic GH releasing activity and their in vitro plasma half-life. This suggests that in vivo additional factors are moderating the expression of activity.

Regulation of growth hormone secretion

The secretion of growth hormone (GH) is controlled by a complex regulatory system. The primary control is exerted by two neuroendocrine hormones, GH-releasing hormone and somatostatin, though other hypothalamic neuropeptides directly and indirectly participate in this process. The regulation of each of these neurohormones is now being clarified at both physiologic and molecular levels, as are their respective roles in the generation of pulsatile GH secretion and in GH feedback regulation. Considerable information has been amassed concerning signal transduction systems mediating the effects of hypothalamic hormones on GH secretion. Although multiple second messengers have been implicated, the adenylate cyclase-cyclic AMP-protein kinase A system appears to exert a predominant role. The developmental regulation of the somatotropes and of GH gene expression is also of importance in determining the GH responses to releasing and inhibiting hormones. The availability of several rodent strains with genetic disorders of growth associated with impaired GH secretion, along with the development of transgenic models, has permitted a more detailed analysis of the role of cytokines and growth factors on both somatotrope biology and hormone secretion. Finally, knowledge gained from studies in animals is permitting a better understanding of the mechanisms involved in physiologic GH secretion and altered GH secretion associated with specific disease states in humans.

Morphologic effects of hGRH gene expression on the pituitary, liver, and pancreas of MT-hGRH transgenic mice. An in situ hybridization analysis

Morphologic changes in the pituitary, liver, and pancreas of mice with the metallothionein-human growth hormone--releasing hormone (MT-hGRH) transgene were analyzed by in situ hybridization histochemistry (ISH). There was progression from somatotroph hyperplasia to neoplasia in pituitaries of transgenic mice. Pituitary neoplasms were present between 9 to 12 months of age in some mice. Magnetic resonance imaging (MRI) readily identified enlarged pituitaries in MT-hGRH transgenic mice. Serum mouse GH and hGRH levels were marked elevated in MT-hGRH transgenic mice. In situ hybridization histochemistry showed mRNA for hGRH in liver, pituitary, pancreas, spleen, and in most other tissues examined. Combined ISH and immunohistochemistry in the pituitary gland showed that some of the GH cells also produced hGRH, and ultrastructural immunohistochemical analysis of pituitaries showed that GH and hGRH were localized in the same cell and within the same secretory granules. Liver cells of MT-hGRH transgenic mice showed evidence of hypertrophy, and the pancreatic islets were hyperplastic with significant increases in the islet cell areas. The morphologic changes in the liver were distinctive enough to separate control littermates from MT-hGRH transgenic mice in all cases. The enlarged pancreatic islets had increased numbers of insulin-producing cells. Immunoreactive hGRH and hGRH mRNA were both localized in islet cells, and an intense hybridization signal of hGRH mRNA, but only weak staining for hGRH protein, were detected in the liver of transgenic mice. These results indicate that excessive hGRH production leads to distinct morphologic changes in various organs in MT-hGRH transgenic mice and that there is temporal progression from hyperplasia to adenomatous somatotrophs in pituitaries with chronic stimulation by hGRH that involves paracrine, endocrine, and autocrine mechanisms.

Impaired generation of cyclic adenosine 3',5'-monophosphate in a somatomammotroph cell line derived from dwarf (dw) rat anterior pituitaries

The dwarf (dw) mutation in rats results in 40-50% growth retardation associated with a selective reduction in pituitary somatotroph number, GH content, and GH mRNA levels and a decreased GH secretory response to GH-releasing factor (GRF). Recent studies in freshly dispersed pituitary cells have provided evidence for a defect in adenylate cyclase-linked GRF signal transduction in dw somatotrophs. To further examine this defect in a more specific cell population, we developed a somatomammotroph cell line (DP) derived from anterior pituitaries of male dw rats. A similar cell line from normal rats (Po) was used as control. We studied acute GH (4-h release) and cAMP (30-min intracellular accumulation) responses to GH secretagogues known to interact with the adenylate cyclase system. Basal GH release in both cell lines was 80-130% of the cell content, thus limiting the capacity for further GH responses. GRF (10(-8) M) produced a doubling of cAMP levels in Po and DP cells (P less than 0.01), but inconsistent effects on GH release. (Bu)2cAMP (5 x 10(-3) M) increased GH secretion by 50-100% in both groups (P less than 0.01). Cholera toxin (10(-9) M) increased GH release by 50% in both Po and DP (P less than 0.01), but the cAMP response in DP cells was only half that in Po cells (P less than 0.01). Forskolin (10(-5) M), a direct stimulator of adenylate cyclase, doubled GH release in both groups (P less than 0.01). However, cAMP generation was impaired in DP, with a maximal response to forskolin less than one third that in Po (P less than 0.01). In somatotrophs, cAMP mediates not only GRF-stimulated GH release, but also GH synthesis and mitogenesis. The impairment in maximal cAMP generation in DP cells, while not affecting acute GH release, may underlie the defect in somatotroph cell number and GH content in the dw pituitary gland.

Biosynthesis of human growth hormone-releasing hormone (hGRH) in the pituitary of hGRH transgenic mice

We have studied the posttranslational processing of prohuman GH-releasing hormone (pro-hGRH) to the mature hormones, hGRH(1-44)-NH2 and hGRH(1-40)-OH, and its carboxyl-terminal peptide (hGCTP) in pituitary cells from transgenic mice bearing a metallothionein-hGRH fusion gene after incubation with [35S]methionine. After separation on HPLC, 35S-labeled and unlabeled hGRH in medium and cell extracts were characterized by RIA and immunoprecipitation with antisera against hGRH and against hGCTP. After a 4-h pulse, unlabeled and [35S]pro-hGRH, hGRH(1-44)-NH2, and hGRH(1-40)-OH were identified in medium and cell extracts by both RIA and immunoprecipitation with anti-hGRH serum. In cell extracts, after a 0.5-h pulse, [35S]pro-hGRH and hGRH(1-44)-NH2 but not [35S]hGRH(1-40)-OH were detectable. After a 0.5-h chase, however, 35S-labeled hGRH(1-40)-OH, pro-hGRH, and [35S]hGRH(1-44)-NH2 were all measurable. After a 4-h chase, comparable levels of [35S]hGRH(1-44)-NH2 and hGRH(1-40)-OH were present, and very little intracellular 35S-pro-hGRH remained. A progressive decrease in the ratio of immunoprecipitable pro-hGRH to mature hGRH peptides and an increase in the ratio of hGRH(1-40)-OH to hGRH(1-44)-NH2 was observed in the two chase periods. In medium, [35S]hGRH(1-44)-NH2 was detectable at all times, whereas only minimal amounts of [35S]hGRH(1-40)-OH were present. Labeled and unlabeled pro-hGRH in cell extracts was also detected with anti-hGCTP serum, and another peak, which coeluted with synthetic hGCTP, was also identified. The low molar ratio of intracellular immunoreactive hGCTP to hGRH (less than 0.02) suggests a more rapid turnover rate of hGCTP than of hGRH. These results demonstrate the processing of hGRH prohormone to both mature forms of hGRH and provide evidence that hGRH(1-40)-OH is derived from hGRH(1-44)-NH2.

Growth hormone and insulin-like growth factor-I measurements in high growth (hg) mice

Effects of a recessive gene causing high growth (hg) were studied on two major components of the growth axis in mice. Plasma and pituitary levels of growth hormone and plasma levels of insulin-like growth factor I (IGF-I) were measured in three lines homozygous for hg, each compared with a control line of alike genetic background but wild type for the hg locus (Hg). Line Gh (hghg) and line GH (HgHg) are from a line which had undergone long-term selection for high postweaning weight gain; line Ch (hghg) and line CH (HgHg) were extracted from the second backcross of Gh to C57BL/6J; line L54 (hghg) was from the sixth backcross to C57BL/6J (B6) (HgHg). Pituitary GH levels and plasma IGF-I levels were measured in both sexes at 3, 4.5, 6 and 9 wk of age. Plasma growth hormone was measured in 8- to 12-wk-old males at hourly intervals from 08.00 to 17.00. Body weight in lines homozygous for hg at 6 and 9 wk of age was 10-30% greater than in control lines. The ontogeny of this increased growth depended on genetic background. Pituitary growth hormone content was 52% lower in the two hghg lines measured (lines Ch and Gh) than in control lines at 4.5, 6 and 9 wk. Plasma growth hormone levels were also much lower in hg mice, with values only 20-30% of those in their respective controls. hg lines showed consistently low plasma growth hormone levels throughout the 9 hr sampling period, while control lines expressed the characteristic pulsatile hormone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a defect in growth hormone-releasing factor signal transduction in the dwarf (dw/dw) rat pituitary

Dwarf (dw/dw) rats exhibit a 40% reduction in body growth, isolated GH deficiency (less than 5% of normal pituitary content), and a decreased number of pituitary somatotrophs (15-20% of normal). Since GH-releasing factor (GRF) stimulates GH synthesis and secretion and somatotroph proliferation, and its effects are probably mediated by cAMP, we have assessed GH secretion and cAMP production in dw rat pituitaries in response to various GH secretagogues. Dispersed pituitary cells from dw rats were less sensitive (2.5-fold) to stimulation of GH secretion by GRF and showed a 25% reduction in the maximal GH response even after normalization of their reduced GH content. Intracellular cAMP was elevated 63-fold over basal levels in normal cells after 4 h in response to maximal GRF stimulation, but only 1.9-fold in dw cells, and even larger differences between the groups were found at earlier time points. The GH responses of dw cells to exogenous cAMP, however, were indistinguishable from normal. Forskolin, a direct stimulator of adenylate cyclase, elicited comparable maximal GH and cAMP responses, but an increased ED50, in dw cells. Activation of GS alpha by cholera toxin showed an increased ED50 and reduced GH and cAMP responses in dw cells, and marked decreases in these responses were observed in response to prostaglandin E1. Phorbol ester stimulation resulted in a reduced maximal GH response in dw cells without a change in sensitivity. These results provide evidence for a defect in the GRF signal transduction pathway associated with a decreased ability of GS alpha to stimulate adenylate cyclase in dw rat somatotrophs that may be causally linked to their GH deficiency.

Synthesis and biological evaluation of mouse growth hormone-releasing factor

The recently described mouse growth hormone-releasing factor (mGRF) was synthesized by the solid phase procedure, purified by 2 stages of preparative high performance liquid chromatography and fully characterized. The biologic activity of the 42-amino acid peptide (H-His-Val-Asp-Ala-Ile-Phe- Thr-Thr-Asn-Tyr- Arg-Lys-Leu-Leu-Ser-Gln-Leu-Tyr-Ala-Arg-Lys-Val-Ile-Gln-Asp-Ile-Met-Asn- Lys- Gln-Gly-Glu-Arg-Ile- Gln-Glu-Gln-Arg-Ala-Arg-Leu-Ser-OH) was assessed in primary cultures of both mouse and rat anterior pituitary cells and compared to synthetic rat (rGRF) and human (hGRF) growth hormone-releasing factors. mGRF was equipotent to rGRF in mouse somatotrophs but slightly less potent in rat somatotrophs, while hGRF was 3-5 times less potent in both rodent species.

Tissue-specific transcription initiation and effects of growth hormone (GH) deficiency on the regulation of mouse and rat GH-releasing hormone gene in hypothalamus and placenta

Hypothalamic GRH gene expression has been shown to be negatively regulated by GH in both rat and mouse. The recent reports of different 5' untranslated sequences in mouse GRH cDNA from hypothalamus and placenta have raised the possibility of tissue-specific regulation of the GRH gene. To provide support for this possibility, we have studied rodent models with GH deficiency due to genetic defects in the pituitary. Complementary DNA probes for the hypothalamic and placental 5' regions were used to determine the tissue specificity of each mRNA. Although the hypothalamic form of GRH mRNA was detected in placenta, it constituted less than 0.7% of total placental GRH mRNA. A placental 5' probe (based on the previously reported sequence) hybridized only with a larger mRNA species and was not tissue specific, indicating that it was not related to GRH and was derived possibly from a cloning artifact. The correct 5' sequence of mouse placental GRH cDNA was determined and shown to be distinct from both that previously reported and the hypothalamic sequence. Although the placental form of GRH mRNA was detected in hypothalamus using the polymerase chain reaction, its levels were undetectable by Northern blotting. The 5' end of rat placental GRH cDNA was similarly sequenced and shown to exhibit no homology with the rat 5' hypothalamic sequence, but a high degree of homology with the corresponding mouse placental sequence. In GH-deficient dwarf (dw/dw) rats, hypothalamic GRH mRNA levels were significantly increased above control levels in both females and males, and pregnancy did not alter the levels in either (dw) or control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue distribution and molecular heterogeneity of human growth hormone-releasing factor in the transgenic mouse

Transgenic mice expressing the human growth hormone-releasing factor (hGRF) gene linked to the metallothionein promoter exhibit high circulating levels of hGRF and GH and increased growth. We have described the distribution of GRF immunoreactivity (GRF-IR) in various tissues and characterized its molecular heterogeneity using gel filtration and high performance liquid chromatography (HPLC) and two separate RIAs that recognized mid-molecule and carboxyl-terminal epitopes of hGRF. The highest levels of GRF-IR were in the pituitary, followed by the pancreas. Intermediate levels were present in hypothalamus and liver, and lower levels in visceral organs, heart, and gonads. The pituitary and brain revealed evidence of the two mature hormone forms [hGRF(1-44)-NH2 and hGRF(1-40)-OH and in addition a more hydrophobic form that is believed to represent the hGRF precursor (proGRF) on the basis of its estimated molecular size (approximately 9,000) and selective recognition by the mid-molecule RIA. The profiles of GRF in pancreas and gut were similar except that only minimal amounts of hGRF(1-40)-OH were detected. In contrast, neither mature hormonal form was present in the liver and plasma contained primarily hGRF(3-44)-NH2, the major circulating metabolite of hGRF. The results provide evidence for variable processing of the hGRF precursor that is tissue specific and indicate that several extrahypothalamic tissues possess the necessary complement of enzymes to generate the mature hormonal forms.

Somatostatin secretion and action in the regulation of growth hormone secretion

The inhibition of growth hormone (GH) secretion by the hypothalamic peptide, somatostatin, is mediated by two critical factors: the concentration of the peptide in hypothalamic portal plasma and the number of somatostatin (SRIF) receptors on the somatotroph. The secretory patterns of SRIF and GH-releasing hormone (GRH) in portal blood of unanesthetized sheep is pulsatile and a close relationship of GRH pulses to those of GH secretion was documented, while those of SRIF appear to have more of a modulatory role on the responses to GRH. Peripheral infusion of SRIF at a rate to provide concentrations comparable to those in the portal system leads to a desensitization of SRIF effects on the somatotroph, likely mediated by down-regulation of SRIF receptors. These effects are believed to modulate the GH responses to GRH secretion in the generation of pulsatile GH secretion.

Growth hormone and prolactin secretion in cultured somatomammotroph cells

A somatomammotropic cell line (P0) derived from adult rat pituitaries has been maintained in culture for 2 yr. Secretion of GH and PRL by this cell line has been studied in response to hypophysiotropic peptides known to affect the release of both hormones as well as agents that affect second messenger systems in an attempt to characterize the stimulus-secretion mechanisms used by these cells. GH and PRL release during short term (4 h) incubations of P0 cells and primary cultures of dispersed rat pituitary cells was initially measured in response to GRF, TRH, vasoactive intestinal peptide (VIP), and SRIF. In P0 cells, the minimal effective dose of each of the hypophysiotropic peptides was comparable with respect to GH and PRL secretion. The effects of TRH and VIP were similar to those in freshly dispersed cells with respect to PRL release, whereas those of GRF and SRIF were less potent with respect to GH release. The stimulation of GH and PRL release in P0 cells by adenylate cyclase-related agents ((Bu)2 cAMP and forskolin) was comparable to that for GH secretion in mature somatotrophs but much greater than that of PRL release in mature lactotrophs. Stimulation of GH and PRL release in P0 cells by protein kinase C-related agents (diacylglycerol and phorbol ester) was also similar to that observed for GH release from mature pituitary cells, whereas minimal or undetectable effects were observed on PRL release from mature cells. The results indicate that the P0 somatomammotropic cell line possesses receptors, second messenger systems, and secretory characteristics of both somatotrophs and lactotrophs, although where differences exist, there is more resemblance to somatotrophs. They also demonstrate that the responses to each of the agents studied are bihormonal and appear to be regulated by a common mechanism.

Measurement of growth hormone-releasing hormone and somatostatin in hypothalamic-portal plasma of unanesthetized sheep. Spontaneous secretion and response to insulin-induced hypoglycemia

To elucidate the role of growth hormone (GH)-releasing hormone (GRH) and somatostatin (SRIH) in the regulation of the growth hormone (GH) secretory pattern, we collected portal blood from five unanesthetized ovariectomized ewes for repeated measurements of GRH and SRIH simultaneous with those of peripheral GH. Hormones were measured at 10-min intervals for 5.5 h and their interrelationships analyzed. Mean portal GRH was 20.4 +/- 6.7 (SD) pg/ml and the estimated overall secretion rate was 13 pg/min. GRH secretion was pulsatile with peaks of 25-40 pg/ml and a mean pulse interval of 71 min. Mean portal SRIH was 72 +/- 33 pg/ml and the estimated overall secretion rate was 32 pg/min. SRIH secretion was also pulsatile with peaks of 65-160 pg/ml and a mean pulse interval of 54 min. The GH pulse interval was 62 min. A significant association was present between GRH and GH secretory peaks though not between GRH and SRIH or SRIH and GH. Insulin hypoglycemia resulted in a rapid and brief stimulation of SRIH secretion followed by a decline in GH levels. No effect was observed on GRH secretion until 90 min, when a slight increase occurred. The results suggest (a) the presence of an independent neural rhythmicity of GRH and SRIH secretion with a primary role of GRH in determining pulsatile GRH secretion, and (b) that the inhibitory effects of insulin hypoglycemia on GH in this species are attributable to a combination of enhanced SRIH secretion and possibly other factors, though without significant inhibition of GRH.

Effects of thyroid hormone deficiency and replacement on rat hypothalamic growth hormone (GH)-releasing hormone gene expression in vivo are mediated by GH

The role of thyroid hormone and GH in the regulation of hypothalamic GH-releasing hormone (GRH) gene expression in the rat was examined after the induction of thyroid hormone deficiency by thyroidectomy. Thyroidectomy resulted in a time-dependent decrease in hypothalamic GRH content, which was significant by 2 weeks postoperatively, and a reduction in pituitary GH content to 1% of the control level by 4 weeks. In contrast, GRH secretion by incubated hypothalami under both basal and K(+)-stimulated conditions was increased after thyroidectomy. Hypothalamic GRH mRNA levels also exhibited a time-dependent increase, which was significant at 1 week and maximal by 2 weeks after thyroidectomy. Administration of antirat GH serum to thyroidectomized rats resulted in a further increase in GRH mRNA levels. T4 treatment of thyroidectomized rats for 5 days, which also partially restored pituitary GH content, lowered the elevated GRH mRNA levels. However, comparable effects on GRH mRNA levels were observed by rat GH treatment alone. These results suggest that the changes in hypothalamic GRH gene expression after thyroidectomy in the rat are due to the GH deficiency caused by thyroidectomy, rather than a direct effect of thyroid hormone on the hypothalamus, since the changes were reversible by GH alone despite persistent thyroid hormone deficiency. In addition, they further support the role of GH as a physiological negative feedback regulator of GRH gene expression.

Cloning and characterization of mouse growth hormone-releasing hormone (GRH) complementary DNA: increased GRH messenger RNA levels in the growth hormone-deficient lit/lit mouse

We have isolated and cloned the full length cDNA for mouse GH-releasing hormone (mGRH) from mouse hypothalamus using a recently described strategy involving the polymerase chain reaction technique (PCR). Degenerate oligonucleotide primers were selected based on short (six amino acids) conserved regions in the human and rat GRH peptides that would recognize DNA sequences encoding similar amino acids regardless of codon usage. Primer-extended cDNA was amplified by PCR on cDNA templates prepared by reverse transcribing total mouse hypothalamic RNA. After cloning and sequencing the initial product, the 3' and 5' ends of mGRH were generated using a separate PCR strategy (RACE protocol). The mGRH cDNA encodes a 103-amino acid reading frame, structurally similar to the human and rat GRH genes, containing a signal sequence, a 42-residue GRH peptide, and a 31-residue C-terminal region. Although the structures of mouse and rat GRH are highly conserved in the signal peptide and C-terminal region, there is considerable diversity in the GRH region, which exhibits nearly comparable homology with the rat (68%) and human (62%) structures. Differences between mouse and rat GRH were also found in the amino acid cleavage sites at the 5' and 3' ends of the mature peptide and at the polyadenylation signal.(ABSTRACT TRUNCATED AT 250 WORDS)

Dipeptidylpeptidase IV and trypsin-like enzymatic degradation of human growth hormone-releasing hormone in plasma

The plasma enzyme responsible for primary proteolytic cleavage of growth hormone-releasing hormone (GRH) at the 2-3 amino acid bond was characterized. Native GRH[GRH(1-44)-NH2 and GRH(1-40)-OH], and COOH-terminally shortened fragments [GRH(1-32)-NH2 and GRH(1-29)-NH2] were rapidly cleaved, while GRH(2-32)-NH2 was not degraded at this site. Moreover, degradation to GRH(3-44)-NH2 was unaffected by an aminopeptidase inhibitor, indicating that this metabolite was generated from a single step cleavage by a dipeptidylpeptidase (DPP) rather than sequential aminopeptidase cleavages. Conversion to GRH(3-44)-NH2 was blocked by diprotin A, a DPP type IV (DPP IV) competitive inhibitor. D-Amino acid substitution at either position 1 or 2 also prevented hydrolysis, characteristic of DPP IV. Analysis of endogenous plasma GRH immunoreactivity from a human GRH transgenic pig revealed that the major peak coeluted with GRH(3-44)-NH2. Native GRH exhibited trypsin-like degradation at the 11-12 position but cleavage at the 12-13 site occurred only with GRH(1-32)-NH2 and GRH(1-29)-NH2. Formation of these metabolites was independent of prior DPP IV hydrolysis but was greatly reduced by trypsin inhibitors. Evaluation of plasma stability of potential GRH super analogues, designed to resist degradation by these enzymes, confirmed that GRH degradation in plasma occurs primarily by DPP IV, and to a lesser extent by trypsin-like enzyme(s).

Regulation of growth hormone-releasing hormone gene expression and biosynthesis

Growth hormone-releasing hormone (GRH) was initially isolated, characterized, sequenced, and cloned from human tumors and subsequently from the hypothalamus of humans and other animal species. Extensive structure-function studies have indicated the amino terminus to be most important for its biologic action, and the primary mechanism of its bioinactivation occurs by cleavage of an amino terminal dipeptide. The GRH gene is expressed primarily in the hypothalamic arcuate nucleus but also in the placenta. Expression of the GRH gene is regulated by growth hormone in a classical feedback manner, with hypophysectomy leading to increased expression that is reversed by growth hormone treatment. GRH gene overexpression in transgenic mice leads to a syndrome similar to that of ectopic GRH secretion with massive pituitary hyperplasia and markedly enhanced growth. The transgenic mouse has been used for studies of GRH biosynthesis and provides a suitable model for the study of precursor processing to the mature hormone.

Comparison of the sensitivity of growth hormone secretion to somatostatin in vivo and in vitro in acromegaly

Somatostatin (SRIH) sensitivity in acromegaly was evaluated in vivo by comparing the inhibition of GHRH (1 microgram/kg, iv)-stimulated GH secretion in eight acromegalic and six normal subjects. A SRIH infusion (50 micrograms/h) that inhibited the mean plasma GH response to GHRH by 74 +/- 5% (+/- SE) in normal subjects had no significant effect in the acromegalic patients. However, when two acromegalic patients in whom SRIH had no suppressive effect were excluded from the analysis, the effect of SRIH in the other six (82 +/- 7%) was comparable to that in the normal subjects. Within the acromegalic group, the percent suppression of basal and GHRH-stimulated GH secretion was inversely correlated with both basal plasma GH (r = -0.751; P = 0.03 and r = -0.727; P = 0.04, respectively) and insulin-like growth factor I (r = -0.800; P = 0.02 and r = -0.727; P = 0.04, respectively) concentrations. The in vitro sensitivity to SRIH was studied in pituitary adenomas from five of the acromegalic patients in 3- to 4-day monolayer cultures of dispersed cells. The SRIH IC50 values were lowest in the tumors (8.6-44 pmol/L) from the three patients who had in vivo SRIH sensitivity (suppression of GHRH-stimulated GH secretion) comparable to that in the normal subjects. The IC50 values were higher in the tumors (150 and 21,000 pmol/L) from the two patients that were least responsive to SRIH in vivo. These results indicate that there is considerable variability of SRIH sensitivity in patients with acromegaly. Although the role of this defect in the pathogenesis of acromegaly is uncertain, it may be an important determinant in the degree of elevation of plasma GH levels.

Inhibitory effect of hypothalamic medial preoptic area somatostatin on growth hormone-releasing factor in the rat

Possible inhibitory effects of somatostatin (SRIF) on GRF were studied by assessing spontaneous GH secretion and GRF content and release in adult male rats depleted of hypothalamic SRIF by anterolateral hypothalamic deafferentation (AHD) or electrolytic lesions in the medial preoptic area (MPO). Plasma GH levels were measured 7 days postoperatively every 20 min in conscious animals with indwelling iv cannulae. Median eminence SRIF was markedly reduced 8 days postoperatively in both AHD and MPO rats, as determined by immunohistochemistry and RIA (P less than 0.01). Although GRF immunoreactivity in the median eminence of AHD and MPO animals appeared well preserved immunocytochemically, hypothalamic GRF content by RIA was significantly decreased at 8 days (P less than 0.01). Spontaneous GH secretion was pulsatile in sham-operated animals. In contrast, basal GH levels in AHD and MPO animals were markedly elevated (P less than 0.01), and secretory pulses were absent. Intravenous injection of specific anti-GRF serum into MPO animals decreased the elevated plasma GH levels (P less than 0.01), indicating increased hypothalamic GRF secretion. GRF release from hypothalamic median eminence-arcuate nucleus complexes in vitro was significantly greater in AHD and MPO animals than in control animals 4 and 8 days postoperatively in response to 30 mM K+ (P less than 0.01), but not under basal conditions. These results suggest that hypothalamic medial preoptic area somatostatinergic neurons play a tonic inhibitory role in the regulation of GRF release and that GH hypersecretion observed after MPO and AHD is attributable to changes in both SRIF and GRF.

Feedback regulation of growth hormone (GH)-releasing hormone gene expression by GH in rat hypothalamus

The role of the pituitary, and in particular, of GH in GH-releasing hormone (GRH) gene expression was studied in hypophysectomized rats with and without GH treatment. Hypothalamic GRH mRNA was 6-fold greater in hypophysectomized than in control rats. Increased levels of GRH mRNA were observed at 3 days and the maximal increase was noted at 7 days, postoperatively. Administration of GH to hypox rats partially reversed the increase in GRH mRNA, suggesting a negative feedback regulation by GH of GRH gene activity at the transcript accumulation level. The overall regulation of GRH gene expression, however, appears more complex since GRH mRNA levels and GRH content exhibited discordant changes after both hypophysectomy and GH treatment, suggesting that factors other than GH are required for efficient translation of GRH mRNA.

Differential effects of somatostatin (SRIH) and a SRIH analog, SMS 201-995, on the secretion of growth hormone and thyroid-stimulating hormone in man

We compared the ability of SRIH and SRIH analog, SMS 201-995 (SMS), to inhibit stimulated GH and TSH secretion in men who received 120-min iv infusions of saline, SRIH (5, 50, and 500 micrograms/h), and SMS (3, 30, and 300 ng/kg.h) together with a bolus iv injection of GHRH (1 microgram/kg) and TRH (500 micrograms). Integrated GH secretion during the 60 min after GHRH plus TRH injection was decreased compared to that after saline by (mean +/- SE) 32 +/- 14% (P = 0.059), 78 +/- 5% (P less than 0.001), and 88 +/- 3% (P less than 0.001) during the 5, 50, and 500 micrograms/h SRIH infusions, and by 13 +/- 7% (P = NS), 50 +/- 15% (P less than 0.05), and 80 +/- 6% (P less than 0.001) during the 3, 30, and 300 ng/kg.h SMS infusions. In contrast, integrated TSH secretion was unaltered during the 5 micrograms/h SRIH and 3 ng/kg.h SMS infusions; it decreased by only 43 +/- 5% (P less than 0.001) and 66 +/- 4% (P less than 0.001) during the 50 and 500 micrograms/h SRIH infusions and by 33 +/- 8% (P less than 0.05) and 50 +/- 3% (P less than 0.001) during the 30 and 300 ng/kg.h SMS infusions. Analysis of the dose-response curves indicated approximately 10- and 5-fold greater potencies of SRIH and SMS, respectively, in inhibiting GH as compared to TSH secretion. These results quantify the effect of SRIH as an inhibitor of GH secretion and suggest that if SRIH has a physiological role in the inhibition of TSH secretion in man, it is limited to conditions associated with marked suppression of GH.

Intravenous medroxalol stimulates prolactin secretion in normal and hypertensive subjects

This study evaluated the effects of medroxalol on prolactin secretion. Twelve normal subjects received medroxalol, 1 mg/kg, intravenously and on a separate occasion, 5% dextrose in water. Integrated prolactin secretion during the 3 hours after medroxalol injection was significantly increased as compared with dextrose (P less than 0.001). Intravenous administration of medroxalol, 2 mg/kg, to 10 hypertensive subjects resulted in significant elevation of mean prolactin levels above basal levels at all time intervals measured from 30 to 240 minutes after injection. Oral medroxalol administration to 11 hypertensive subjects for up to 15 months did not alter mean prolactin levels. Medroxalol neither stimulated prolactin release nor decreased dopamine suppression of prolactin release from pituitary cell cultures. In conclusion, intravenous medroxalol stimulates prolactin secretion in both normal and hypertensive subjects. This effect is not likely mediated by a direct action of the drug on the pituitary but rather by an effect either within the central nervous system or of a drug metabolite.

Effect of hypophysectomy on hypothalamic growth hormone-releasing factor content and release in the rat

The possible regulation of hypothalamic GH-releasing factor (GRF) by GH was studied in untreated and GH-treated hypophysectomized rats by measurement of rat hypothalamic GRF content and release in vitro with a specific GRF RIA. Two weeks after hypophysectomy, animals not receiving hormone replacement showed a marked reduction in hypothalamic GRF content (to 28% of sham-operated control values; P less than 0.001). Replacement therapy with T4, cortisone, and testosterone for 9 days did not correct the decrease in hypothalamic GRF content in hypophysectomized rats, though the addition of GH therapy partially restored GRF levels (to 55% of control values; P less than 0.001). GRF release from incubated mediobasal hypothalamic fragments of hypophysectomized rats was decreased both basally and in response to 30 mM K+. This defect was completely corrected by prior GH treatment. The results suggest an impairment of GRF synthesis and release in the presence of GH deficiency.

Rapid enzymatic degradation of growth hormone-releasing hormone by plasma in vitro and in vivo to a biologically inactive product cleaved at the NH2 terminus

The effect of plasma on degradation of human growth hormone-releasing hormone (GRH) was examined in vitro and in vivo using high performance liquid chromatography (HPLC), radioimmunoassay (RIA), and bioassay. When GRH(1-44)-NH2 was incubated with human plasma, the t1/2 of total GRH immunoreactivity was 63 min (RIA). However, HPLC revealed a more rapid disappearance (t1/2, 17 min) of GRH(1-44)-NH2 that was associated with the appearance of a less hydrophobic but relatively stable peptide that was fully immunoreactive. Sequence analysis indicated its structure to be GRH(3-44)-NH2. Identity was also confirmed by co-elution of purified and synthetic peptides on HPLC. Biologic activity of GRH(3-44)-NH2 was less than 10(-3) that of GRH(1-44)-NH2. After intravenous injection of GRH(1-44)-NH2 in normal subjects, a plasma immunoreactive peak with HPLC retention comparable to GRH(3-44)-NH2 was detected within 1 min and the t1/2 of GRH(1-44)-NH2 (HPLC) was 6.8 min. The results provide evidence for GRH inactivation by a plasma dipeptidylaminopeptidase that could limit its effect on the pituitary.

Decreased hypothalamic growth hormone-releasing hormone content and pituitary responsiveness in hypothyroidism

The effects of thyroidectomy (Tx) and thyroxine replacement (T4Rx) on pituitary growth hormone (GH) secretion and hypothalamic GH-releasing hormone (GRH) concentration were compared to define the mechanism of hypothyroid-associated GH deficiency. Thyroidectomized rats exhibited a complete loss of pulsatile GH secretion with extensive reduction in GRH responsiveness and pituitary GH content. Cultured pituitary cells from Tx rats exhibited reduced GRH sensitivity, maximal GH responsiveness, and intracellular cyclic AMP accumulation to GRH, while somatostatin (SRIF) suppressive effects on GH secretion were increased. Hypothalamic GRH content was also markedly reduced. T4Rx completely restored hypothalamic GRH content and spontaneous GH secretion despite only partial recovery of pituitary GH content, GRH and SRIF sensitivity, and intracellular cyclic AMP response to GRH. The results indicate multiple effects of hypothyroidism on GH secretion and suggest that a critical role of T4 in maintaining normal GH secretion, in addition to restoring GH synthesis, is related to its effect on hypothalamic GRH.

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.

Identification of a putative GH-releasing factor (GRF) batch as predominantly ovine CRF with a small quantity of human GRF

The existence of discordant results regarding the effects of intracerebroventricular (icv) administration of GRF on GH secretion prompted a reexamination of the central actions of GRF and a detailed chemical characterization of the peptide designated as hpGRF-44-NH2. The icv injection of 10 micrograms rat (r) GRF to freely-moving rats caused an acute stimulation of GH release, whereas 10 micrograms of the putative hpGRF peptide icv continued to suppress spontaneous GH secretion. Through a series of biochemical and immunologic studies we demonstrate that peptide hpGRF-44-NH2, code number 92-81-5G-41-47, is predominantly ovine (o) CRF and also contains a small amount (3-5%) human (h) GRF-44-NH2. We conclude that the major effect of icv administered GRF, at high doses, is to stimulate GH release and that the central actions previously attributed to the hpGRF peptide are, in fact, due to the oCRF component of this compound.

Effects of intravenous, subcutaneous, and intranasal administration of growth hormone (GH)-releasing hormone-40 on serum GH concentrations in normal men

In addition to stimulating GH release in normal subjects, GH-releasing hormone-40 (GHRH-40) stimulates GH secretion in some adults and children with GH deficiency. Recognizing that GHRH-40 may have potential as a therapeutic agent for the treatment of GH deficiency, we examined the effects of iv, sc, and intranasal (in) GHRH-40 administration on GH secretion and measured the plasma levels of immunoreactive GHRH achieved after the administration of the peptide via these different routes. Normal men were given vehicle or GHRH-40 iv (0.003, 0.01, 0.03, and 0.1 micrograms/kg; n = 10), sc (1, 3.3, and 10 micrograms/kg; n = 8), or in (3, 10, 30, and 100 micrograms/kg; n = 5). No subject had any symptoms after administration of vehicle or GHRH-40. During the 2-h period after iv administration of GHRH-40, the maximal increment in serum GH levels above basal (nanograms per ml; mean +/- SD) after the 0.1 micrograms/kg dose was 15.5 +/- 10.4 compared to 2.4 +/- 4.1 after vehicle (P = 0.0017). During the 3-h period after sc administration, when compared to the maximal increment in serum GH above basal after vehicle alone (10.2 +/- 12.9), the maximal increments above basal in serum GH were increased after both the 3.3 micrograms/kg (26.2 +/- 23.1; P = 0.022) and 10 micrograms/kg (63.6 +/- 53.5; P = 0.0003) doses. During the 3-h period after in administration, when compared to the maximal increment in serum GH above basal after vehicle alone (2.8 +/- 6.4), the maximal increments above basal in GH were higher after both the 30 micrograms/kg (18.5 +/- 10.4; P = 0.0053) and 100 micrograms/kg (21.7 +/- 8.1; P = 0.0028) doses. In addition, significant dose-response relationships were documented between the maximal increments above basal in serum GH and GHRH-40 administered by all routes. The mean (+/- SEM) peak plasma level of IR-GHRH (nanograms per ml) achieved after administration of 10 micrograms/kg GHRH-40, iv, as reported previously (66.6 +/- 17.6), was approximately 60- and 500-fold higher than the mean levels in the current study after administration of the same dose sc (1.11 +/- 0.39) or in (0.14 +/- 0.02), respectively. In summary, although GHRH-40 stimulates GH release when administered iv, sc, or in, significantly higher doses were required using the sc and in routes to achieve responses comparable to those obtained with iv administration.

An in vitro model for studies of intercellular communication in cultured rat anterior pituitary cells

The formation of intimate associations among different hormone-secreting cells within the rat adenohypophysis may serve as a possible site for physiologic regulation. In this report we describe a high density plating method which enables us to study cell-to-cell interactions within anterior pituitary cell cultures. Trypsin-dispersed pituitary cell suspensions attach rapidly (within 6 hr) and quantitatively (95-97%) to glass or plastic surfaces when plated in medium containing microM calcium concentrations (pH 7.6-7.8). Freshly plated cell suspensions obtained from female pituitary glands contained subpopulations of mammotrophs 49.3%, somatotrophs 30.3%, gonadotrophs 12.6%, corticotrophs 3.4% and thyrotrophs 1.5%. Epithelial cell colonies were formed during a 3-day culture period as the cells flattened and re-established contacts with neighboring cells. Freeze-fracture electron microscopic analysis of these colonies produced morphological evidence for direct intercellular contacts among the hormone-secreting cells. Large areas of tight junctions and small gap junctions were identified on the membranes of the epithelial cells within these colonies. Cells which contained tight junctions usually contained microvilli and morphological signs of active hormone secretion. Small junctional plaques containing tightly packed intramembrane particles were also occasionally found on the membranes of cells which were actively secreting pituitary hormones. The high density plating procedure which is described in this report provides greater opportunity for cell-cell interaction and thus may prove to be a useful model for evaluating the role of intercellular communication within this tissue.

Fluorometric quantification of DNA in cells and tissue

The validation of a simple and rapid DNA solubilization procedure is described. Quantitative extraction of intact, polymerized DNA was achieved by cell lysis or tissue homogenization in an ammonium hydroxide-Triton X-100 solution. The solubilization procedure inactivates endogenous DNAase and increases the fluorescence-enhancement activity of the extracted DNA, thereby eliminating the need for enzyme treatment or exposure to high salt solutions. The extracts can be utilized directly in a sensitive fluorescence-enhancement assay with bisbenzimidazole (Hoechst 33258) reagent. Estimates of DNA cell content were unaffected by the number of cells lysed or the volume of lysate employed in the assay. In all cases, the solubilized DNA estimates were linear and parallel to the bovine DNA standard. The optimum range for estimation of DNA in this assay is 5-150 ng. In addition, estimates of DNA obtained with this method and the standard diphenylamine assay were in excellent agreement. This simple, one-step DNA extraction procedure can be utilized in conjunction with Hoechst reagent to obtain quantitative estimates of DNA levels in cell or tissue extracts.