Resistance of growth hormone secretion to hypoglycemia in the mouse

Rhythmic growth hormone secretion in physiological and pathological conditions: Lessons from rodent studies

Evolutionally conserved in all mammalians, the release of GH occurs in a rhythmic pattern, characterized by several dominant surges (pulsatile GH) with tonic low inter-pulse levels (tonic GH). Such pulsatile secretion pattern is essential for many physiological actions of GH on different tissues with defined gender dimorphism. Rhythmic release of pulsatile GH is tightly controlled by hypothalamic neurons as well as peripheral metabolic factors. Changes of GH pattern occur within a range of sophisticated physiological and pathological settings and significantly contribute to growth, ageing, survival and disease predispositions. Precise analysis of GH secretion pattern is vitally important for a comprehensive understanding of the function of GH and the components that regulate GH secretion pattern.

Central Mechanisms of Glucose Sensing and Counterregulation in Defense of Hypoglycemia

Glucose homeostasis requires an organism to rapidly respond to changes in plasma glucose concentrations. Iatrogenic hypoglycemia as a result of treatment with insulin or sulfonylureas is the most common cause of hypoglycemia in humans and is generally only seen in patients with diabetes who take these medications. The first response to a fall in glucose is the detection of impending hypoglycemia by hypoglycemia-detecting sensors, including glucose-sensing neurons in the hypothalamus and other regions. This detection is then linked to a series of neural and hormonal responses that serve to prevent the fall in blood glucose and restore euglycemia. In this review, we discuss the current state of knowledge about central glucose sensing and how detection of a fall in glucose leads to the stimulation of counterregulatory hormone and behavior responses. We also review how diabetes and recurrent hypoglycemia impact glucose sensing and counterregulation, leading to development of impaired awareness of hypoglycemia in diabetes.

2016 Laboratory guidelines for postvasectomy semen analysis: Association of Biomedical Andrologists, the British Andrology Society and the British Association of Urological Surgeons

Post-vasectomy semen analysis (PVSA) is the procedure used to establish whether sperm are present in the semen following a vasectomy. PVSA is presently carried out by a wide variety of individuals, ranging from doctors and nurses in general practitioner (GP) surgeries to specialist scientists in andrology laboratories, with highly variable results.

Key recommendations are that: (1) PVSA should take place a minimum of 12 weeks after surgery and after a minimum of 20 ejaculations. (2) Laboratories should routinely examine samples within 4 h of production if assessing for the presence of sperm. If non-motile sperm are observed, further samples must be examined within 1 h of production. (3) Assessment of a single sample is acceptable to confirm vasectomy success if all recommendations and laboratory methodology are met and no sperm are observed. Clearance can then be given. (4) The level for special clearance should be <100 000/mL non-motile sperm. Special clearance cannot be provided if any motile sperm are observed and should only be given after assessment of two samples in full accordance with the methods contained within these guidelines. Surgeons are responsible both preoperatively and postoperatively for the counselling of patients and their partners regarding complications and the possibility of late recanalisation after clearance. These 2016 guidelines replace the 2002 British Andrology Society (BAS) laboratory guidelines and should be regarded as definitive for the UK in the provision of a quality PVSA service, accredited to ISO 15189:2012, as overseen by the United Kingdom Accreditation Service (UKAS).

Insulin-like growth factor 1 mediates negative feedback to somatotroph GH expression via POU1F1/CREB binding protein interactions

Circulating insulin-like growth factor 1 (IGF-1) has been shown to act as a negative feedback regulator of growth hormone (GH) gene expression; however, the mechanism of this negative feedback is poorly understood. Activation and regulation of GH gene expression require the binding of the transcription factor POU1F1 to the GH promoter along with cyclic AMP (cAMP) response element binding protein (CREB) binding protein (CBP). We investigate the role of CBP as a target of IGF-1 somatotroph regulation using the MtT/S somatotroph cell line. IGF-1 significantly inhibits basal GH mRNA levels but not POU1F1 levels. Chromatin immunoprecipitation assays demonstrate inhibition of CBP binding to the GH promoter after IGF-1 treatment. We hypothesized that IGF-1 receptor (IGF-1R) signaling disrupts the POU1F1/CBP complex to inhibit gene expression. In support, the use of a mutant CBP (S436A) construct, which lacks a critical phosphorylation site, leads to the loss of IGF-1 inhibition. The studies of CBP (S436A) knock-in mice show elevated serum GH levels, a greater response to GH releasing hormone (GHRH) stimulation along with lower weight gain, and decreased body fat. Our data confirm the inhibitory effects of IGF-1 on GH expression at the level of the promoter and provide evidence of CBP's role as a target of IGF-1R signaling.

Targeted deletion of somatotroph insulin-like growth factor-I signaling in a cell-specific knockout mouse model

The role of IGF-I in the negative regulation of GH expression and release is demonstrated by in vitro and in vivo models; however, the targets and mechanisms of IGF-I remain unclear. We have developed a cell-specific knockout mouse in which the IGF-I receptor was ablated from the somatotroph in order to validate and characterize IGF-I negative regulation; we termed this the somatotroph IGF-I receptor knockout (SIGFRKO) mouse. The SIGFRKO mice demonstrated increased GH gene expression and secretion as well as increased serum IGF-I. Compensatory changes were noted with decreased GHRH and increased somatostatin mRNA expression levels. SIGFRKO mice had normal linear growth, but by 14 wk of age weighed significantly less than controls. Furthermore, metabolic studies revealed SIGFRKO mice had significantly less fat mass and body percent fat. These data support somatotroph IGF-I negative regulation and suggest that hypothalamic feedback limits the extent of GH release. The SIGFRKO mouse is a model delineating the mechanisms of IGF-I regulation in the hypothalamic-pituitary axis and demonstrates compensatory mechanisms that mediate growth and metabolic function in mammals.

Counterregulatory deficits occur within 24 h of a single hypoglycemic episode in conscious, unrestrained, chronically cannulated mice

Hypoglycemia-induced counterregulatory failure is a dangerous complication of insulin use in diabetes mellitus. Controlled hypoglycemia studies in gene knockout models, which require the use of mice, would aid in identifying causes of defective counterregulation. Because stress can influence counterregulatory hormones and glucose homeostasis, we developed glucose clamps with remote blood sampling in conscious, unrestrained mice. Male C57BL/6 mice implanted with indwelling carotid artery and jugular vein catheters were subjected to 2 h of hyperinsulinemic glucose clamps 24 h apart, with a 6-h fast before each clamp. On day 1, blood glucose was maintained (euglycemia, 178 +/- 4 mg/dl) or decreased to 62 +/- 1 mg/dl (hypoglycemia) by insulin (20 mU x kg(-1) x min(-1)) and variable glucose infusion. Donor blood was continuously infused to replace blood sample volume. Baseline plasma epinephrine (32 +/- 8 pg/ml), corticosterone (16.1 +/- 1.8 microg/dl), and glucagon (35 +/- 3 pg/ml) were unchanged during euglycemia but increased significantly during hypoglycemia, with a glycemic threshold of approximately 80 mg/dl. On day 2, all mice underwent a hypoglycemic clamp (blood glucose, 64 +/- 1 mg/dl). Compared with mice that were euglycemic on day 1, previously hypoglycemic mice had significantly higher glucose requirements and significantly lower plasma glucagon and corticosterone (n = 6/group) on day 2. Epinephrine tended to decrease, although not significantly, in repeatedly hypoglycemic mice. Pre- and post-clamp insulin levels were similar between groups. We conclude that counterregulatory responses to acute and repeated hypoglycemia in unrestrained, chronically cannulated mice reproduce aspects of counterregulation in humans, and that repeated hypoglycemia in mice is a useful model of counterregulatory failure.

Insulin hypoglycemia and growth hormone secretion in sheep: a paradox revisited

Although insulin-induced hypoglycemia is a potent stimulus for growth hormone (GH) secretion in humans, hypoglycemia was reported to suppress GH in sheep. We investigated whether GH suppression in sheep during insulin hypoglycemia resulted from the dose of insulin administered or the fed state of the animal. Saline or insulin (0.05, 0.2, 1.0, or 5.0 U/kg) intravenous boluses were administered to eight fasted ewes in a crossover experiment. In another experiment, four sheep were fed 2 h before intravenous administrations of either 0.2 or 5 U/kg of insulin. All doses of insulin resulted in comparable hypoglycemia, although the duration of hypoglycemia increased directly with insulin dose. Hypoglycemia in fasted animals stimulated GH secretion. The GH rise above baseline was inversely related to the insulin dose, and the insulin doses of 1 and 5 U/kg resulted in late suppression of GH below baseline concentrations. Insulin administration to fed animals caused an identical degree of hypoglycemia but no increase in GH. Insulin-hypoglycemia stimulates GH secretion in sheep in a manner similar to humans, and the response is dependent on both fed state and insulin dose.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

CNS sites involved in sympathetic and parasympathetic control of the pancreas: a viral tracing study

The viral transneuronal tracing method was used to identify the CNS cell groups that regulate the parasympathetic and sympathetic outflow systems of the pancreas. Pseudorabies virus (PRV) was injected into the pancreas of vagotomized rats and after 6 days survival, the pattern of transneuronal labeling in the CNS sympathetic regulatory regions was determined. The converse experiment was performed in order to elucidate the central parasympathetic cell groups that regulate the pancreas. Immunohistochemical methods were used to identify putative neuropeptide- and catecholamine-containing CNS neurons involved in these regulatory circuits. The major finding of this study indicates that five brain regions, viz., paraventricular hypothalamic nucleus, perifornical hypothalamic region, A5 catecholamine cell group, rostral ventrolateral medulla, and lateral paragigantocellular reticular nucleus, contain a considerable amount of overlap in cell body labeling. In addition, the ventrolateral part of the periaqueductal gray matter and gigantocellular reticular nucleus, ventral part also showed a similar overlap, but the numbers of neurons found in these areas were considerably lower than the five major regions. These data suggest that these brain regions may provide parallel and possibly redundant, autonomic pathways affecting glucagon and adrenaline release.

Induction of Fos protein in the rat hypothalamus elicited by insulin-induced hypoglycemia

To evaluate the responses to insulin-induced hypoglycemia of neurons in vivo, we studied Fos protein induction in the brain by means of immunohistochemistry. The induction of Fos protein was maximum after the first injection of insulin for 3 h. This induction was found in the parvocellular division of paraventricular nucleus (PVN), the periventricular, dorsomedial and arcuate nuclei and the lateral hypothalamic area of the hypothalamus. These findings show the activation of specific subsets of neurons in areas of the hypothalamus following hypoglycemic stimulation.