Breed differences in insulin sensitivity and insulinemic responses to oral glucose in horses and ponies of moderate body condition score

Breed-related differences may occur in the innate insulin sensitivity (SI) of horses and ponies, an important factor believed to be associated with the risk of laminitis. The aim of this study was to measure the glucose and insulin responses of different breeds of horses and ponies in moderate body condition to a glucose-containing meal and to compare these responses with the indices of SI as determined by a frequently sampled intravenous glucose tolerance test (FSIGT). Eight Standardbred horses, 8 mixed-breed ponies, and 7 Andalusian-cross horses with a mean ± SEM BCS 5.0 ± 0.3 of 9 were used in this study. Each animal underwent an oral glucose tolerance test (OGTT) in which they were fed a fiber-based ration (2.0 g/kg BW) containing 1.5 g/kg BW added glucose, as well as a standard FSIGT with minimal model analysis. The glucose response variables from the OGTT were similar between groups; however, the peak insulin concentration was higher in ponies (94.1 ± 29.1 μIU/mL; P = 0.003) and Andalusians (85.3 ± 18.6; P = 0.004) than in Standardbreds (21.2 ± 3.5). The insulin area under the curve was also higher in ponies (13.5 ± 3.6 IU · min · L(-1); P = 0.009) and Andalusians (15.0 ± 2.7; P = 0.004) than in Standardbreds (3.1 ± 0.6). Insulin sensitivity, as determined by the FSIGT, was lower in Andalusians (0.99 ± 0.18 × 10(-4)/[mIU · min]) than in Standardbreds (5.43 ± 0.94; P < 0.001) and in ponies (2.12 ± 0.44; P = 0.003) than in Standardbreds. Peak insulin concentrations from the OGTT were negatively correlated with SI (P < 0.001; rs = -0.75). These results indicate that there are clear breed-related differences in the insulin responses of horses and ponies to oral and intravenous glucose. All animals were in moderate body condition, indicating that breed-related differences in insulin dynamics occurred independent of obesity.

Effects of the rate of insulin infusion during isoglycemic, hyperinsulinemic clamp procedures on measures of insulin action in healthy, mature thoroughbred mares

The objective of this study was to determine whether the rate of insulin infusion during isoglycemic hyperinsulinemic clamp procedures affected measures of insulin action, including glucose disposal and plasma non-esterified fatty acid, endothelin-1, and nitric oxide concentrations, in mature, healthy horses. Eight thoroughbred mares were studied during a 2-h hyperinsulinemic clamp procedure, conducted at each of 4 rates of insulin infusion: 0 (CON), 1.2 (LOWINS), 3 (MEDINS), and 6 (HIGHINS) mU · kg(-1) · min(-1). The infusion rate of a dextrose solution was adjusted throughout the clamp procedures to maintain blood glucose levels within 10% of baseline glucose concentrations. Plasma insulin concentrations were measured throughout the clamp procedures, and used with the rate of glucose infusion to calculate the plasma insulin concentration-to-rate of glucose infusion ratio, a measure of insulin action on glucose disposal. The rate of glucose infusion increased with rate of insulin infusion (P < 0.05). The plasma insulin concentration-to-rate of glucose infusion ratio was highest for the LOWINS treatment (P < 0.05) and decreased by 62% (P < 0.05) and 84% (P < 0.05) for the MEDINS and HIGHINS treatments, respectively. Although plasma non-esterified fatty acid concentrations were lower than baseline by t = 30 min of the clamp procedures in the LOWINS, MEDINS, and HIGHINS treatments (P < 0.05), the decline was similar for all 3 rates of insulin infusion. Jugular vein plasma nitric oxide and endothelin-1 concentrations were not affected by insulin infusion rate (P > 0.05). The data indicate that it is important to standardize insulin infusion rate if data are to be compared between hyperinsulinemic clamp studies.

Insulin dysregulation

Abnormalities of insulin metabolism include hyperinsulinaemia and insulin resistance, and these problems are collectively referred to as insulin dysregulation in this review. Insulin dysregulation is a key component of equine metabolic syndrome: a collection of endocrine and metabolic abnormalities associated with the development of laminitis in horses, ponies and donkeys. Insulin dysregulation can also accompany prematurity and systemic illness in foals. Causes of insulin resistance are discussed, including pathological conditions of obesity, systemic inflammation and pituitary pars intermedia dysfunction, as well as the physiological responses to stress and pregnancy. Most of the discussion of insulin dysregulation to date has focused on insulin resistance, but there is increasing interest in hyperinsulinaemia itself and insulin responses to feeding. An oral sugar test or in-feed oral glucose tolerance test can be performed to assess insulin responses to dietary carbohydrates, and these tests are now recommended for use in clinical practice. Incretin hormones are likely to play an important role in postprandial hyperinsulinaemia and are the subject of current research. Insulin resistance exacerbates hyperinsulinaemia, and insulin sensitivity can be measured by performing a combined glucose-insulin test or i.v. insulin tolerance test. In both of these tests, exogenous insulin is administered and the rate of glucose uptake into tissues measured. Diagnosis and management of hyperinsulinaemia is recommended to reduce the risk of laminitis. The term insulin dysregulation is introduced here to refer collectively to excessive insulin responses to sugars, fasting hyperinsulinaemia and insulin resistance, which are all components of equine metabolic syndrome.

Effects of intravenous lipopolysaccharide infusion on glucose and insulin dynamics in horses with equine metabolic syndrome

OBJECTIVE

To test the hypothesis that glucose and insulin dynamics during endotoxemia differ between healthy horses and horses with equine metabolic syndrome (EMS).

ANIMALS

6 healthy adult mares and 6 horses with EMS.

PROCEDURES

Each horse randomly received an IV infusion of lipopolysaccharide (20 ng/kg [in 60 mL of sterile saline {0.9% NaCl} solution]) or saline solution, followed by the other treatment after a 7-day washout period. Baseline insulin-modified frequently sampled IV glucose tolerance tests were performed 27 hours before and then repeated at 0.5 and 21 hours after infusion. Results were assessed via minimal model analysis and area under the curve values for plasma glucose and serum insulin concentrations.

RESULTS

Lipopolysaccharide infusion decreased insulin sensitivity and increased area under the serum insulin concentration curve (treatment × time) in both healthy and EMS-affected horses, compared with findings following saline solution administration. The magnitude of increase in area under the plasma glucose curve following LPS administration was greater for the EMS-affected horses than it was for the healthy horses. Horses with EMS that received LPS or saline solution infusions had decreased insulin sensitivity over time.

CONCLUSIONS AND CLINICAL RELEVANCE

Glucose and insulin responses to endotoxemia differed between healthy horses and horses with EMS, with greater loss of glycemic control in EMS-affected horses. Horses with EMS also had greater derangements in glucose and insulin homeostasis that were potentially stress induced. It may therefore be helpful to avoid exposure of these horses to stressful situations.

High glycemic and insulinemic responses to meals affect plasma growth hormone secretory characteristics in Quarter Horse weanlings

Growth hormone is a key component of the somatotropic axis and is critical for the interplay between nutrition, regulation of metabolic functions, and subsequent processes of growth. The objective of this study was to investigate potential relations between meal feeding concentrates differing in the glycemic responses they elicit and GH secretory patterns in young growing horses. Twelve Quarter Horse weanlings (5.4 ± 0.4 mo of age) were used in a crossover design, consisting of two 21-d periods and two treatments, a high-glycemic (HG) or low-glycemic (LG) concentrate meal, fed twice daily. Horses were individually housed and fed hay ad libitum. On the final day of each period, quarter-hourly blood samples were drawn for 24 h to measure plasma glucose, insulin, non-esterified fatty acids, and GH. Growth hormone secretory characteristics were estimated with deconvolution analysis. After a meal, HG-fed horses exhibited a longer inhibition until the first pulse of GH secretion (P = 0.012). During late night hours (1:00 AM to 6:45 AM), HG horses secreted a greater amount of pulsatile GH than LG horses (P = 0.002). These differences highlight the potential relations between glycemic and insulinemic responses to meals and GH secretion. Dietary energy source and metabolic perturbations associated with feeding HG meals to young, growing horses have the potential to alter GH secretory patterns compared with LG meals. This may potentially affect the developmental pattern of various tissues in the young growing horse.

Hyperinsulinaemia increases vascular resistance and endothelin-1 expression in the equine digit

REASONS FOR PERFORMING STUDY

Insulin leads to overexpression of endothelin-1 (ET-1) in the endothelium of insulin-resistant rodents. If this is also the case in equine laminar tissue, this could explain the predisposition of insulin-resistant horses to laminitis.

OBJECTIVES

To investigate the effect of hyperinsulinaemia on metabolism and vascular resistance of the isolated equine digit in a model of extracorporeal perfusion.

STUDY DESIGN

Randomised, controlled study with interventional group, with blinded evaluation of histology results.

METHOD

After exsanguination, equine digits (n = 11) and autologous blood were collected at an abattoir. One digit served as a hyperinsulinaemic pilot limb, 5 digits were assigned to the hyperinsulinaemic perfusion (IP) group and 5 to the control perfusion (CP) group. Digits were perfused for 10 h at a defined perfusion rate of 12 ml/min/kg. After the first hour of perfusion (equilibration period), insulin was added to the reservoir of the IP digits. Perfusion pressure, glucose consumption, lactate and lactate dehydrogenase were monitored. Vascular resistance was calculated as perfusion pressure (in millimetres of mercury) in relation to the flow rate (in millilitres per minute). After perfusion, histology samples of the dorsal hoof wall (haematoxylin & eosin or periodic acid-Schiff) were evaluated. Immunohistology with a polyclonal rabbit-derived anti-endothelin antibody was used for detection of ET-1.

RESULTS

In the IP group, the mean insulin concentration in the plasma of the perfusate was 142 ± 81 μiu/ml, while insulin concentration was <3 μiu/ml in the CP group. Mean vascular resistance was significantly higher (P<0.01) in the IP group (2.04 ± 1.13 mmHg/ml/min) than in the CP group (1.31 ± 0.55 mmHg/ml/min). Histology of the IP group samples showed significantly more vessels with an open lumen, increased width of the secondary epidermal lamellae and formation of oedema. In the lamellar vessels (veins and arteries) and nerve fibres, ET-1 expression was much more prominent in the IP group than in the CP group samples.

CONCLUSIONS

Short-term hyperinsulinaemia leads to increased vascular resistance in the equine digit and increased expression of ET-1 in the laminar tissue.

Metabolic responses of horses and ponies to high and low glycaemic feeds: implications for laminitis

Equine laminitis is the painful condition resulting from disruption of the laminar bonds within the foot. Understanding the dietary triggers may facilitate strategies to prevent laminitis in susceptible animals. The purpose of this review is to examine how dietary factors may lead to insulin resistance and/or excessive insulin production from the pancreas, and why certain breeds or types of horses are more predisposed to this form of laminitis than others. Understanding these relationships will be very important when considering appropriate feeds and the dietary countermeasures necessary for preventing this condition. It is important to note the breed type when considering the likely metabolic effects of dietary carbohydrate, because there are major differences between the Thoroughbred/Standardbred type and some other breeds of horses and ponies. Ponies and certain breeds of horses produce excessive amounts of insulin in response to dietary carbohydrates and this may lead to the development of the three main features of the equine metabolic syndrome, namely obesity, insulin resistance and laminitis. Relative glycaemic index or glycaemic load may be useful in predicting peak plasma insulin (with due consideration for breed type), but carbohydrates such as starch and fructans may have particularly marked effects on insulin sensitivity. Although it is normal for ponies and certain horse breeds to be relatively insulin resistant, it may be possible to reduce the likelihood of exacerbating insulin resistance in obese animals with careful dietary modification. This may help to some extent in reducing hyperinsulinaemia and thereby reducing the risk of laminitis.

Prevalence of overconditioning in mature horses in southwest Virginia during the summer

BACKGROUND

The prevalence of obesity in horses in the eastern United States is not well documented.

OBJECTIVE

To determine body condition and risk factors for obesity in horses in Southwest Virginia during summer.

ANIMALS

A sample of 300 mature (4-20 years old), light breed horses (140 mares, 151 geldings, and 9 stallions) from the VMRCVM Equine Field Service practice equine database. The horses were from 114 farms and 138 owners.

METHODS

Horses were evaluated over a 60-day period in this cross-sectional, prospective study. A questionnaire was completed for each horse. Body condition score (BCS) was assigned using a scale of 1 (emaciated) to 9 (obese) by 2 independent scorers. Morphometric measurements included average neck circumference (ANC), girth, body length, and height at the withers. Horses were categorized based on BCS as underconditioned (BCS < 4), optimal condition (BCS 4-6), overconditioned (BCS 7), and obese condition (BCS 8-9).

RESULTS

Five horses (1.7%) were underconditioned, 142 horses (47.3%) were optimally conditioned, 97 horses (32.3%) were overconditioned, and 56 (18.7%) were obese. Estimated body weight (EBW) (r = 0.14, P = .015), body mass index (BMI) (r = 0.46, P < .001), and neck circumference to height ratio (NCHR) (r = 0.50, P = .001) increased with increasing BCS.

CONCLUSIONS AND CLINICAL IMPORTANCE

The prevalence of overconditioned and obese horses in this population was higher than reported in previous studies and indicates that obesity might be an emerging problem in horses.

Variations of plasma leptin in show horses during a work season

Leptin is an adipocytokine mainly expressed by adipose tissue. Secretion of leptin in healthy animals is closely related to fat mass and metabolic activity. The aim of this study was to investigate plasma leptin variations, in relation to nutritional and exercise parameters in adult show horses during a work season. EDTA-blood samples were taken at rest from 37 Iberian horses. Body weight, body condition score and fat percentage determined by ultrasonic measurement of rump fat thickness were measured. Plasma leptin was determined with a multi-species RIA kit. Linear mixed effects model was used to assess relationship between plasma leptin and other biological parameters. Plasma leptin concentration was <12.6 ng/ml (mean = 2.8 ± 1.6 ng/ml) and was significantly higher during training periods (p < 0.0001) (4.5 ± 1.7 ng/ml) than in show periods (2.0 ± 1.1 ng/ml), despite a significant increase (p < 0.0001) in energy intake. The body weight remained almost constant. The plasma leptin concentrations were significantly affected by exercise (p < 0.0001), body weight (p = 0.04) and BCS (p < 0.0001), but were not affected by percentage of fat. In conclusion, the marked decrease in leptin values observed during a period of intense (i.e. excessive) exercise could result from an adaptation to cumulative alterations in energy balance, to exercise per se or to a combination of both.

A 90-day adaptation to a high glycaemic diet alters postprandial lipid metabolism in non-obese horses without affecting peripheral insulin sensitivity

High glycaemic feeds are associated with the development of insulin resistance in horses. However, studies that evaluated the effect of high glycaemic feeds used horses that either ranged in body condition from lean to obese or were fed to increase body condition over a period of months; thus, the ability of high glycaemic feeds to induce insulin resistance in lean horses has not been determined. This study evaluated the insulin sensitivity of 18 lean horses fed a 10% (LO; n = 6), 20% (MED; n = 6) or 60% (HI; n = 6) non-structural carbohydrate complementary feed for 90 days. Although both the MED and HI diets increased insulinaemic responses to concentrate feeding in relation to the LO diet (p > 0.05), neither induced insulin resistance, as assessed by glucose tolerance test, following the 90-day feeding trial. Interestingly, the post-feeding suppression of plasma non-esterified fatty acids was less pronounced in HI-fed horses (p = 0.054) on days 30 and 90 of the study, potentially indicating that insulin-induced suppression of adipose tissue lipolysis was reduced. As insulin-resistant animals often have elevated plasma lipid concentrations, it is possible that altered lipid metabolism is an early event in the development of insulin resistance. The effects of high glycaemic feeds that are fed for a longer duration of time, on glucose and lipid metabolism, should be investigated further.

Plasma leptin level in hyperlipidemic mares and their newborn foals

The aim of this study was to evaluate the relationship between plasma leptin and lipid levels in breeding mares and their newborn foals. The study was conducted on 17 Polish cold-blood mares and their newborn foals. The mares were divided into two groups, according to the course of delivery. These groups were seven mares which had a normal delivery and 10 mares which had required veterinary intervention during their delivery. Blood samples were taken from the jugular vein of both the mares and their foals. The blood samples were taken within 30 min after delivery, as well as on four successive mornings directly afterwards. In obtained plasma samples, the leptin, triacylglycerols (TG) and free fatty acid (FFA) concentrations were measured. The leptin and TG levels in the plasma sampled on the day of delivery and the two following days were significantly higher in mares which received veterinary intervention during the delivery, when compared to the results obtained from the plasma of mares which had normal delivery. The results obtained from foals did not show any statistical differences. In conclusion, the conducted tests have shown that post-parturient hyperlipidemia and hyperleptinemia in mares did not influence the lipid status and plasma leptin level in newborn foals.

Equine metabolic syndrome

The concept of an equine metabolic syndrome (EMS) was first proposed in 2002. This concept has developed over time, and EMS was recently described in a consensus statement released by the American College of Veterinary Internal Medicine. In human medicine, metabolic syndrome (MetS) refers to a set of risk factors that predict the risk of cardiovascular disease, including obesity, glucose intolerance and insulin resistance (IR), dyslipidemia, microalbuminuria, and hypertension. EMS shares some of the features of MetS, including increased adiposity, hyperinsulinemia, IR, but differs in that laminitis is the primary disease of interest.

Effects of hyperinsulinemia on glucose and lipid transporter expression in insulin-sensitive horses

Plasma insulin concentrations are elevated (hyperinsulinemia) in horses with obesity-associated insulin resistance. In other species, insulin resistance is partly due to reduced levels of insulin receptor and the insulin-sensitive glucose transporter, and, in vitro, chronic hyperinsulinemic conditions reduce the expression of these proteins. Consumption of grain-based concentrate feeds results in postprandial hyperinsulinemia in horses, and adaptation to these diets is associated with insulin resistance. As such, it is possible that the repeated, chronic postprandial hyperinsulinemia associated with these diets could contribute to the development of insulin resistance. The purpose of the current study was to investigate the influence of a 6-h insulin infusion that increased plasma insulin concentrations to >1,000 mIU/L, on the expression of insulin receptor and glucose and lipid transporters in skeletal muscle and adipose tissue of lean, insulin-sensitive horses. Insulin infusion decreased transcript abundance of the glucose transporter 4 (P<0.05), glucose transporter 1 (GLUT1; P<0.09), and the insulin receptor (P<0.001) in adipose tissue, while increasing transcript abundance of GLUT1 (P<0.09) and decreasing protein abundance of the insulin receptor (P<0.09) in skeletal muscle. The acute, 6 hyperinsulinemic conditions achieved in this experiment resulted in alterations to mechanisms of glucose transport that could promote insulin resistance via reduced insulin-stimulated glucose disposal. Insulin infusion also reduced transcript abundance of the lipid transporters CD36 (P<0.001) and fatty acid transporter protein (FATP; P<0.05) in adipose tissue while increasing FATP (P<0.05) and lipoprotein lipase (P<0.01) in skeletal muscle. The reduction in adipose tissue lipid transporters could have been due to the decreased plasma lipid concentrations, whereas the increase in skeletal muscle may indicate that insulin stimulates lipid uptake into equine skeletal muscle. This report provides preliminary evidence that severe hyperinsulinemia alters glucose and lipid transporter expression that could promote an insulin-resistant state; these should be further investigated in horses consuming grain-based concentrates.

Assessment of body fat in the pony: part I. Relationships between the anatomical distribution of adipose tissue, body composition and body condition

REASONS FOR PERFORMING STUDY

Evaluation of equine body fat content is important for nutritional and clinical purposes. However, our understanding of total body fat and its regional distribution in the body is sparse. Currently, body fat evaluation relies on the subjective assessment of body condition score (BCS), which has never been validated against 'gold standard' chemical analysis or dissection measurements in ponies.

OBJECTIVES

To define the relationships between subjective (BCS), objective (morphometric) indices of body fat and 'gold standard' measurements of actual body composition.

HYPOTHESES

BCS and morphometry offer valid, noninvasive methods for determination of body fat in equids.

METHODS

Seven mature (mean ± s.e. 13 ± 3 years, 212 ± 14 kg, BCS 1.25-7/9), Welsh Mountain pony mares, destined for euthanasia (for nonresearch purposes), were used. For all ponies, body mass (BM), BCS and various morphometric measurements were recorded. Following euthanasia, all ponies were systematically dissected. Discrete white adipose tissue (WAT) depots were independently described. Gross, body chemical composition was determined by proximate analyses.

RESULTS

Total somatic soft tissues increased linearly (r(2) = 1.00), whereas body WAT content (1-26% live BM) increased exponentially (r(2) = 0.96), with BCS. WAT was equally distributed between internal and external sites in all animals irrespective of BCS. Nuchal fat was a poor predictor of total WAT (r(2) = 0.66). Periorbital WAT did not alter with BCS (r(2) = 0.01). Heart girth:withers height and ultrasonic retroperitoneal fat depth were closely associated with total, chemically-extracted lipid which comprised 1-29% live BM (r(2) = 0.91 and 0.88, respectively).

CONCLUSIONS AND POTENTIAL RELEVANCE

The exponential relationship between BCS and total body WAT/lipid suggests that BCS is unlikely to be a sensitive index of body fat for animals in moderate-obese states. Morphometric measurements (body girths and retroperitonel fat depth) may be useful to augment subjective BCS systems.

Effects of the insulin-sensitizing drug pioglitazone and lipopolysaccharide administration on insulin sensitivity in horses

BACKGROUND

Obesity and insulin resistance increase the risk of laminitis in horses. Pioglitazone (PG) is an insulin-sensitizing drug used in humans that is absorbed after oral administration to horses.

HYPOTHESIS

PG treatment will increase insulin sensitivity and transcript abundance of glucose and lipid transporters in adipose and skeletal muscle tissues.

ANIMALS

Sixteen lean, healthy horses.

METHODS

Eight horses were administered PG (1 mg/kg bodyweight PO) for 12 days before induction of insulin resistance through IV administration of lipopolysaccharide (LPS). Treated and untreated controls (CN; n = 8) were subjected to testing of peripheral insulin sensitivity and biopsies of both subcutaneous (nuchal ligament) adipose tissue and skeletal muscle before and after treatment, and 24 hours after LPS administration.

RESULTS

PG treatment did not improve basal insulin sensitivity (CNs: 1.4 ± 0.3, PG-treated: 1.9 ± 1.3; P > .4) or mitigate LPS-induced insulin resistance (CNs: 0.4 ± 0.3, PG-treated: 0.4 ± 0.3); however, transcript abundance of glucose and lipid transporters was altered in both skeletal muscle and subcutaneous adipose tissue.

CONCLUSIONS AND CLINICAL IMPORTANCE

Either a higher dose or longer treatment period might be required for physiological effects to be observed. PG is a novel therapeutic agent requiring further investigation in horses in order to determine treatment efficacy.

Dietary supplementation with short-chain fructo-oligosaccharides improves insulin sensitivity in obese horses

Obesity and insulin resistance are risk factors for laminitis in horses and ponies, and diet can play an important role in modulating these risk factors. Dietary supplementation with prebiotic fibers, such as short-chain fructo-oligosaccharides (scFOS), has resulted in improvement of insulin sensitivity in obese dogs and rodents. Thus, we hypothesized that scFOS may reduce insulin resistance in obese horses and designed a study to evaluate the effect of dietary supplementation with scFOS on insulin sensitivity. Eight mature Arabian geldings (BW = 523.0 ± 56.5 kg) with an average BCS of 8 were included in a crossover study. In each period, 4 horses were provided 45 g/d per horse of maltodextrin (control) and 4 horses received the same amount of scFOS for 6 wk, with a 3-wk washout between periods. Resting plasma concentrations of glucose, insulin, triglycerides, and leptin were measured. Minimal model analysis of a frequently sampled intravenous glucose tolerance test was used to evaluate insulin sensitivity, glucose effectiveness, acute insulin response to glucose, and disposition index. Without affecting BW and BCS, dietary supplementation with scFOS increased (P < 0.05) insulin sensitivity and reduced (P < 0.05) acute insulin response to glucose in comparison with maltodextrin but did not alter (P > 0.05) glucose effectiveness and disposition index. Resting serum insulin concentration also was reduced (P < 0.05) by scFOS supplementation but not by maltodextrin (P > 0.05). There was no effect (P > 0.05) of scFOS supplementation on plasma glucose or serum triglyceride and leptin concentrations. This study demonstrated that scFOS can moderately improve insulin sensitivity of obese horses, a finding that has potential relevance to the dietary management of obese, insulin-resistant horses at increased risk for laminitis.

Proinflammatory cytokine and chemokine gene expression profiles in subcutaneous and visceral adipose tissue depots of insulin-resistant and insulin-sensitive light breed horses

BACKGROUND

Insulin resistance has been associated with risk of laminitis in horses. Genes coding for proinflammatory cytokines and chemokines are expressed more in visceral adipose tissue than in subcutaneous adipose tissue of insulin-resistant (IR) humans and rodents.

HYPOTHESIS/OBJECTIVES

To investigate adipose depot-specific cytokine and chemokine gene expression in horses and its relationship to insulin sensitivity (SI).

ANIMALS

Eleven light breed mares.

METHODS

Animals were classified as IR (SI=0.58+/-0.31x10(-4) L/min/mU; n=5) or insulin sensitive (IS; SI=2.59+/-1.21x10(-4) L/min/mU; n=6) based on results of a frequently sampled intravenous glucose tolerance test. Omental, retroperitoneal, and mesocolonic fat was collected by ventral midline celiotomy; incisional nuchal ligament and tail head adipose tissue biopsy specimens were collected concurrently. The expression of tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, IL-6, plasminogen activator inhibitor-1 (PAI-1), and monocyte chemoattractant protein-1 (MCP-1) in each depot was measured by real-time quantitative polymerase chain reaction. Data were analyzed by 2-way analysis of variance for repeated measures (P<.05).

RESULTS

No differences in TNF-alpha, IL-1beta, IL-6, PAI-1, or MCP-1 mRNA concentrations were noted between IR and IS groups for each depot. Concentrations of mRNA coding for IL-1beta (P=.0005) and IL-6 (P=.004) were significantly higher in nuchal ligament adipose tissue than in other depots.

CONCLUSIONS AND CLINICAL IMPORTANCE

These data suggest that the nuchal ligament depot has unique biological behavior in the horse and is more likely to adopt an inflammatory phenotype than other depots examined. Visceral fat may not contribute to the pathogenesis of obesity-related disorders in the horse as in other species.

Endocrinopathic laminitis: reducing the risk through diet and exercise

Certain individual animals appear predisposed to recurrent pasture-associated laminitis, but the exact mechanisms underlying their predisposition remain a fundamental question in laminitis research. The risk of endocrinopathic laminitis can be reduced if steps are taken to improve insulin sensitivity and reduce inflammation using strategies based on exercise and diet. Exercise has been shown to reduce insulin resistance and suppress inflammation, and dietary manipulation can significantly affect insulin sensitivity.

Effects of exercise training on adiposity, insulin sensitivity, and plasma hormone and lipid concentrations in overweight or obese, insulin-resistant horses

OBJECTIVE

To determine effects of exercise training without dietary restriction on adiposity, basal hormone and lipid concentrations and glucose and insulin dynamics in overweight or obese, insulin-resistant horses.

ANIMALS

12 overweight or obese (body condition score > or = 7), insulin-resistant (insulin sensitivity < or = 1.2 x 10(-4) L/min/mU) geldings.

PROCEDURES

4 horses remained sedentary, and 8 horses were exercised for 4 weeks at low intensity and 4 weeks at higher intensity, followed by 2 weeks of detraining. Prior to and after each training period, frequently sampled IV glucose tolerance tests with minimal model analysis were performed and baseline plasma insulin, glucose, triglycerides, non-esterified fatty acids, and leptin concentrations were analyzed. Adiposity was assessed by use of morphometrics, ultrasonic subcutaneous fat thickness, and estimation of fat mass from total body water (deuterium dilution method).

RESULTS

Body weight and fat mass decreased by 4% (mean +/- SD, 20 +/- 8 kg) and 34% (32 +/- 9 kg), respectively, compared with pre-exercise values, with similar losses during low- and higher-intensity training. There was no effect of exercise training on subcutaneous fat thickness, plasma hormone and lipid concentrations, or minimal model parameters of glucose and insulin dynamics.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that moderate exercise training without concurrent dietary restriction does not mitigate insulin resistance in overweight or obese horses. A more pronounced reduction in adiposity or higher volume or intensity of exercise may be necessary for improvement in insulin sensitivity in such horses.