Leptin administration improves skeletal muscle insulin responsiveness in diet-induced insulin-resistant rats.

In addition to suppressing appetite, leptin may also modulate insulin secretion and action. Leptin was administered here to insulin-resistant rats to determine its effects on secretagogue-stimulated insulin release, whole body glucose disposal, and insulin-stimulated skeletal muscle glucose uptake and transport. Male Wistar rats were fed either a normal (Con) or a high-fat (HF) diet for 3 or 6 mo. HF rats were then treated with either vehicle (HF), leptin (HF-Lep, 10 mg. kg(-1). day(-1) sc), or food restriction (HF-FR) for 12-15 days. Glucose tolerance and skeletal muscle glucose uptake and transport were significantly impaired in HF compared with Con. Whole body glucose tolerance and rates of insulin-stimulated skeletal muscle glucose uptake and transport in HF-Lep were similar to those of Con and greater than those of HF and HF-FR. The insulin secretory response to either glucose or tolbutamide (a pancreatic beta-cell secretagogue) was not significantly diminished in HF-Lep. Total and plasma membrane skeletal muscle GLUT-4 protein concentrations were similar in Con and HF-Lep and greater than those in HF and HF-FR. The findings suggest that chronic leptin administration reversed a high-fat diet-induced insulin-resistant state, without compromising insulin secretion.

Calorigenic actions of leptin are additive to, but not dependent on, those of thyroid hormones.

We examined a possible mechanistic interaction between leptin and thyroid hormones in rats with hypothyroidism induced by thyroidectomy (TX) and propylthiouracil administration. In study 1, the TX rats were treated by vehicle (V, n = 9) or by recombinant murine leptin (L, 0.3 mg. kg(-1). day(-1), n = 9) or were pair-fed (PF, n = 9) against L. In study 2, the TX rats were all given 3, 3'5'-triiodo-L-thyronine (T(3)) replacement (T, 5 microg. kg(-1). day(-1)) to correct hypothyroidism. They were then subdivided into three groups, namely, vehicle (T+V, n = 9), leptin (T+L, n = 10), and pair-feeding (T+PF, n = 9), similar to study 1 except for T(3) (T). Reduced food consumption and weight gain in the TX rats were reversed by T(3) replacement. Leptin suppressed food intake in the TX rats regardless of T(3) replacement. O(2) consumption (VO(2)) and CO(2) production (VCO(2)) were reduced in TX rats (P < 0.05 vs. normal) but were normalized by either T(3) or leptin treatment. T+L additively increased VO(2) and VCO(2) (P < 0.05 vs. TX, T(3), and L). The respiratory exchange ratio was unaltered in TX rats, with and without T(3), but was significantly reduced by L or T+L treatments. These results indicate that the metabolic actions of leptin are not dependent on a normal thyroid status and that the effects of leptin and T(3) on oxidative metabolism are additive.

Leptin restores euglycemia and normalizes glucose turnover in insulin-deficient diabetes in the rat.

Leptin has been shown to improve insulin sensitivity and glucose metabolism in normoinsulinemic healthy or obese rodents. It has not been determined whether leptin may act independently of insulin in regulating energy metabolism in vivo. The present study was designed to examine the effects of leptin treatment alone on glucose metabolism in insulin-deficient streptozotocin (STZ)-induced diabetic rats. Four groups of STZ-induced diabetic rats were studied: 1) rats treated with recombinant methionine murine leptin subcutaneous infusion with osmotic pumps for 12-14 days (LEP; 4 mg x kg(-1) x day(-1), n = 10); 2) control rats infused with vehicle (phosphate-buffered saline) for 12-14 days (VEH; n = 10); 3) pair-fed control rats given a daily food ration matching that of LEP rats for 12-14 days (PF; n = 8); and 4) rats treated with subcutaneous phloridzin for 4 days (PLZ; 0.4 g/kg twice daily, n = 10). Phloridzin treatment normalizes blood glucose without insulin and was used as a control for the effect of leptin in correcting hyperglycemia. All animals were then studied with a hyperinsulinemic-euglycemic clamp (6 mU x kg(-1) x min(-1). Our study demonstrates that leptin treatment in the insulin-deficient diabetic rats restored euglycemia, minimized body weight loss due to food restriction, substantially improved glucose metabolic rates during the postabsorptive state, and restored insulin sensitivities at the levels of the liver and the peripheral tissues during the glucose clamp. The effects on glucose turnover are largely independent of food restriction and changes in blood glucose concentration, as evidenced by the minimal improvement of insulin action and glucose turnover parameters in the PF and PLZ groups. Our results suggest that the antidiabetic effects of leptin are achieved through both an insulin-independent and an insulin-sensitizing mechanism.

Differential effects of leptin in regulation of tissue glucose utilization in vivo.

We have recently shown that leptin enhances systemic insulin sensitivity and whole body glucose utilization in the rat. This study examines our hypothesis that leptin has differential effects in regulating glucose utilization among the tissues, i.e. stimulating glucose utilization in brown adipose tissue (BAT) and skeletal muscle but suppressing glucose utilization in white adipose tissue (WAT) in normal male rats (275-350 g BW). The rats were treated with s.c. infusion of recombinant murine leptin (4 mg/kg x day) or vehicle (V) with Alzet osmotic pumps or with vehicle and pair-feeding (PF) for 7 days. Leptin significantly decreased food intake (leptin, 11.5 +/- 0.4 g/day; V, 16.8 +/- 1.5 g/day; P < 0.05) and body weight (maximum change, 5.0 +/- 0.2%; P < 0.05 vs. V) and lowered plasma triglyceride, insulin, and glucose levels, but raised beta-hydroxybutyrate levels. Glucose utilization by individual tissues was determined with an i.v. bolus of [1-(14)C]2-deoxyglucose (2-DG) after a 90-min hyperinsulinemic (2 mU/kg x min) euglycemic clamp. With leptin treatment, the 2-DG-determined glucose utilization in interscapular BAT was almost 3-fold that in V-treated rats and 70% greater than that in PF rats. In contrast, in the epididymal WAT, glucose utilization was reduced by leptin treatment to only 34% that in V-treated rats and 45% that in PF rats. Leptin increased 2-DG uptake by extensor digitorum longus muscle and soleus muscle compared with that in the V and PF groups. With leptin treatment, the GLUT4 glucose transporter mRNA and protein levels were increased in BAT, but decreased in WAT (both P < 0.05). There was no significant change in GLUT4 mRNA and protein expression in extensor digitorum longus muscle and soleus muscle. Oxygen consumption was significantly increased (32.1 +/- 7.4%) in BAT (139.0 +/- 8.2 nmole O2/30 min x 10(6) cells) of leptin-treated rats vs. that in V control rats (105.3 +/- 6.7 nmole O2/30 min x 10(6) cells). In conclusion, leptin has differential, tissue-specific effects on glucose and oxygen utilization, which contribute to the reduction in whole body adiposity by enhancing energy consumption in BAT and muscle while attenuating energy storage in WAT.

Combined inhibition of interleukin-1 and tumor necrosis factor in rodent endotoxemia: improved survival and organ function.

The interleukin (IL)-1 receptor antagonist (Ra) and a polyethylene glycol-linked dimer of the type I soluble receptor of tumor necrosis factor (TNF), PEG-(rsTNF-RI)2, were used to determine whether maximal protection against lethality and organ dysfunction is achieved by single or dual cytokine inhibition under rigorous conditions of rodent endotoxemia. Inhibition of IL-1 or TNF alone protected maximally against lethality when inhibitors were given simultaneously with lipopolysaccharide (LPS) under minimal lethal conditions. Combined inhibition of IL-1 and TNF was necessary to maximally protect against lethality when treatment was delayed until 7 h after LPS injection under minimal lethal conditions or when treatment was begun immediately after LPS injection under supralethal conditions. Improved survival in IL-1Ra- plus PEG-(rsTNF-RI)2-treated rats was associated with enhanced protection against renal and metabolic dysfunctions. Thus, under very severe conditions of endotoxemia, TNF and IL-1 may act independently to mediate lethality and some organ dysfunctions.

The interleukin-1 receptor antagonist (rhIL-1ra) protects against cerebral infarction in a rat model of hypoxia-ischemia.

We assessed the cerebral protective effects of the competitive interleukin-1 antagonist rhIL-1ra in 7-day-old rats that were subjected to brain hypoxia-ischemia by unilateral carotid artery ligation and subsequent exposure to 2 h of 7.5% O2-balanced N2. This procedure leads to atrophy in the cerebral hemisphere ipsilateral to carotid occlusion, with prominent foci of neuronal infarction in the striatum. Systemic administration of 100 mg/kg of rhIL-1ra before and/or after the hypoxic exposure limited the insult. The results indicate that rhIL-1ra has potent neuroprotective properties against morphologic brain injury from hypoxia-ischemia. rhIL-1ra may prove to be clinically useful in protecting against hypoxia-ischemia-related disorders.

Detection of Mycoplasma pulmonis in arthritic joints of rats by indirect immunoperoxidase staining.

Neonatal and 8-week-old rats were inoculated with Mycoplasma pulmonis. A portion of the animals developed polyarthritis. Indirect immunoperoxidase staining was used to identify the localization of M. pulmonis within arthritic joints. M. pulmonis antigen was most often observed within cartilage in the neonatal group and in synovial tissue in the 8-week-old group.

Mycoplasma pneumoniae-induced hydrocephalus in hamsters.

Hydrocephalus was induced in neonatal hamsters after intracerebral inoculation of Mycoplasma pneumoniae. Examination of the ependyma from affected animals by electron microscopy did not reveal mycoplasma. However, in an ependymal organ culture system, M. pneumoniae cytadsorbed to ependymal cells.

Localization of Mycoplasma pulmonis in cartilage.

Transmission electron microscopy of articular cartilage in Mycoplasma pulmonis-infected neonatal rats revealed the presence of mycoplasmas within the matrix and lacunae. The mycoplasmas appeared to have a tropism for the chondrocytes and induced lysis of both the chondrocytes and matrix of the cartilage.

Cytadsorption of Mycoplasma pulmonis to rat ependyma.

Mycoplasma pulmonis was inoculated intracerebrally into neonatal rats. Hydrocephalus was induced, and the lateral ventricles and aqueduct were examined by scanning electron microscopy. Mycoplasmas were observed to be cytadsorbed to the ependymal surface.

Pathogenicity of influenza A virus in ependymal organ culture.

Ependymal organ culture was used as a model to study the effect of influenza A virus on the ciliated ependyma of the rat. One-square-milli-meter portions of cerebellum from newborn rats were harvested in roller tubes containing Ham's F-12 medium calf serum, and glutamine. Ciliary activity was monitored by stereomicroscopy and, after vigorous ciliary motion was established, tubes were inoculated with 0.1 ml of an inoculum containing 35 plaque-forming units (PFU) of neurotropic influenza A virus (WSN strain). Explants were examined by scanning electron microscopy (SEM) at 2 and 7 days and by transmission electron microscopy (TEM) at 7 days. When compared to noninfected controls, SEM showed that at 2 days the density of microvilli and cilia was decreased in influenza A virus-infected explants. At 7 days, the ependymal cells were nearly denuded of cilia and microvilli, and macrophage like cells were frequently resting upon the ependymal surface. TEM showed numerous viral particles, both budding from the cells surface and located extracellularly near the cell surface.

A new model of congenital hydrocephalus in the rat.

Hydrocephalic neonates were observed in a small breeding colony of rats. Normal rats from this colony were obtained and brother-sister mated for seven generations. The overall prevalence of hydrocephalics was approximately 23%; however, in one subline, the prevalence approached 50%. Breeding data suggested the trait to be polygenic. Hydrocephalics could be detected at 1-2 days of age, and survived for 4-5 weeks. Dilatation of the ventricles was restricted to the lateral ventricles. No evidence of developmental anomalies was seen within the ventricles. Preliminary evidence suggested that the pathophysiology may be related to poorly developed veins in the periosteal-dural layers and to underdeveloped pia-arachnoid cells. The hydrocephalus was classified as being of the communicating type.

Pathogenicity of Mycoplasma pulmonis in ependymal organ culture.

Ependymal organ culture was used as a model to study the effect of Mycoplasma pulmonis on the ciliated ependyma of the rat. Reduced ciliary activity or ciliostasis occurred 48 to 72 h after infection. Scanning electron microscopy showed that numerous organisms were associated with the cytoplasmic membrane of host cells beginning at 24 h, and transmission electron microscopy indicated that M. pulmonis cytadsorbs to the cell surface. Lesions observed in the organ cultures were limited to changes at the cell surface. These changes included reduction in microvilli density, matting of cilia into bundles, collapse of the cilia onto the cell surface, deciliation of the ependymal cells, and flattening of the cell surface. The results indicated that the response of the ependymal cell to M. pulmonis is similar to that which occurrs in Mycoplasma-infected tracheal and oviduct organ culture systems.