Somatomedin-C mediates growth hormone negative feedback by effects on both the hypothalamus and the pituitary.

Somatomedin-C stimulates somatostatin release to a maximum of 390 percent of basal release during short-term (20-minute) incubation of rat hypothalamus. It has no effect on basal or stimulated growth hormone release from primary cultures of rat adenohypophyseal cells during a 4-hour incubation, but inhibits stimulated release by more that 90 percent after 24 hours. These findings suggest that somatomedin-C participates in the growth hormone negative feedback loop with an immediate effect on hypothalamic somatostatin and a delayed effect on the anterior pituitary.

Release of somatostatin-like immunoreactivity from incubated rat hypothalamus and cerebral cortex. Effects of glucose and glucoregulatory hormones.

Somatostatin (SRIF) is localized in the hypothalamus, extrahypothalamic brain, and throughout the gastrointestinal tract. Release of gastrointestinal SRIF-like immunoreactivity (SRIF-LI) is under nutrient regulation but the effect of nutrients on neural SRIF-LI is unknown. The present studies examined the effects of glucose uptake and metabolism and hormones influencing glucose disposition on SRIF-LI release from medial basal hypothalamus (MBH) and cerebral cortex (Cx) incubated in Krebs-Ringer bicarbonate containing bacitracin. After a preincubation to achieve stable secretion, tissues were incubated for 20 min in 14 mM glucose (basal) and then, for 20 min in fresh medium with test materials. MBH SRIF-LI release was inversely related to medium glucose concentration with release in the absence of glucose (235+/-42 pg/MBH per 20 min) more than five times that in the presence of 25 mM glucose (46+/-4 pg/20 min). In the presence of 14 mM glucose MBH SRIF-LI release was stimulated above basal by agents interfering with glucose uptake including 3-O-methyl-d-glucose (42 mM; 70+/-5 vs. 42+/-3 pg/20 min, P < 0.05), phlorizin (50 mM; 351+/-63 vs. 29+/-2 pg/20 min, P < 0.001) or cytochalasin B (20 muM; 110+/-7 vs. 22+/-2 pg/20 min, P < 0.001). Inhibition of glucose metabolism by 2-deoxy-d-glucose resulted in dose-related stimulation of MBH SRIF-LI release (maximal at 28 mM; 201+/-28 pg/20 min vs. 32+/-4 pg/20 min, P < 0.001). Viability of MBH was unimpaired by incubation in the absence of glucose or following exposure to 2-deoxy-d-glucose as determined by retention of SRIF-LI responsiveness to stimulation by potassium (60 mM) or neurotensin (5 muM). In contrast, Cx SRIF-LI release was slightly inhibited by decreases in medium glucose and unaffected by inhibition of glucose uptake or metabolism. These results provide evidence for nutrient regulation of MBH but not Cx SRIF-LI release and may explain inhibition of growth hormone seen in the rat in response to hypoglycemia. Insulin (10 nM-1 muM) stimulated MBH but not Cx SRIF-LI release while glucagon was without effect. Our previous demonstration that MBH SRIF-LI release was stimulated by somatomedin-C, but not insulin at physiologic concentrations, is consistent with an action of insulin through the somatomedin-C receptor at the doses studied. Our studies indicate a regional specificity for the control of SRIF secretion within the brain and suggests the possibility of a role for hypothalamic SRIF in metabolic regulation.

Somatostatin-like immunoreactivity in rat blood. Characterization, regional differences, and responses to oral and intravenous glucose.

Somatostatin-like immunoreactivity (SLI) has been demonstrated by radioimmunoassay (RIA) in rat serum using an antiserum specific for somatostatin and cross-reacting maximally with the biologically important area on the peptide. The RIA has a sensitivity of 35 pg/ml. SLI dilutes in parallel with synthetic somatostatin standard in the RIA and shows characteristics similar to synthetic somatostatin on Sephadex G-25 (f) gel chromatography eluting largely as a single peak with 1 M acetic acid. Significant regional differences in serum SLI are present. A positive gradient was found in paired samples from aorta (mean+/-SEM, 0.304+/-0.024 ng/ml) and portal vein (0.495+/-0.047 ng/ml) consistent with the known presence of somatostatin in gut and pancreas, and a negative gradient was noted between paired samples from portal vein (0.523+/-0.076 ng/ml) and hepatic vein (0.290+/-0.048 ng/ml) indicating hepatic clearance. No significant differences were demonstrated between aorta and confluence of cerebral venous sinuses or between aorta and inferior vena cava (IVC). After intragastric glucose, a significant and marked elevation of portal SLI was observed, maximal at 5 min (0.416+/-0.137 vs. 1.55+/-0.30 ng/ml at 5 min). A significant biphasic elevation of portal SLI also occurred after intravenous glucose. After both routes of glucose administration, the patterns of portal SLI followed closely those of portal glucose and insulin. By contrast, IVC SLI failed to reflect these changes.Thus, SLI in the rat shows chromatographic similarity with synthetic somatostatin. Regional differences in serum levels are marked; the highest concentrations being found in the portal venous effluent of pancreas and gut. Furthermore, glucose causes elevation of portal SLI in a pattern similar to portal insulin and glucose and without concomitant elevation in IVC. This differential elevation of SLI after glucose is consistent with a hormonal action within the portal system as a direct effect of somatostatin on the liver has previously been demonstrated. In addition, the liver is important in the clearance of portal SLI, possibly to prevent extraportal effects in response to gut and pancreatic stimulation. Finally, it is clear that regional sampling of serum for SLI measurement may be critical in the investigation of the putative physiological roles for somatostatin.

Partial purification and characterization of a peptide with growth hormone-releasing activity from extrapituitary tumors in patients with acromegaly.

Growth hormone (GH)-releasing activity has been detected in extracts of carcinoid and pancreatic islet tumors from three patients with GH-secreting pituitary tumors and acromegaly. Bioactivity was demonstrated in 2 N acetic acid extracts of the tumors using dispersed rat adenohypophyseal cells in primary monolayer culture and a rat anterior pituitary perifusion system. The GH-releasing effect was dose responsive and the greatest activity was present in the pancreatic islet tumor. Small amounts of activity were also found in two other tumors (carcinoid and small cell carcinoma of lung) unassociated with GH hypersecretion. Each of the tumors contained somatostatin-like immunoreactivity but the levels did not correlate with the net biologic expression of the tumor. Sephadex G-75 gel filtration indicated the GH-releasing activity to have an apparent molecular size of slightly greater than 6,000 daltons. The GH-releasing activity was adsorbed onto DEAE-cellulose at neutral pH and low ionic strength, from which it could be eluted by increasing ionic strength. The GH-releasing activity was further purified by high pressure liquid chromatography using an acetonitrile gradient on a cyanopropyl column to yield a preparation that was active at 40 ng protein/ml. Partially purified GH-releasing activity, from which most of the bioactive somatostatin had been removed, increased GH release by pituitary monolayer cultures to five times base line. Enzymatic hydrolysis studies revealed that the GH-releasing activity was resistant to carboxypeptidase, leucine-aminopeptidase, and pyroglutamate-amino-peptidase but was destroyed by trypsin and chymotrypsin, indicating that internal lysine and/or arginine and aromatic amino acid residues are required for biologic activity and that the NH2-terminus and CO9H-terminus are either blocked or not essential. The results provide an explanation for the presence of GH-secreting tumors in some patients with the multiple endocrine neoplasia syndrome, type I, and warrant the addition of GH-releasing activity to the growing list of hormones secreted by tumors of amine precursor uptake and decarboxylation cell types.

Hepatic and renal metabolism of somatostatin-like immunoreactivity. Simultaneous assessment in the dog.

The hepatic and renal metabolism of somatostatin-like immunoreactivity (SLI) was assessed simultaneously in an in vivo dog model. The hepatic extraction of this peptide was 29.4 +/- 2.3% and was similar for endogenous and infused exogenous SLI. The renal extraction was 62.3 +/- 5%. The renal clearance of SLI was significantly greater than that of inulin indicating that the peptide is handled by peritubular uptake from postglomerular blood in addition to glomerular filtration. In both organs SLI extraction was not saturable even at arterial concentrations in excess of 100 times physiological range. The overall metabolic clearance rate of SLI was 19.7 +/- 1.6 ml/kg per minute of which 32.7 +/- 4.6% was contributed by hepatic and 37 +/- 4.9% by renal uptake mechanisms. The plasma half disappearance time of exogenously infused SLI was 1.9 +/- 0.3 min. The studies indicate that in the dog, the liver and kidney are both major sites of SLI metabolism, together accounting for 70.0 +/- 8.7% of the metabolic clearance of the peptide.

Motivation to change in recent onset and long-standing bulimia nervosa: are there differences?

UNLABELLED: REASON FOR THE STUDY: Little is known about how motivation to change evolves over the course of an eating disorder. The present study compared 'stage of change' and motivation, confidence and readiness to change in two groups of patients with bulimia nervosa (BN), adolescents with a short duration of illness and adults with a long duration of illness. METHOD: Patients completed the Severity of eating disorder symptomatology scale, Hospital Anxiety and Depression Scale and measures of stage of change and motivation, readiness and confidence to change their bulimic symptomatology at pre-treatment. MAIN FINDINGS: Short- and long duration groups did not differ in illness severity, comorbidity, stage of change, motivation, readiness, and confidence to change. There were, however, some differences between groups in terms of the relationship between motivational measures, illness severity, duration and comorbidity. CONCLUSIONS: There seem to be more similarities than differences between adolescents with short duration of illness and those with well-established BN in terms of their motivation to change.

Evidence for multiple molecular weight forms of somatostatin-like material in Escherichia coli.

Extracts of Escherichia coli grown in defined medium contain somatostatin-related material (1-10 pg/g wet weight of cells). Preconditioned medium had no immunoactive somatostatin whereas, conditioned medium had 110-150 pg/l. Following purification of the extracted material on Sep-pak C18, Bio-Gel P-6 and HPLC, multiple molecular weight forms of somatostatin- (SRIF-) related material were identified. The material in one peak reacted in both the N-terminal and C-terminal SRIF immunoassay and coeluted on HPLC with SRIF-28, whereas that in a second peak eluted near SRIF-14 and was reactive only in the C-terminal SRIF assay. The two peaks are thus similar to SRIF-28 and SRIF-14 of vertebrates. These findings add support to the suggestion that vertebrate-type peptide hormones and neuropeptides have early evolutionary origins.

Classification and nomenclature of somatostatin receptors.

There is considerable controversy about the classification and nomenclature of somatostatin receptors. To date, five distinct receptor genes have been cloned and named chronologically according to their respective publication dates, but two were unfortunately given the same appellation (SSTR4). Consensually, a nomenclature for the recombinant receptors has been agreed according to IUPHAR guidelines (sst1, sst2, sst3, sst4, and sst5). However, a more informative classification is to be preferred for the future, employing all classification criteria in an integrated scheme. It is already apparent that the five recombinant receptors fall into two classes or groups, on the basis of not only structure but also pharmacological characteristics. One class (already referred to by some as SRIF1) appears to comprise sst2, sst3 and sst5 receptor subtypes. The other class (SRIF2) appears to comprise the other two recombinant receptor subtypes (sst1 and sst4). This promising approach is discussed but it is acknowledged that much more data from endogenous receptors in whole tissues are needed before further recommendations on somatostatin receptor nomenclature can be made.

Immunoreactive neurotensin in the pancreas of genetically obese and diabetic mice. A longitudinal study.

Immunoreactive neurotensin (IR-NT) content in 2 N acetic acid extracts of pancreas was measured in genetically diabetic (C57BL/KsJ db/db and ob/ob) and obese (C57BL/6J ob/ob and db/db) mice and normal littermate controls from 5 to 24 wk of age to determine the relationship of any changes to the development of metabolic abnormalities. Pancreatic IR-NT in obese mice showed no consistent change compared with lean littermate controls. In contrast, diabetic mice demonstrated an increase in pancreatic IR-NT that occurred at 6-8 wk of age, and maximal about the time of islet B-cell failure (8-10 wk), and persisted over the study period. Pancreatic IR-NT eluted in two peaks on reverse phase high-pressure liquid chromatography, one of which exhibited a retention time similar to that of synthetic NT. These findings suggest that pancreatic IR-NT concentration is regulated by insulin, with elevated levels occurring in association with insulin deficiency and its metabolic consequences but not with insulin resistance. Taken together with the previous demonstration that NT influences pancreatic islet hormone secretion, the present findings support a possible role of endogenous NT in islet hormone regulation.

Temporal relationship of tissue somatostatin-like immunoreactivity to metabolic changes in genetically obese and diabetic mice.

Somatostatin-like immunoreactivity (SRIF-LI) content in 2 N acetic acid extracts of hypothalamus, gastric antrum, and pancreas was measured in genetically obese (C57BL/6J ob/ob and db/db) and diabetic (C57BL/KsJ db/db and ob/ob) mice and normal littermate controls from 5 to 24 wk to determine the relationship of previously reported changes to the development of metabolic abnormalities. Hypothalamic SRIF-L concentration was similar in control, diabetic, and obese mice at all ages and increased progressively with age in all groups. Gastric antrum SRIF-LI was similar in all groups of mice at all ages. Obese mice gained weight progressively and showed moderate hyperglycemia and marked hyperinsulinemia from 5 wk of age. Pancreatic SRIF-LI content in obese (C57BL/6J) animals was similar to that in lean littermate controls, but pancreatic SRIF-LI concentration (expressed by weight or protein content) was decreased until 8 (6J ob/ob) and 10 (6J db/db) wk. Diabetic (C57BL/KsJ) mice showed a similar metabolic pattern until 10 wk with no change in pancreatic SRIF-LI content or concentration. Thereafter a progressive fall in serum insulin and a marked rise in serum glucose was associated with increasing pancreatic SRIF-LI content and concentration. These studies suggest that the genetically hyperphagic syndromes are unassociated with any change in hypothalamic or gastric SRIF-LI; that pancreatic SRIF-LI increases occur in response to, rather than as the cause of, relative hypoinsulinemia; and that the genetic background of the mice (KsJ or 6J) rather than the mutant gene (db or ob) determines the defect in carbohydrate metabolism and the pancreatic SRIF-LI response.

The characterization of somatostatin-like immunoreactivity in human serum.

We describe the characterization of somatostatin-like immunoreactivity (SRIF-LI) found by radioimmunoassay (RIA) to be present in normal human serum. Degradation by serum of 125I-Tyr1 SRIF in the assay, as assessed by chromatoelectrophoresis and immunoprecipitation, was overcome by using EDTA in the assay buffer and Trasylol in the blood samples. Serum samples thus obtained from 48 normal subjects revealed a bimodal distribution of SRIF-LI; 92 per cent (group 1) had a mean level of 0.274 +/- 0.009 ng. per milliliter. What was measured in these sera showed identity to synthetic SRIF on serial dilutions, Sephadex G-25 chromatography, and thin-layer chromatography, and it was shown to be immunoreactive by an antibody-Sepharose affinity system. Higher levels (1.0 +/- 0.041 ng. per milliliter) were found in 8 per cent of the sera; 50 per cent of this material behaved identically as serum SRIF-LI from group 1. The remainder proved to be heterogeneous, consisting of two peaks of large molecular weight, both of which shared immunologic identity with synthetic SRIF as shown by binding to the antibody-Sepharose affinity system. Their further nature is unknown.

Tissue and serum somatostatin-like immunoreactivity in fed, 15-h-fasted, and 72-h-fasted rats.

Somatostatin-like immunoreactivity (SLI) was measured in extracts of gastric antrum, colon, pancreas, and central nervous system, as well as in unextracted portal and inferior vena caval serum from fed, 15-h-fasted, and 72-h-fasted rats. No differences were found in SLI in the central nervous system of the three groups. However, striking variations were found in the gastrointestinal tract and pancreas; the antrum, colon, and pancreas of 15-h-fasted rats contained the least SLI, the content being significantly elevated in these three areas after feeding and after a 72-h fast. Portal serum levels were highest after feeding but lowest in 72-h-fasted rats, in spite of high intestinal and pancreatic SLI content in both. These tissue and serum differences suggest a physiologic role for SLI in nutrient homeostasis not only at tissue level, but also putatively as a hormone in the portal system.

Sorting of pancreatic islet cell subpopulations by light scattering using a fluorescence-activated cell sorter.

Methods have been developed for the preparation of suspensions of viable rat pancreatic islet cells and their analysis and sorting in the fluorescence-activated cell sorter (FACS III or IV). Histograms of cell number versus light scattering in a near forward angle (1-15 degrees) demonstrated that viable islet cells produce a broad peak that is distinctly separated from the peaks generated by exocrine cells, erythrocytes, and nonviable cells. Electron microscopic examination and radioimmunoassay of hormone content in fractions collected across the peak showed that glucagon-containing (A) cells scatter less intensely and are concentrated within the left side of the islet cell peak, while somatostatin-containing (D) cells are localized to the far right side, indicating a higher intrinsic light scattering property of the D-cells. The more abundant insulin-containing (B) cells define the center of the islet cell peak. Sodium dodecyl sulfate slab gel electrophoresis and radioautography of 35S-methionine labeled cellular proteins confirmed that sorted cells are viable. Cells from the far left region contained increased amounts of labeled 18 Kd proglucagon and its 13-Kd and 10-Kd conversion intermediates, while cells from the right side were relatively enriched in labeled 12.4 Kd prosomatostatin. These results demonstrate that intrinsic light scattering alone can be used to prepare A- or D-cell enriched fractions from islets for biochemical analysis.

Regulation of hypothalamic neuropeptide expression by peripheral metabolism.

The mechanisms of appetite and body-weight regulation by peripheral signals are highly complex in vertebrates and remain poorly understood. It is intuitively apparent that such regulation must involve interactions between peripheral metabolic status and the brain, but what are the signals recognized by the brain to initiate feeding? The hypothalamus has long been recognized as central in "recognition" of peripheral nutrient and metabolic signals (and, perhaps, body weight status) and in "regulation" of hunger and satiety responses and, therefore, is a logical site on which to focus research aimed at understanding interactions between and regulation of the periphery and central nervous system. Recent studies demonstrating modulation of hypothalamic neurotransmitter expression by peripheral metabolic status may yield insights into regulation of appetite and metabolism in obesity and aberrant metabolic homeostasis. This review concentrates on summarizing data regarding regulation of expression of neuropeptide Y and growth hormone-releasing hormone as model peptide systems for addressing questions relating peripheral metabolism and hypothalamic neuropeptide expression.

Differential effects of insulin- and proinsulin-induced hypoglycemia on pituitary hormone and catecholamine secretion.

The effects of insulin- and proinsulin-induced hypoglycemia on pituitary hormone and catecholamine secretion were compared in normal men to search for possible hypothalamic or pituitary inhibitory effects of proinsulin on glucocounterregulatory responses. When subjects received 0.1 U/kg i.v. human insulin and 25-38 micrograms/kg i.v. human proinsulin on separate occasions, plasma glucose decreased more rapidly after insulin, and the nadir was slightly lower, but integrated hypoglycemic responses were similar. Cortisol, growth hormone (GH), prolactin, epinephrine, and norepinephrine responses occurred more rapidly after insulin than after proinsulin. Peak and integrated cortisol, GH, and catecholamine responses to insulin and proinsulin were similar, but those of prolactin were reduced after proinsulin when compared with insulin by 42% (P less than .01) and 34% (P less than .05), respectively. When euglycemia was maintained by a variable glucose infusion rate after the injection of insulin and proinsulin, no differences were observed in plasma levels of any of the hormones. The intravenous injection of a dose of proinsulin (6 micrograms/kg), which did not produce hypoglycemia but was the molar equivalent of insulin used in the first protocol, failed to modify the GH or prolactin responses to a combined injection of GH-releasing hormone (1 microgram/kg) and thyrotropin-releasing hormone (500 micrograms). Our results indicate that the onset of pituitary hormone and catecholamine responses to hypoglycemia are related to the rate of plasma glucose decline, with the slower responses to proinsulin reflecting a more gradual onset of hypoglycemia. The magnitude of the cortisol, GH, and catecholamine responses, however, was comparable with proinsulin- and insulin-induced hypoglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative efficacy of a once-daily controlled-release formulation of glipizide and immediate-release glipizide in patients with NIDDM.

OBJECTIVE: To compare the efficacy and safety of controlled-release glipizide (glipizide-GITS [gastrointestinal therapeutic system]) and immediate-release glipizide in patients with non-insulin-dependent diabetes mellitus (NIDDM). RESEARCH DESIGN AND METHODS: In a multicenter, open-label, randomized, two-way crossover study, 132 patients with NIDDM received daily doses of 5, 20, or 40 mg of either glipizide-GITS or immediate-release glipizide for 8 weeks followed by 8 weeks of the alternate formulation. Plasma glucose, serum insulin, C-peptide, and plasma glipizide levels were measured at fasting and post-Sustacal challenge at the end of 1 and 8 weeks of each treatment phase. HbA1c was measured at the end of weeks 7 and 8 of each treatment phase. RESULTS: Both formulations of glipizide yielded similar mean HbA1c values. However, mean fasting plasma glucose (FPG) levels were significantly lower with glipizide-GITS treatment than with immediate-release glipizide at the end of week 1 (11.0 vs. 11.6 mmol/l; P < 0.01) and at the end of the 8-week treatment phase (10.9 vs. 11.7 mmol/l; P < 0.001). Fasting insulin and C-peptide levels were lower after 5 mg glipizide-GITS vs. immediate-release glipizide. Glucose responses to Sustacal were similar after both formulations of glipizide; however, serum insulin (P < 0.01) and C-peptide responses (P < 0.05) were lower with glipizide-GITS than with immediate-release glipizide treatment at the end of the 8-week treatment phase. Mean plasma glipizide concentrations were stable by the end of week 1, and the concentrations increased proportionately with dose. Once-daily Glipizide-GITS provided effective mean glipizide concentrations (> 50 ng/ml) 24 h after dosing, even at the lowest (5 mg) dose level. Both formulations were well tolerated. CONCLUSIONS: Glipizide-GITS was significantly more effective than immediate-release glipizide in reducing FPG levels. Both formulations reduced postprandial plasma glucose levels equally; however, glipizide-GITS exerted its control in the presence of lower plasma glipizide concentrations in addition to significantly lower insulin and C-peptide levels. This suggests that glipizide-GITS improves insulin sensitivity.

Covalent labeling of the somatostatin receptor in rat anterior pituitary membranes.

The molecular characteristics of the somatostatin (SRIF) receptor were investigated by covalently cross-linking [125I-Tyr11]SRIF to rat anterior pituitary membranes using three heterobifunctional cross-linking agents, N-5-azido-2-nitrobenzoyloxysuccinimide, N-hydroxysuccinimidyl-4-azidobenzoate, and N-succinimidyl-6-(4'-azido-2'-nitrophenylamino) hexanoate, and the homobifunctional agent disuccinimidyl suberate. Sodium dodecyl sulfate-gel electrophoresis followed by autoradiography revealed two SRIF-binding proteins with apparent mol wt (Mr) of 69,000 and 66,000 that were selectively labeled by the four cross-linking agents. When cross-linking was performed with N-5-azido-2-nitrobenzoyloxysuccinimide, both proteins migrated as a broad band centered at 68,000; however, with N-hydroxysuccinimidyl-4-azidobenzoate, the band was resolved into 69,000 and 66,000 Mr components. N-Succinimidyl-6-(4'-azido-2'-nitrophenylamino) hexanoate covalently labeled the 69,000 Mr protein and a minor species with a Mr of 45,000-47,000. Cross-linking with disuccinimidyl suberate labeled only the 66,000 Mr band. Labeling of both bands was specific, since affinity labeling with each of the four agents was abolished when 1 microM cyclic SRIF was included in the binding reaction. Binding of [125I-Tyr11]SRIF to membranes and labeling of the 69,000 and 66,000 Mr SRIF-binding species were similarly inhibited in a dose-dependent manner by unlabeled SRIF. Radiolabeling of both proteins was specifically displaced by 1 microM SRIF-28 and [D-Trp8,D-Cys14]SRIF, but not by oxytocin. Moreover, the extent of radiolabel incorporation into both components was dependent on the concentration of [125I-Tyr11]SRIF in the binding reaction. These results demonstrate the presence of two SRIF-binding proteins in rat anterior pituitary membranes that show characteristics of the SRIF receptor.

Glucopenia-mediated release of somatostatin from incubated rat hypothalamus: monosaccharide specificity and role of glycolytic intermediates.

Rat hypothalamic somatostatin (SRIF) release in vitro is inversely related to medium glucose concentration and is stimulated by agents interfering with neural glucose uptake or metabolism. The present studies examined monosaccharide specificity of this effect by comparing the ability of glucose and nonmetabolized sugars (ribose, fructose, and galactose) to reverse glucopenia-mediated SRIF release. Removal of glucose from incubation medium resulted in a 7-fold stimulation of hypothalamic SRIF release (24.9 +/- 3.0 to 169.0 +/- 46.2 pg/5 min) that was promptly and fully reversed by readdition of glucose (21.5 +/- 6.0 pg/5 min). Depolarizing concentrations of potassium (40 mM) added to medium after incubation in the presence or absence of glucose produced similar 4-fold biphasic stimulation of SRIF release, suggesting that tissue viability was unimpaired by short term glucopenia. By contrast, nonmetabolized sugars (ribose, fructose, or galactose) at concentrations up to 28 mM were unable to reverse glucopenia-mediated hypothalamic SRIF release. These findings suggest that neural metabolism of glucose specifically influences hypothalamic SRIF release, an effect likely to depend on glycolysis with resulting energy production. This explanation is supported by the finding that the glycolytic intermediates dihydroxyacetone and glyceraldehyde were partly, and pyruvate completely, able to restore glucopenia-mediated SRIF release toward basal, with a maximal effect at 14 mM. Our present studies suggest that rat hypothalamic SRIF release reflects ambient glucose status, is stimulated by glucopenia, and may explain inhibition of GH secretion in the rat during hypoglycemia.

Regulation of rat hypothalamic preprogrowth hormone-releasing factor messenger ribonucleic acid by dietary protein.

To further evaluate nutrient regulation of GRF synthesis, we measured hypothalamic preproGRF messenger (m) RNA in food-deprived rats refed diets varying in nutrient composition by nuclease protection analysis. Adult male Sprague-Dawley rats were allowed free access to food (Fed), food deprived for 72 h (72-h FD), or 72 h FD then refed for 72 h with either a normal (NF) diet or isocaloric diets containing no protein (PF), carbohydrate (CF), or fat (FF). Seventy-two-hour FD rats displayed the expected 80% reduction in hypothalamic preproGRF mRNA. Upon refeeding, levels were normalized in rats refed NF, CF, or FF diets. In contrast, preproGRF mRNA in rats refed a PF diet was similar to that in 72-h FD rats. Rats refed a PF diet failed to gain weight and consumed less food than animals refed NF, CF, or FF diets. However, the lack of the GRF response to the PF diet was due to protein deprivation rather than caloric restriction, since hypothalamic preproGRF mRNA returned to 66% of Fed values in rats refed an equivalent amount (grams per day) of a NF diet. In 72-h FD rats refed isocaloric diets containing 4%, 8%, or 12% protein, preproGRF mRNA was restored to Fed values in a protein concentration-dependent manner being completely restored by the 12% diet. A lack of dietary protein was sufficient to regulate hypothalamic preproGRF mRNA since feeding rats a PF diet without prior food deprivation resulted in 70% reduction in preproGRF mRNA, whereas CF and FF diets were without effect. These data indicate that decreased hypothalamic preproGRF mRNA expression in 72-h FD rats occurs as a result of dietary protein deprivation.

Dexamethasone regulates somatostatin receptor subtype messenger ribonucleic acid expression in rat pituitary GH4C1 cells.

Ligand binding studies have shown that glucocorticoids down-regulate somatostatin receptor (sst) concentration in several endocrine target cells and cell lines including GH4C1 cells. However, it has not been determined whether this decrease in sst number occurred via transcriptional and/or posttranscriptional events. In the present study, we have investigated the effect of dexamethasone (Dex) treatment (1 microM) of GH4C1 cells for up to 48 h on the steady state level of messenger RNA (mRNA) for sst1, sst2, and sst3, the predominant isoforms expressed in this cell line, by solution hybridization-nuclease protection analysis. Exposure of GH4C1 cells to Dex for 2 h increased sst1 mRNA levels 2.5-fold and sst2 1.5-fold compared with controls (Con). Prolonged exposure, however, resulted in a decrease in mRNA levels of sst1 to 50% and sst2 to 30% of Con by 24-48 h. In contrast, sst3 mRNA levels were unchanged at 2 h, decreased to 30% of Con by 6 h, and remained decreased for up to 24 h. Longer exposure resulted in a dramatic increase in expression, reaching 350% of Con by 48 h. The Dex effect on expression of all subtypes was dose dependent, maximal at 10 nM. Steroid hormone regulation of sst mRNA expression in GH4C1 cells proved to be complex. Exposure to Dex for 24 h, as expected, decreased expression of all subtypes. Progesterone, however, increased sst1 mRNA levels, decreased sst3 levels, but was without effect on sst2; treatment with estrogen and testosterone increased expression of all three subtypes. Nuclear run-on assays indicated that the Dex-induced changes in sst1 and sst2 mRNA levels were associated with congruent changes in the transcription rate of sst genes. Thus, glucocorticoids regulate sst expression in GH4C1 cells, at least in part, by controlling the rate of transcription of sst genes.