Differential responses of the growth hormone axis in two rat models of streptozotocin-induced insulinopenic diabetes

Comparative analysis of effects of conditioned mediums obtained from 2D or 3D cultured mesenchymal stem cells on kidney functions of diabetic rats: Early intervention could potentiate transdifferentiation of parietal epithelial cell into podocyte precursors

AIM

The secretome of mesenchymal stem cells (MSCs) could be a potential therapeutic intervention for diabetes and associated complications like nephropathy. This study aims to evaluate the effects of conditioned mediums (CMs) collected from umbilical cord-derived MSCs incubated under 2-dimensional (2D) or 3D culture conditions on kidney functions of rats with type-I diabetes (T1D).

MAIN METHODS

Sprague-Dawley rats were treated with 20 mg/kg streptozocin for 5 consecutive days to induce T1D, and 12 doses of CMs were applied intraperitoneally for 4 weeks. The therapeutic effects of CMs were comparatively investigated by biochemical, physical, histopathological, and immunohistochemical analysis.

KEY FINDINGS

3D-CM had significantly higher total protein concentration than the 2D-CM Albumin/creatinine ratios of both treatment groups were significantly improved in comparison to diabetes. Light microscopic evaluations showed that glomerular and cortical tubular damages were significantly ameliorated in only the 3D-CM applied group compared to the diabetes group, which were correlated with transmission electron microscopic observations. The nephrin and synaptopodin expressions increased in both treatment groups compared to diabetes. The WT1, Ki-67, and active caspase-3 expressions in glomeruli and parietal layers of the treatment groups suggest that both types of CMs suppress apoptosis and promote possible parietal epithelial cells' (PECs') transdifferentiation towards podocyte precursor cells by switching on WT1 expression in parietal layer rather than inducing new cell proliferation.

SIGNIFICANCE

3D-CM was found to be more effective in improving kidney functions than 2D-CM by ameliorating glomerular damage through the possible mechanism of transdifferentiation of PECs into podocyte precursors and suppressing glomerular apoptosis.

Upregulation in Inflammation and Collagen Expression in Perirenal but Not in Mesenteric Adipose Tissue from Diabetic Munich Wistar Frömter Rats

Perirenal adipose tissue (PRAT) surrounding the kidney is emerging as a player and novel independent risk factor in diabetic kidney disease (DKD); DKD is a complication of diabetes and is a major cause of increased cardiovascular (CV) risk and CV mortality in affected patients. We determined the effect of diabetes induction on (i) kidney and CV damage and (ii) on the expression of proinflammatory and profibrotic factors in both the PRAT and the mesenteric adipose tissue (MAT) of Munich Wistar Frömter (MWF) rats. The 16-week-old male MWF rats (n = 10 rats/group) were fed standard chow (MWF-C) or a high-fat/high-sucrose diet for 6 weeks together with low-dose streptozotocin (15 mg/kg i.p.) at the start of dietary exposure (MWF-D). Phenotyping was performed at the end of treatment through determining water intake, urine excretion, and oral glucose tolerance; use of the homeostatic model assessment-insulin resistance index (HOMA-IR) evidenced the development of overt diabetes manifestation in MWF-D rats. The kidney damage markers Kim-1 and Ngal were significantly higher in MWF-D rats, as were the amounts of PRAT and MAT. A diabetes-induced upregulation in IL-1, IL-6, Tnf-α, and Tgf-β was observed in both the PRAT and the MAT. Col1A1 was increased in the PRAT but not in the MAT of MWF-D, whereas IL-10 was lower and higher in the PRAT and the MAT, respectively. Urinary albumin excretion and blood pressure were not further increased by diabetes induction, while heart weight was higher in the MWF-D. In conclusion, our results show a proinflammatory and profibrotic in vivo environment in PRAT induced by diabetes which might be associated with kidney damage progression in the MWF strain.

Shortened Daily Photoperiod Alleviates Anxiety-like Behaviour by Antioxidant Effect and Changes Serum Fatty Acid Profile in Diabetic Rats

The aim of our study was to investigate the effects of a shortened daily photoperiod on anxiety-like behaviour, brain oxidative stress, lipid status and fatty acid composition of serum lipids in a streptozotocin (STZ)-induced model of diabetes mellitus in rats. Male Wistar rats were divided into the following groups: first group-control group (C_12/12); second group-diabetic group (DM_12/12; 100 mg/kg STZ); third group-control group exposed to a light/dark cycle 6/18 h (C_6/18); fourth group-diabetic group exposed to a light/dark cycle 6/18 h (DM_6/18). Anxiety-like behaviour was tested three weeks following STZ injection by elevated plus maze (EPM) and open-field test (OFT). Oxidative stress parameters were measured in the cortex, hippocampus and thalamus, while lipid status and fatty acid methyl esters (FAMEs) were measured in the serum. Both EPM and OFT showed a lower degree of anxiety-like behaviour in the DM_6/18 vs. DM_12/12 group. Lipid peroxidation in the cortex, hippocampus and thalamus was significantly lower in the DM_6/18 vs. DM_12/12 group (p < 0.05), associated with an increased level of antioxidant enzymes and protein thiols in the cortex and thalamus. In the DM_6/18 group, oleic, vaccenic, dihomo-γ-linolenic and docosahexaenoic acid concentrations were significantly higher in comparison to the DM_12/12 group. A shortened daily photoperiod alleviates anxiety-like behaviour in diabetic rats by reduced lipid peroxidation and changes in the serum fatty acids profile.

2D and 3D cultured human umbilical cord-derived mesenchymal stem cell-conditioned medium has a dual effect in type 1 diabetes model in rats: immunomodulation and beta-cell regeneration

BACKGROUND

Type 1 diabetes (T1D) is a T-cell-mediated autoimmune disease characterized by the irreversible destruction of insulin-producing β-cells in pancreatic islets. Helper and cytotoxic T-cells and cytokine production, which is impaired by this process, take a synergetic role in β-cell destruction, and hyperglycemia develops due to insulin deficiency in the body. Mesenchymal stem cells (MSCs) appear like an excellent therapeutic tool for autoimmune diseases with pluripotent, regenerative, and immunosuppressive properties. Paracrine factors released from MSCs play a role in immunomodulation by increasing angiogenesis and proliferation and suppressing apoptosis. In this context, the study aims to investigate the therapeutic effects of MSC's secretomes by conditioned medium (CM) obtained from human umbilical cord-derived MSCs cultured in 2-dimensional (2D) and 3-dimensional (3D) environments in the T1D model.

METHODS

First, MSCs were isolated from the human umbilical cord, and the cells were characterized. Then, two different CMs were prepared by culturing MSCs in 2D and 3D environments. The CM contents were analyzed in terms of total protein, IL-4, IL-10, IL-17, and IFN-λ. In vivo studies were performed in Sprague-Dawley-type rats with an autoimmune T1D model, and twelve doses of CM were administered intraperitoneally for 4 weeks within the framework of a particular treatment model. In order to evaluate immunomodulation, the Treg population was determined in lymphocytes isolated from the spleen after sacrification, and IL-4, IL-10, IL-17, and IFN-λ cytokines were analyzed in serum. Finally, β-cell regeneration was evaluated immunohistochemically by labeling Pdx1, Nkx6.1, and insulin markers, which are critical for the formation of β-cells.

RESULTS

Total protein and IL-4 levels were higher in 3D-CM compared to 2D-CM. In vivo results showed that CMs induce the Treg population and regulate cytokine release. When the immunohistochemical results were evaluated together, it was determined that CM application significantly increased the rate of β-cells in the islets. This increase was at the highest level in the 3D-CM applied group.

CONCLUSION

The dual therapeutic effect of MSC-CM on immunomodulation and homeostasis/regeneration of β-cells in the T1D model has been demonstrated. Furthermore, this effect could be improved by using 3D scaffolds for culturing MSCs while preparing CM.

Zinc Chloride Enhances the Antioxidant Status, Improving the Functional and Structural Organic Disturbances in Streptozotocin-Induced Diabetes in Rats

Background and Objectives: Diabetes mellitus (DM) is a complex disease affecting the whole metabolic balance of the body and resulting in multiple organ complications: cardiovascular, neuronal, renal, etc. Our study focuses on investigating the effect of zinc chloride (Zn) on certain blood parameters suggestive for assessing the metabolic disturbances, the liver and kidney function, the oxidative stress and the immune defense capacity in experimental-induced DM with streptozotocin (STZ) and cholesterol in rats. Materials and Methods: The animals were assigned to three groups, as follows: Group 1 (Control): buffer citrate solution 0.1 mL/100 g body; Group 2 (STZ): 20 mg/kg body STZ and fat diet (10 g cholesterol/100 g diet); Group 3 (STZ+Zn): 20 mg/kg body STZ + 5 mg/kg body Zn chloride and the same fat diet. DM was induced by administering STZ in a single take daily, for three consecutive days, Zn and citrate buffer were administered orally for a month. The protocol was approved by the Ethics Committee of the University 'Grigore T Popa' Iasi, in agreement with the International Regulations about the handling of laboratory animals. Results: The use of STZ in rats fed with cholesterol was correlated with important weight gain, hyperglycemia, the intensification of the transaminases activity and the increase in serum alkaline phosphatase, cholesterol, triglyceride, urea, creatinine and in malondialdehyde. Conclusions: The treatment with Zn resulted in weight loss and a decrease in blood sugar in diabetic rats. Supplementation with Zn notably reduced oxidative stress, preserved the pancreatic architecture and restored the liver and kidney function and structure in STZ-induced DM in rats.

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.

Identification and applications of neuroactive silk proteins: a narrative review

In traditional medicine, natural silk is regarded as a cognitive enhancer and a cure for ameliorating the symptoms of heart disease, atherosclerosis, and metabolic disorders. In this review, general characteristics of both silk proteins, fibroin and sericin, extracted from silkworm Bombyx mori and their potential use in the neuronal disorders was discussed. Evidence shows that silk proteins exhibit neuroprotective effects in models of neurotoxicity. The antioxidant, neuroprotective, and acetylcholinesterase inhibitory mechanisms of silk proteins could prove promising in the treatment of neurodegenerative diseases. Owing to their excellent neurocompatibility and physicochemical properties, silk proteins have been used as scaffolds and drug delivery materials in the neuronal tissue engineering. These data support the potential of silk proteins as an effective complementary agent for central and peripheral neurological disorders.

Rodent models of diet-induced type 2 diabetes mellitus: A literature review and selection guide

Several research teams have focused on finding the "ideal" animal model that reflects the pathophysiological changes and closely simulates the metabolic characteristics of patients with type 2 diabetes. However, the multitude of studies on this topic has resulted in large variations, making the models difficult to compare, as the measured parameters vary significantly. Additionally, selecting the appropriate animal model for a new study has become more difficult due to the increasing number of background variables. This article gives a detailed overview of the literature, covering the entire range of animal models and model characteristics, and most importantly, provides guidance for selecting the most suitable model for specific research goals in the future.

A high linoleic acid diet exacerbates metabolic responses and gut microbiota dysbiosis in obese rats with diabetes mellitus

Dietary polyunsaturated fatty acid (PUFA) levels may affect inflammatory responses and lipid metabolism.

Growth Hormone Response to Oral Glucose Load: From Normal to Pathological Conditions

The exact physiological basis of acute growth hormone (GH) suppression by oral glucose is not fully understood. Glucose-mediated increase in hypothalamic somatostatin seems to be the most plausible explanation. Attempts to better understand its underlying mechanisms are compromised by species disparities in the response of GH to glucose load. While in humans, glucose inhibits GH release, the acute elevation of circulating glucose levels in rats has either no effect on GH secretion or may be stimulatory. Likewise, chronic hyperglycemia alters GH release in both humans and rats nonetheless in opposite directions. Several factors influence nadir GH concentrations including, age, gender, body mass index, pubertal age, and the type of assay used. Besides the classical suppressive effects of glucose on GH release, a paradoxical GH increase to oral glucose may be observed in around one third of patients with acromegaly as well as in various other disorders. Though its pathophysiology is poorly characterized, an altered interplay between somatostatin and GH-releasing hormone has been suggested and a link with pituitary ectopic expression of glucose-dependent insulinotropic polypeptide receptor has been recently demonstrated. A better understanding of the dynamics mediating GH response to glucose may allow a more optimal use of the OGTT as a diagnostic tool in various conditions, especially acromegaly.

Enhanced Pulsatile Growth Hormone Secretion and Altered Metabolic Hormones by in Vivo Hexarelin Treatment in Streptozotocin-Induced Diabetic Rats

Significant growth hormone (GH) reductions have been reported in diabetic animal models with disturbed metabolic balance coinciding with GH deficiency. Therefore, enhanced GH secretion may have beneficial effects in controlling diabetes. Thus, we aim to investigate the effect of hexarelin, a synthetic GH secretagogue (GHS), on GH secretion in streptozotocin (STZ, 65 mg/kg)-induced diabetic rats. Daily hexarelin (100 μg/kg) treatment was performed for two weeks in four-week-long STZ-diabetic and vehicle control rats. Pulsatile GH secretion in STZ-rats was significantly reduced in total, pulsatile, basal, and mass of GH secretion per burst. In addition, impaired GH secretion was followed by an increase in fasting-level free fatty acids (FFAs) and a decrease in insulin-like growth factor 1 (IGF-1) compared to control rats. After hexarelin treatment, pulsatile GH secretion in STZ-rats was significantly increased in total, pulsatile, and basal, but not in the mass GH secretion per burst, compared to STZ-rats without hexarelin treatment. However, there was no significant elevation in GH secretion in the hexarelin-treated control group. In addition, hexarelin-treated STZ-rats showed a significant decrease in fasting level FFAs, whereas suppression of fasting level for IGF-1 was maintained. These results suggest that STZ-induced diabetic rats have impaired pulsatile GH secretion, causing increased FFAs and decreased IGF-1 levels in circulation. Hexarelin injections for two weeks is able to normalize impaired pulsatile GH secretion with normal fasting levels of FFAs, but fails to recover IGF-1 levels.

Early detection of cardiac dysfunction in the type 1 diabetic heart using speckle-tracking based strain imaging

Enhanced sensitivity in echocardiographic analyses may allow for early detection of changes in cardiac function beyond the detection limits of conventional echocardiographic analyses, particularly in a small animal model. The goal of this study was to compare conventional echocardiographic measurements and speckle-tracking based strain imaging analyses in a small animal model of type 1 diabetes mellitus. Conventional analyses revealed differences in ejection fraction, fractional shortening, cardiac output, and stroke volume in diabetic animals relative to controls at 6-weeks post-diabetic onset. In contrast, when assessing short- and long-axis speckle-tracking based strain analyses, diabetic mice showed changes in average systolic radial strain, radial strain rate, radial displacement, and radial velocity, as well as decreased circumferential and longitudinal strain rate, as early as 1-week post-diabetic onset and persisting throughout the diabetic study. Further, we performed regional analyses for the LV and found that the free wall region was affected in both the short- and long-axis when assessing radial dimension parameters. These changes began 1-week post-diabetic onset and remained throughout the progression of the disease. These findings demonstrate the use of speckle-tracking based strain as an approach to elucidate cardiac dysfunction from a global perspective, identifying left ventricular cardiac regions affected during the progression of type 1 diabetes mellitus earlier than contractile changes detected by conventional echocardiographic measurements.

Diabetes mellitus and its management with medicinal plants: A perspective based on Iranian research

ETHNOPHARMACOLOGICAL RELEVANCE

Complementary and alternative medicine has been increasingly used to treat chronic illnesses, such as diabetes mellitus. However, various limitations in terms of their application and efficacies exist. Furthermore, there is still much to be done to discover the right herbal medicine for diabetes.

AIM OF THE STUDY

This paper aims to evaluate previous herbal studies on the management of diabetes mellitus, to address their strengths and weaknesses and propose a general framework for future studies.

APPROACH AND METHODS

Data sources such as PubMed, ScienceDirect, Scopus, SpringerLink, and Wiley were searched, limited to Iran, using 36 search terms such as herbal, traditional, medicine, and phytopharmacy in combination with diabetes and related complications. Reviewed articles were evaluated regarding the use of botanical nomenclature and included information on (1) identity of plants and plant parts used, (2) the processing procedure, and (3) the extraction process. The main outcomes were extracted and then surveyed in terms of the efficacies of herbs in the management of diabetes mellitus. Then a comparative study was performed between Iranian and non-Iranian studies with respect to herbs best studied in Iran.

RESULTS

Of the 82 herbs studied in Iran, only six herbs were endemic and 19 were studied in detail. Although most of the reviewed herbs were found to decrease the level of blood glucose (BG) and/or glycated hemoglobin (HbA1C) in both Iranian and non-Iranian studies, information on their pharmacological mechanisms is scarce. However, the level of HbA1C was measured in a limited number of clinical trials or animal studies. Available information on both short- and long-term use of studied herbs on diabetes related complications and functions of involved organs as well as comorbid depression and/or simultaneous changes in lifestyle is also insufficient. Furthermore, little or no information on their phytochemical, toxicological, and herb-drug interaction properties is available. It is worth noting that the efficacy of the reviewed herbs has been studied scarcely in both humans and animals regarding both Iranian and non-Iranian studies. A significant number of reviewed articles failed to cite the scientific name of herbs and include information on the processing procedure and the extraction process.

CONCLUSIONS

Treatment of diabetes mellitus as a multifactorial disease using herbal medicines requires a comprehensive approach. In order to discover the right herbal medicine for the management of diabetes many other important factors than the levels of BG, HbA1C and insulin should be considered. According to our criteria, all the reviewed herbs suffered from inadequate investigation in human, animal and in vitro models in this respect, whereas they are worth investigating further. However, more research on endemic plants and the traditional history of herbal medicine is warranted. In our opinion, the pharmacological, toxicological, and phytochemical information should be obtained before clinical trials. Furthermore, information such as botanical scientific nomenclature, side effects, and toxicity will improve the quality and validity of publications in herbal research. In particular, designing a database covering all valid information about herbs and/or diseases will decrease unnecessary costs and increase the efficiency of research.

Experimental diabetes induced by alloxan and streptozotocin: The current state of the art

Diabetes mellitus is a chronic metabolic disorder with a high prevalence worldwide. Animal models of diabetes represent an important tool in diabetes investigation that helps us to avoid unnecessary and ethically challenging studies in human subjects, as well as to obtain a comprehensive scientific viewpoint of this disease. Although there are several methods through which diabetes can be induced, chemical methods of alloxan- and streptozotocin-induced diabetes represent the most important and highly preferable experimental models for this pathological condition. Therefore, the aim of this article was to review the current knowledge related to quoted models of diabetes, including to this point available information about mechanism of action, particular time- and dose-dependent protocols, frequent problems, as well as major limitations linked to laboratory application of alloxan and sterptozotocin in inducing diabetes. Given that diabetes is known to be closely associated with serious health consequences it is of fundamental importance that current animal models for induction of diabetes should be continuously upgraded in order to improve overall prevention, diagnosis and treatment of this pathological condition.

No effect of NOS inhibition on skeletal muscle glucose uptake during in situ hindlimb contraction in healthy and diabetic Sprague-Dawley rats

Nitric oxide (NO) has been shown to be involved in skeletal muscle glucose uptake during contraction/exercise, especially in individuals with Type 2 diabetes (T2D). To examine the potential mechanisms, we examined the effect of local NO synthase (NOS) inhibition on muscle glucose uptake and muscle capillary blood flow during contraction in healthy and T2D rats. T2D was induced in Sprague-Dawley rats using a combined high-fat diet (23% fat wt/wt for 4 wk) and low-dose streptozotocin injections (35 mg/kg). Anesthetized animals had one hindlimb stimulated to contract in situ for 30 min (2 Hz, 0.1 ms, 35 V) with the contralateral hindlimb rested. After 10 min, the NOS inhibitor, NG-nitro-l-arginine methyl ester (l-NAME; 5 μM) or saline was continuously infused into the femoral artery of the contracting hindlimb until the end of contraction. Surprisingly, there was no increase in skeletal muscle NOS activity during contraction in either group. Local NOS inhibition had no effect on systemic blood pressure or muscle contraction force, but it did cause a significant attenuation of the increase in femoral artery blood flow in control and T2D rats. However, NOS inhibition did not attenuate the increase in muscle capillary recruitment during contraction in these rats. Muscle glucose uptake during contraction was significantly higher in T2D rats compared with controls but, unlike our previous findings in hooded Wistar rats, NOS inhibition had no effect on glucose uptake during contraction. In conclusion, NOS inhibition did not affect muscle glucose uptake during contraction in control or T2D Sprague-Dawley rats, and this may have been because there was no increase in NOS activity during contraction.

Effect of sericin on diabetic hippocampal growth hormone/insulin-like growth factor 1 axis

Previous studies have shown that sericin extracted from silk cocoon significantly reduces blood glucose levels and protects the nervous system against diabetes mellitus. In this study, a rat type 2 diabetes mellitus model was established by intraperitoneal injection of 25 mg/kg streptozotocin for 3 successive days, following which the rats were treated with sericin for 35 days. After treatment, the blood glucose levels of the diabetic rats decreased significantly, the growth hormone level in serum and its expression in the hippocampus decreased significantly, while the insulin-like growth factor-1 level in serum and insulin-like growth factor-1 and growth hormone receptor expression in the hippocampus increased significantly. The experimental findings indicate that sericin improves disorders of the growth hormone/insulin-like growth factor 1 axis to alleviate hippocampal damage in diabetic rats.

Modeling type 2 diabetes in rats using high fat diet and streptozotocin

The pathology of type 2 diabetes is complex, with multiple stages culminating in a functional β‐cell mass that is insufficient to meet the body's needs. Although the broad outlines of the disease etiology are known, many critical questions remain to be answered before next‐generation therapeutics can be developed. In order to further elucidate the pathobiology of this disease, animal models mimicking the pathology of human type 2 diabetes are of great value. One example of a type 2 diabetes animal model is the high‐fat diet‐fed, streptozotocin (HFD/STZ)‐treated rat model. The present review first summarizes the current understanding of the metabolic profile and pathology involved in the different stages of the type 2 diabetes disease progression in humans. Second, the known characteristics of the HFD/STZ rat model are reviewed and compared with the pathophysiology of human type 2 diabetes. Next, the suitability of the HFD/STZ model as a model of type 2 diabetes with a focus on identifying critical caveats and unanswered questions about the model is discussed. The improved understanding of refined animal models will hopefully lead to more relevant preclinical studies and development of improved therapeutics for diabetes. Depending on the amount of residual functional β‐cells mass, the HFD/STZ rat model might be a suitable animal model of the final stage of type 2 diabetes.

Anti-inflammatory and hypoglycemic efficacy of Poria cocos and Dioscorea opposita in prediabetes mellitus rats

Poria cocos (Fu Ling) and Dioscorea opposita (Chinese Yam) were suggested to have potential benefits in blood sugar control.

QTLs influencing IGF-1 levels in a LOU/CxFischer 344F2 rat population. Tracks towards the metabolic theory of Ageing

OBJECTIVE

Since a reduction of the insulin/IGF-1 signaling cascade extends life span in many species and IGF-1 signaling might partly mediate the effects of caloric restriction (CR), an experimental intervention for increasing longevity, the purpose of the present study was to use quantitative trait loci (QTL) analysis, an unbiased genetic approach, to identify particular regions of the genome influencing plasma IGF-1 levels in an F2 intercross between F344 and LOU/C rats; the latter being an inbred strain of Wistar origin, considered as a model of healthy aging since it resists to age (and diet)-induced obesity.

DESIGN

F1 hybrids were obtained by crossbreeding LOU/C with F344 rats, and then F1 were bred inter se to obtain the F2 population, of which 93 males and 94 females were studied. Total plasma IGF-1 levels were determined by radioimmunoassay. A genome scan of the F2 population was made with 100 microsatellite markers) selected for their polymorphism between LOU/C and F344 strains (and by covering evenly the whole genome.

RESULTS

By simple interval mapping sex-dependent QTLs were found on chromosome 17 in males and on chromosome 18 in females. By multiple interval mapping, additional QTLs were found on chromosomes 1, 4, 5, 6, 12, 15 and 19 in males and on chromosomes 3, 5, 6, 12 and 17 in females. Only the markers D1Rat196 and D12Mgh5 were found in both males and females. The majority of QTLs corresponded to metabolic syndrome (cardiac function: n = 45 (30%), obesity/diabetes: n = 22 (15%), inflammation: n = 19 (13%) and only a limited number to body weight: n = 13 (9%), proliferation (n = 10 (7%) or ossification: n = 7 (5%). Ninety-six candidate genes were located on the different QTLs. A significant proportion of these genes are connected to IGF-1 production and receptor pathways (n = 18) or metabolic syndrome (n = 11).

CONCLUSIONS

Subsequent studies are necessary to determine whether the genetic networks underscored are also involved in age-associated obesity, diabetes and inflammation as well as cardiovascular impairments.

High-intensity endurance training results in faster vessel-specific rate of vasorelaxation in type 1 diabetic rats

This study examined the effects of 6 weeks of moderate- (MD) and high-intensity endurance training (HD) and resistance training (RD) on the vasorelaxation responsiveness of the aorta, iliac, and femoral vessels in type 1 diabetic (D) rats. Vasorelaxation to acetylcholine was modeled as a mono-exponential function. A potential mediator of vasorelaxation, endothelial nitric oxide synthase (e-NOS) was determined by Western blots. Vessel lumen-to-wall ratios were calculated from H&E stains. The vasorelaxation time-constant (τ) (s) was smaller in control (C) (7.2±3.7) compared to D (9.1±4.4) and it was smaller in HD (5.4±1.5) compared to C, D, RD (8.3±3.7) and MD (8.7±3.8) (p<0.05). The rate of vasorelaxation (%·s⁻¹) was larger in HD (2.7±1.2) compared to C (2.0±1.2), D (2.0±1.5), RD (2.0±1.0), and MD (2.0±1.2) (p<0.05). τ vasorelaxation was smaller in the femoral (6.9±3.7) and iliac (6.9±4.7) than the aorta (9.0±5.0) (p<0.05). The rate of vasorelaxation was progressively larger from the femoral (3.1±1.4) to the iliac (2.0±0.9) and to the aorta (1.3±0.5) (p<0.05). e-NOS content (% of positive control) was greater in HD (104±90) compared to C (71±64), D (85±65), RD (69±43), and MD (76±44) (p<0.05). e-NOS normalized to lumen-to-wall ratio (%·mm⁻¹) was larger in the femoral (11.7±11.1) compared to the aorta (3.2±1.9) (p<0.05). Although vasorelaxation responses were vessel-specific, high-intensity endurance training was the most effective exercise modality in restoring the diabetes-related loss of vascular responsiveness. Changes in the vasoresponsiveness seem to be endothelium-dependent as evidenced by the greater e-NOS content in HD and the greater normalized e-NOS content in the smaller vessels.

Vessel-specific rate of vasorelaxation is slower in diabetic rats

The rate of adjustment of endothelium-dependent vasorelaxation was examined in the aorta, iliac and femoral arteries of eight control and eight diabetic rats with and without supplementation with vitamin C. Vessels were constricted using 10(-5) M phenylephrine (PE) and relaxed with 10(-4) M acetylcholine (ACh condition) or 10(-4) M ACh plus 10(-4) M vitamin C (ACh + vitamin C condition) in a myography system. Vasorelaxation was modelled as a mono-exponential function using a non-linear regression analysis. The adjustment (τ) of vasorelaxation was faster in control (6.6 ± 3.2 s) compared to diabetic rats (8.4 ± 3.4 s) (p < 0.05). The time-to-steady-state tended to be shorter in control (32.0 ± 13.9 s) compared to diabetic rats (38.0 ± 15.0 s) (p = 0.1). ACh + vitamin C did not speed the vasorelaxation response. The τ for vasorelaxation was shorter in the femoral (6.5 ± 2.7 s) and iliac (6.8 ± 2.5 s) compared to the aorta (9.2 ± 4.2 s) (p < 0.05). The rate of vasorelaxation was greater in the femoral (3.2 ± 1.4%·s(-1)) compared to the iliac (2.0 ± 1.0%·s(-1)) and aorta (1.1 ± 0.4%·s(-1)) in both groups and in the iliac compared to the aorta (p < 0.05) in the control group. In conclusion, the vasorelaxation response was vessel specific with a slower rate of adjustment in diabetic compared to control animals.

Insulin and insulin-like growth factor prevent brain atrophy and cognitive impairment in diabetic rats

There are an estimated 36 million dementia patients worldwide. The anticipated tripling of this number by year 2050 will negatively impact the capacity to deliver quality health care. The epidemic in diabetes is particularly troubling, because diabetes is a substantial risk factor for dementia independently of cerebrovascular disease. There is an urgent need to elucidate the pathogenesis of progressive brain atrophy, the cause of dementia, to allow rational design of new therapeutic interventions. This review summarizes recent tests of the hypothesis that the concomitant loss of insulin and insulin-like growth factors (IGFs) is the dominant cause for age-dependent, progressive brain atrophy with degeneration and cognitive decline. These tests are the first to show that insulin and IGFs regulate adult brain mass by maintaining brain protein content. Insulin and IGF levels are reduced in diabetes, and replacement of both ligands can prevent loss of total brain protein, widespread cell degeneration, and demyelination. IGF alone prevents retinal degeneration in diabetic rats. It supports synapses and is required for learning and memory. Replacement doses in diabetic rats can cross the blood-brain barrier to prevent hippocampus-dependent memory impairment. Insulin and IGFs are protective despite unabated hyperglycemia in diabetic rats, severely restricting hyperglycemia and its consequences as dominant pathogenic causes of brain atrophy and impaired cognition. These findings have important implications for late-onset alzheimer's disease (LOAD) where diabetes is a major risk factor, and concomitant decline in insulin and IGF activity suggest a similar pathogenesis for brain atrophy and dementia.

Does the pituitary somatotrope play a primary role in regulating GH output in metabolic extremes?

Circulating growth hormone (GH) levels rise in response to nutrient deprivation and fall in states of nutrient excess. Because GH regulates carbohydrate, lipid, and protein metabolism, defining the mechanisms by which changes in metabolism alter GH secretion will aid in our understanding of the cause, progression, and treatment of metabolic diseases. This review will summarize what is currently known regarding the impact of systemic metabolic signals on GH-axis function. In addition, ongoing studies using the Cre/loxP system to generate mouse models with selective somatotrope resistance to metabolic signals will be discussed, where these models will serve to enhance our understanding of the specific role the somatotrope plays in sensing the metabolic environment and adjusting GH output in metabolic extremes.

Ileal apical Na+-dependent bile acid transporter ASBT is upregulated in rats with diabetes mellitus induced by low doses of streptozotocin

Increased intestinal bile acid absorption and expansion of the bile acid pool has been implicated in the hypercholesterolemia associated with diabetes mellitus. However, the molecular basis of the increase in bile acid absorption in diabetes mellitus is not fully understood. The ileal apical Na(+)-dependent bile acid transporter (ASBT) is primarily responsible for active reabsorption of the majority of bile acids. Current studies were designed to investigate the modulation of ASBT function and expression in streptozotocin (STZ)-induced diabetes mellitus in rats and to examine the effect of insulin on rat ASBT promoter by insulin. Diabetes mellitus was induced in Sprague-Dawley rats by intraperitoneal injection of low doses of STZ (20 mg/kg body wt) on five consecutive days. Human insulin (10 U/day) was given to a group of diabetic rats for 3 days before euthanasia. RNA and protein were extracted from mucosa isolated from the small intestine and ASBT expression was assessed by real-time quantitative RT-PCR and Western blotting. Our data showed that ASBT mRNA and protein expression were significantly elevated in diabetic rats. Insulin treatment of diabetic rats reversed the increase in ASBT protein expression to control levels. Consistently, ileal Na(+)-dependent [(3)H]taurocholic uptake in isolated intestinal epithelial cells was significantly increased in diabetic rats. In vitro studies utilizing intestinal epithelial Caco-2 cells demonstrated that ASBT expression and promoter activity were significantly decreased by insulin. These studies demonstrated that insulin directly influences ASBT expression and promoter activity and that ASBT function and expression are increased in rats with STZ-induced diabetes mellitus. The increase in ASBT expression may contribute to disturbances in cholesterol homeostasis associated with diabetes mellitus.

Evaluation of neonatally-induced mild diabetes in rats: Maternal and fetal repercussions

Many experimental studies have been performed to evaluate mild diabetes effects. However, results are divergent regarding glycemia and insulin measurement, fetal macrossomia, and placental weights. The aim was to investigate repercussions of neonatally-induced mild diabetes on the maternal organism and presence of congenital defects in their offspring in other mild diabetes model. On the day of birth, female offspring were distributed into two groups: Group streptozotocin (STZ): received 100 mg STZ/kg body weight, and Control Group: received vehicle in a similar time period. Maternal weights and glycemias were determined at days 0, 7, 14 and 21 of pregnancy. At day 21 of pregnancy, the rats were anesthetized and a laparotomy was performed to weigh and analyze living fetuses and placentas. The fetuses were classified as small (SPA), appropriate (APA) and large (LPA) for pregnancy age. Fetuses were also analyzed for the presence of external anomalies and processed for skeletal anomaly and ossification sites analysis. Statistical significance was considered as p < 0.05. In STZ group, there was increased glycemia at 0 and 14 days of pregnancy, lower weights throughout pregnancy, higher placental weight and index, an increased proportion of fetuses classified as SPA and LPA, and their fetuses presented with an increased frequency of abnormal sternebra, and absent cervical nuclei, which were not enough to cause the emergence of skeletal anomalies. Thus, this study shows that mild diabetes altered fetal development, characterized by intrauterine growth restriction. Further, the reached glycemia does not lead to any major congenital defects in the fetuses of streptozotocin-induced mild diabetic rats.

Inducible nitric oxide synthase-mediated decrease of intestinal P-glycoprotein expression under streptozotocin-induced diabetic conditions

AIMS

P-glycoprotein (P-gp), one of the important drug-efflux pumps, is known to be affected by pathological conditions such as inflammation or infection. Recently, it is reported that high glucose or hyperglycemia can alternate P-gp expression levels at the blood-brain barrier or in the kidney, although the details are still unknown. Here, we analyzed the alteration of intestinal P-gp expression and function in the development of diabetes and elucidated the mechanisms.

MAIN METHODS

Type 1 diabetes was induced in male ddY mice by an i.p. injection of streptozotocin (STZ) (230 mg/kg). We analyzed ileal P-gp expression and function using Western blot analysis and an in situ closed loop method, respectively.

KEY FINDINGS

A significant reduction of P-gp expression level in ileum was found 9 days after STZ administration. In contrast, a remarkable decrease in P-gp function was observed on the 3rd and 9th days. Interestingly, nitric oxide synthase (NOS) activity in ilea was significantly increased on the 9th day. The decrease of P-gp expression levels observed on the 9th day was completely suppressed by L-N(G)-nitroarginine methyl ester (L-NAME), a broad range NOS inhibitor, or aminoguanidine, a specific inducible NOS (iNOS) inhibitor.

SIGNIFICANCE

These results indicate the possibility that nitric oxide (NO), produced by iNOS in the ileum, is involved in the reduction of ileal P-gp expression under STZ-induced diabetic conditions.

Animal models for clinical and gestational diabetes: maternal and fetal outcomes

BACKGROUND

Diabetes in pregnant women is associated with an increased risk of maternal and neonatal morbidity and remains a significant medical challenge. Diabetes during pregnancy may be divided into clinical diabetes and gestational diabetes. Experimental models are developed with the purpose of enhancing understanding of the pathophysiological mechanisms of diseases that affect humans. With regard to diabetes in pregnancy, experimental findings from models will lead to the development of treatment strategies to maintain a normal metabolic intrauterine milieu, improving perinatal development by preventing fetal growth restriction or macrosomia. Based on animal models of diabetes during pregnancy previously reported in the medical literature, the present study aimed to compare the impact of streptozotocin-induced severe (glycemia >300 mg/dl) and mild diabetes (glycemia between 120 and 300 mg/dl) on glycemia and maternal reproductive and fetal outcomes of Wistar rats to evaluate whether the animal model reproduces the maternal and perinatal results of clinical and gestational diabetes in humans.

METHODS

On day 5 of life, 96 female Wistar rats were assigned to three experimental groups: control (n = 16), severe (n = 50) and mild diabetes (n = 30). At day 90 of life, rats were mated. On day 21 of pregnancy, rats were killed and their uterine horns were exposed to count implantation and fetus numbers to determine pre- and post-implantation loss rates. The fetuses were classified according to their birth weight.

RESULTS

Severe and mild diabetic dams showed different glycemic responses during pregnancy, impairing fetal glycemia and weight, confirming that maternal glycemia is directly associated with fetal development. Newborns from severe diabetic mothers presented growth restriction, but mild diabetic mothers were not associated with an increased rate of macrosomic fetuses.

CONCLUSION

Experimental models of severe diabetes during pregnancy reproduced maternal and fetal outcomes of pregnant women presenting uncontrolled clinical diabetes. On the other hand, the mild diabetes model caused mild hyperglycemia during pregnancy, although it was not enough to reproduce the increased rate of macrosomic fetuses seen in women with gestational diabetes.

Long-term physical training increases liver IGF-I in diabetic rats

Diabetes reduces the serum levels of insulin-like growth factor-I (IGF-I) and physical training may prevent this reduction. Almost all circulating IGF-I is produced and secreted by the liver. To examine the influence of moderate physical training on liver IGF-1 levels in diabetes, male Wistar rats were given a single dose of alloxan (30 mg/kg b.w.) to induce diabetes and then randomly allocated to sedentary or trained groups. The training protocol consisted of a 1h swimming session/day, five days/week for eight weeks with a load corresponding to 5% of the body weight. These two groups were compared with sedentary or trained non-diabetic rats (controls). A subcutaneous insulin tolerance test (ITT) was performed at the 6th week of experiment. At the end of the training period, the rats in all groups were sacrificed and blood was collected for the quantification of hematocrit and serum glucose, insulin, triglycerides, albumin, GH and IGF-1. Skeletal muscle and hepatic glycogen levels and hepatic triglyceride, protein, DNA and IGF-I concentrations were also determined. Diabetes reduced the serum insulin, GH and IGF-I concentrations, and the hepatic protein/DNA ratio and IGF-I concentrations, but increased serum glucose and triglyceride levels. Serum glucose removal during ITT was increased in the trained diabetic animals compared to sedentary control. Physical training reduced the serum glucose and triglyceride levels but increased the muscle glycogen content and restored the hepatic protein/DNA ratio and serum and hepatic IGF-I in diabetic rats. In conclusion, long-term chronic exercise improved the metabolic state and attenuated the reduction in serum and hepatic IGF-I concentrations caused by diabetes.

Diabetic cardiomyopathy-associated dysfunction in spatially distinct mitochondrial subpopulations

Diabetic cardiomyopathy is the leading cause of heart failure among diabetic patients, and mitochondrial dysfunction has been implicated as an underlying cause in the pathogenesis. Cardiac mitochondria consist of two spatially, functionally, and morphologically distinct subpopulations, termed subsarcolemmal mitochondria (SSM) and interfibrillar mitochondria (IFM). SSM are situated beneath the plasma membrane, whereas IFM are embedded between myofibrils. The goal of this study was to determine whether spatially distinct cardiac mitochondrial subpopulations respond differently to a diabetic phenotype. Swiss-Webster mice were subjected to intraperitoneal injections of streptozotocin or citrate saline vehicle. Five weeks after injections, diabetic hearts displayed decreased rates of contraction, relaxation, and left ventricular developed pressures (P < 0.05 for all three). Both mitochondrial size (forward scatter, P < 0.01) and complexity (side scatter, P < 0.01) were decreased in diabetic IFM but not diabetic SSM. Electron transport chain complex II respiration was decreased in diabetic SSM (P < 0.05) and diabetic IFM (P < 0.01), with the decrease being greater in IFM. Furthermore, IFM complex I respiration and complex III activity were decreased with diabetes (P < 0.01) but were unchanged in SSM. Superoxide production was increased only in diabetic IFM (P < 0.01). Oxidative damage to proteins and lipids, indexed through nitrotyrosine residues and lipid peroxidation, were higher in diabetic IFM (P < 0.05 and P < 0.01, respectively). The mitochondria-specific phospholipid cardiolipin was decreased in diabetic IFM (P < 0.01) but not SSM. These results indicate that diabetes mellitus imposes a greater stress on the IFM subpopulation, which is associated, in part, with increased superoxide generation and oxidative damage, resulting in morphological and functional abnormalities that may contribute to the pathogenesis of diabetic cardiomyopathy.

The characterization of high-fat diet and multiple low-dose streptozotocin induced type 2 diabetes rat model

Aim. Based on the previously established method, we developed a better and stable animal model of type 2 diabetes mellitus by high-fat diet combined with multiple low-dose STZ injections. Meanwhile, this new model was used to evaluate the antidiabetic effect of berberine. Method. Wistar male rats fed with regular chow for 4 weeks received vehicle (control groups), rats fed with high-fat diet for 4 weeks received different amounts of STZ once or twice by intraperitoneal injection (diabetic model groups), and diabetic rats were treated with berberine (100 mg/kg, berberine treatment group). Intraperitoneal glucose tolerance test and insulin tolerance test were carried out. Moreover, fasting blood glucose, fasting insulin, total cholesterol, and triglyceride were measured to evaluate the dynamic blood sugar and lipid metabolism. Result. The highest successful rate (100%) was observed in rats treated with a single injection of 45 mg/kg STZ, but the plasma insulin level of this particular group was significantly decreased, and ISI has no difference compared to control group. The successful rate of 30 mg/kg STZ twice injection group was significantly high (85%) and the rats in this group presented a typical characteristic of T2DM as insulin resistance, hyperglycemia, and blood lipid disorder. All these symptoms observed in the 30 mg/kg STZ twice injection group were recovered by the treatment of berberine. Conclusion. Together, these results indicated that high-fat diet combined with multiple low doses of STZ (30 mg/kg at weekly intervals for 2 weeks) proved to be a better way for developing a stable animal model of type 2 diabetes, and this new model may be suitable for pharmaceutical screening.

Effects of physical training on serum and pituitary growth hormone contents in diabetic rats

The present study investigated the effects of moderate physical training on some of the parameters in the GH-IGF axis in experimental diabetic rats. Male Wistar rats were allocated into the following groups: sedentary control, trained control, sedentary diabetic, trained diabetic. Diabetes was induced by alloxan (32 mg/kg, b.w. iv). The physical training protocol consisted of 1 h swimming session/day, 5 days/week for 8 weeks supporting a load corresponding to 90% of maximal lactate steady state. After the experimental period, blood was collected to measure serum glucose, insulin, triglycerides, albumin, insulin-like growth factors-I (IGF-I), and growth hormone (GH). Pituitary gland was removed for GH quantification. Diabetes increased blood glucose and triglycerides and decreased insulin, IGF-I, serum and pituitary GH. Physical training decreased glucose and triglycerides, and also counteracted the reduction of serum IGF-I in diabetic rats. In conclusion, physical training recovered serum IGF-I showing no alteration of serum or pituitary GH levels.

Role of Glucocorticoids in Fasting-induced Changes in Hypothalamic and Pituitary Components of the Growth Hormone (GH)-axis

To directly test if elevated glucocorticoids are required for fasting-induced regulation of growth hormone (GH)-releasing hormone (GHRH), GHRH receptors (GHRH-R) and ghrelin receptors (GHS-R) expression, male rats were bilaterally adrenalectomized or sham operated. After 7 days, animals were fed ad libitum or fasted for 48 h. Bilateral adrenalectomy increased hypothalamic GHRH to 146% and decreased neuropeptide Y (NPY) mRNA to 54% of SHAM controls. Pituitary GHRH-R and GHS-R mRNA levels were decreased by adrenalectomy to 30% and 80% of sham-operated controls. In shamoperated rats, fasting suppressed hypothalamic GHRH (49%) and stimulated NPY (166%) mRNA levels, while fasting increased pituitary GHRH-R (391%) and GHS-R (218%) mRNA levels. However, in adrenalectomized rats, fasting failed to alter pituitary GHRH-R mRNA levels, while the fasting-induced suppression of GHRH and elevation of NPY and GHS-R mRNA levels remained intact. In fasted adrenalectomized rats, corticosterone replacement increased GHRH-R mRNA levels and intensified the fasting-induced decrease in GHRH, but did not alter NPY or GHS-R response. These data suggest that elevated glucocorticoids mediate the effects of fasting on hypothalamic GHRH and pituitary GHRH-R expression, while glucocorticoids are likely not the major determinant in fasting-induced increases in hypothalamic NPY and pituitary GHS-R expression.

Acute effects of glucagon-like peptide-1 on hypothalamic neuropeptide and AMP activated kinase expression in fasted rats

Intracerebroventricular (icv) administration of glucagon-like peptide-1 (GLP-1) inhibits food intake and induces c-fos expression in the hypothalamus. However, the effects of GLP-1 on hypothalamic neuronal activity or neuropeptide mRNA expression are unknown. In this study, we examined the effects of GLP-1 on fasting-induced changes in the expression of hypothalamic orexigenic and anorexigenic neuropeptide. Food intake was significantly inhibited after icv injection of GLP-1 in 48 h fasted rats. Hypothalamic neuropeptide Y (NPY) and agouti-related peptide (AgRP) mRNAs were significantly increased by fasting, whereas icv GLP-1 treatment significantly attenuated these fasting-induced increases. Both proopiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) mRNA levels were decreased by fasting, while GLP-1 treatment attenuated fasting-induced decreases in POMC and CART expression. We also determined the mRNA levels of AMP-activated kinase (AMPK) and found that fasting resulted in a significant stimulation of hypothalamic AMPKalpha2 mRNA. Fasting-induced increase in AMPKalpha2 mRNA was almost completely prevented by GLP-1 treatment. Analysis of phosphorylated AMPKalpha and acetyl CoA carboxylase showed similar results. Taken together, our observation suggests that the decreased food intake by GLP-1 is caused by preventing the fasting-induced increase in hypothalamic NPY and AgRP and the fasting-induced decrease in hypothalamic POMC and CART. Our results also suggest that the food intake lowering effect of GLP-1 is caused by reversing the fasting-induced increase in hypothalamic AMPK activity. Therefore we conclude that the anorectic effect of GLP-1 seems to be mediated by, at least in part, by the hypothalamus.

Impact of energy intake and expenditure on neuronal plasticity

The Roman poet Horace was among the first to recognize that when "clogged with yesterday's excess, the body drags the mind down with it." Although considerable attention has been paid in neuroscience to the enhancement of neuronal function by wheel running and caloric restriction, far less is known about the other side of this issue. What are the consequences of unhealthy habits to central nervous system function? Prolonged exposure to excessive caloric intake impairs neuronal function and also contributes to obesity and other risk factors for diabetes. Diabetes, a disease characterized by reduced sensitivity to glucose and insulin, is also associated with deficits in brain structure and function. In contrast, enhancement of somatic metabolism by wheel running or caloric restriction improves central neuroplasticity. Generalizing across studies reveals a relationship between global metabolic efficiency and neuroplasticity in the hippocampus, a brain region that is essential for learning and memory. The specific principles upheld by these findings are suggestive of a continuum, with global metabolic alterations fluctuating in concert with neuroplasticity in the hippocampus.

Role of endogenous somatostatin in regulating GH output under basal conditions and in response to metabolic extremes

Somatostatin (SST) was first described over 30 years ago as a hypothalamic neuropeptide which inhibits GH release. Since that time a large body of literature has accumulated describing how endogenous SST mediates its effects on GH-axis function under normal conditions and in response to metabolic extremes. This review serves to summarize the key findings in this field with a focus on recent progress, much of which has been made possible by the availability of genetically engineered mouse models and SST receptor-specific agonists.

Effects of a high-glucose environment on the pituitary growth hormone-releasing hormone receptor: type 1 diabetes compared with in vitro glucotoxicity

The present study investigated the effects of diabetes and high glucose on GHRH receptor (GHRH-R) mRNA and protein levels in the pituitary of diabetic rats 2, 21, and 60 days post-streptozotocin (post-STZ) administration. Two days post-STZ, the 2.5-kb GHRH-R mRNA transcript was increased. Twenty-one days post-STZ, both the 2.5- and 4-kb transcripts and a 72-kDa (125)I-GHRH-GHRH-R complex were elevated. Sixty days post-STZ, the 4-kb transcript remained increased and the 45-kDa (125)I-GHRH-GHRH-R complex (functional receptor) was decreased. Hypothalamic GHRH mRNA and serum total IGF-I levels were reduced at all three time points. To better understand the role of high glucose on GHRH-R regulation, time-course effects of 33 compared with 6 mM d-glucose (DG) were examined in cultured anterior pituitary cells from 2-mo-old healthy rats. Membrane lipoperoxidation was present in 33 mM DG, and GHRH-R mRNA levels were diminished after 24 h, Fluo-GHRH internalization was marginal after 16-24 h, and GHRH-induced cAMP levels were decreased after 24 and 48 h. Altogether, these results indicate that the increase of the 2.5-kb GHRH-R mRNA transcript in vivo could be a consequence of a decrease of hypothalamic GHRH mRNA levels in STZ rats. Since it does not affect primarily functional GHRH-R levels, the initial diminution of circulating IGF-I levels could result from a decreased GHRH-R stimulation by GHRH. Thus, the effect of glucotoxicity would be related to a decrease of functional GHRH-R protein, as observed in rats 60 days post-STZ and in cultured pituitary cells from healthy rats exposed to a high-glucose environment.

Ghrelin improves growth hormone responses to growth hormone-releasing hormone in a streptozotocin-diabetic model of delayed onset

GH secretion is markedly altered in diabetes mellitus (DM) in both rats and humans, albeit in opposite directions. In the rat, diabetes suppresses pulsatile GH secretion, especially high amplitude pulses, and decreases GH responses to secretagogue, depending inversely on severity of metabolic alteration. In the present study, we wanted to address the GH responses to GHRH and low doses of ghrelin in a streptozotocin (STZ) model of diabetes characterized by the delayed onset of the metabolic alterations. We have shown that the administration of high doses of STZ (90 mg/kg in 0.01 M solution of chloride-sodium, ip) to five-day-old pups (n5-STZ) can induce the appearance of a characteristic diabetic syndrome in adult age, the diabetic triad, with elevated plasma glucose levels: polyuria, polydipsia, hyperphagia, and reduced body weight gain. At the age of 3 months, in these n5-STZ male and female rats the GH responses to GHRH (1 microg/kg) and GHRH combined with ghrelin (1+3 microg/kg) had diminished both in punctual times and in the area under the curve (AUC). However, the combined administration of GHRH and ghrelin, being the more potent stimulus, elicited a synergistic GH response. Thus, male and female rats with delayed onset diabetes displayed an altered GH response to GHRH, although the combined administration of GHRH and ghrelin was able to restore the GH secretion with a synergistic effect.