Nicholas Frank

Effects of diet-induced weight gain on insulin sensitivity and plasma hormone and lipid concentrations in horses

OBJECTIVE To determine the effects of diet-induced weight gain on glucose and insulin dynamics and plasma hormone and lipid concentrations in horses. ANIMALS 13 adult geldings. PROCEDURES Horses were fed 200% of their digestible energy requirements for maintenance for 16 weeks to induce weight gain. Frequently sampled IV glucose tolerance tests were performed before and after weight gain to evaluate glucose and insulin dynamics. Adiposity (assessed via condition scoring, morphometric measurements, and subcutaneous fat depth) and plasma concentrations of insulin, glucose, nonesterified fatty acids, triglycerides, and leptin were measured on a weekly or biweekly basis. RESULTS Mean + or - SD body weight increased by 20% from 440 + or - 44 kg to 526 + or - 53 kg, and body condition score (scale, 1 to 9) increased from 6 + or - 1 to 8 + or - 1. Plasma glucose, triglyceride, and nonesterified fatty acid concentrations were similar before and after weight gain. Leptin and insulin concentrations increased with weight gain. Mean + or - SD insulin sensitivity decreased by 71 + or - 28%, accompanied by a 408 + or - 201% increase in acute insulin response to glucose, which resulted in similar disposition index before and after weight gain. CONCLUSIONS AND CLINICAL RELEVANCE Diet-induced weight gain in horses occurred concurrently with decreased insulin sensitivity that was effectively compensated for by an increase in insulin secretory response. Obesity resulted in hyperinsulinemia and hyperleptinemia, compared with baseline values, but no changes in lipid concentrations were apparent. Preventing obesity is a potential strategy to help avoid insulin resistance, hyperinsulinemia, and hyperleptinemia in horses.

Metabolic syndrome—From human organ disease to laminar failure in equids

Obesity, insulin resistance, hyperinsulinemia and hypertriglyceridemia are components of an equine metabolic syndrome phenotype associated with increased laminitis risk in horses. Links between these conditions and laminitis must still be elucidated, but human medicine provides candidate mechanisms for future study, including inflammation associated with obesity, vascular compromise induced by insulin resistance, and endothelial dysfunction. Just as metabolic syndrome has been linked to cardiovascular disease in humans, studies are now required to determine the exact mechanisms responsible for the increased predisposition to laminitis observed in horses with equine metabolic syndrome.

Effects of an intravenous endotoxin challenge on glucose and insulin dynamics in horses

OBJECTIVE To evaluate the effects of endotoxin administered IV on glucose and insulin dynamics in horses. ANIMALS 16 healthy adult mares. PROCEDURES Each week of a 2-week randomized crossover study, each horse received an IV injection (duration, 30 minutes) of Escherichia coli O55:B5 lipopolysaccharide (LPS) in 60 mL of sterile saline (0.9% NaCl) solution (20 ng/kg) or sterile saline solution alone (control treatment). Frequently sampled IV glucose tolerance test procedures were performed at 24 hours before (baseline) and 24 and 48 hours after injection; glucose and insulin dynamics were assessed via minimal model analysis. RESULTS 13 of 16 horses had a clinical response to LPS, which was characterized by mild colic and leukopenia. Before treatment, mean +/- SD insulin sensitivity was 2.9 +/- 1.9 x 10(4) L x min(1) x mU(1); this significantly decreased to 0.9 +/- 0.9 x 10(4) L x min(1) x mU(1) 24 hours after treatment (69% reduction) and was 1.5 +/- 0.9 x 10(4) L x min(1) x mU(1) 48 hours after treatment. At baseline, mean +/- SD acute insulin response to glucose was 520 +/- 196 mU x min x L(1); this significantly increased to 938 +/- 620 mU x min x L(1) (80% increase) and 755 +/- 400 mU x min x L(1) (45% increase) at 24 and 48 hours after LPS treatment, respectively. CONCLUSIONS AND CLINICAL RELEVANCE Compared with baseline values, insulin sensitivity was decreased for 24 hours after IV injection of LPS, and affected horses had a compensatory pancreatic response. These disturbances in glucose and insulin dynamics may contribute to development of laminitis in horses.

Effects of long-term oral administration of levothyroxine sodium on glucose dynamics in healthy adult horses.

OBJECTIVE To determine the effects of long-term oral administration of levothyroxine sodium (L-T(4)) on glucose dynamics in adult euthyroid horses. ANIMALS 6 healthy adult mares. PROCEDURES Horses received L-T(4) (48 mg/d) orally for 48 weeks. Frequently sampled IV glucose tolerance test procedures were performed on 3 occasions (24-hour intervals) before and at 16, 32, and 48 weeks during the treatment period. Data were assessed via minimal model analysis. The repeatability of measurements was evaluated. RESULTS During treatment, body weight decreased significantly from the pretreatment value; mean +/- SD weight was 49 +/- 14 kg, 43 +/- 7 kg, and 25 +/- 18 kg less than the pretreatment value at weeks 16, 32, and 48, respectively. Compared with pretreatment findings, 1.8-, 2.4-, and 1.9-fold increases in mean insulin sensitivity (SI) were detected at weeks 16, 32, and 48, respectively; SI was negatively correlated with body weight (r = -0.42; P < 0.001). During treatment, glucose effectiveness increased and the acute insulin response to glucose decreased. Overall mean within-horse coefficients of variation were 5% and 29% for plasma glucose and serum insulin concentrations, respectively, and 33%, 26%, and 23% for SI, glucose effectiveness, and the acute insulin response to glucose, respectively. CONCLUSIONS AND CLINICAL RELEVANCE Long-term administration of L-T(4) was associated with weight loss and increased SI in adult euthyroid horses, although other factors may have confounded results. Levothyroxine sodium may be useful for the treatment of obesity and insulin resistance in horses, but further studies are required.

Association of Season and Pasture Grazing with Blood Hormone and Metabolite Concentrations in Horses with Presumed Pituitary Pars Intermedia Dysfunction

BACKGROUND Pituitary pars intermedia dysfunction (PPID) is a risk factor for pasture-associated laminitis, which follows a seasonal pattern. HYPOTHESIS Hormonal responses to season differ between PPID and unaffected horses. ANIMALS Seventeen horses aged 8-30 years (14 horses ≥ 20 years of age). METHODS Longitudinal observational study. Blood was collected monthly from August 2007 until July 2008 after pasture grazing and again after overnight stall confinement. Blood hormone and metabolite concentrations were measured and pasture grass samples were analyzed to determine carbohydrate content. Analysis of variance analysis for repeated measures was performed. RESULTS Mean ACTH concentrations varied significantly over time (P < .001), with higher concentrations detected in August, September, and October compared with November-April. Pasture × time effects were detected for glucose and insulin concentrations, with peaks observed in September. Horses were retrospectively allocated to PPID (n = 8) and control (n = 9) groups on the basis of plasma ACTH concentrations. Changes in insulin concentrations over time differed in the PPID group when compared with the control group. Insulin concentrations were positively correlated with grass carbohydrate composition. CONCLUSIONS AND CLINICAL IMPORTANCE PPID did not affect the timing or duration of the seasonal increase in ACTH concentrations, but higher values were detected in affected horses. Insulin concentrations differed between groups, but hyperinsulinemia was rarely detected. Glucose and insulin concentrations peaked in September when horses were grazing on pasture, which could be relevant to the seasonal pattern of laminitis.

Measurement of C‐peptide concentrations and responses to somatostatin, glucose infusion, and insulin resistance in horses

REASONS FOR PERFORMING STUDY Hyperinsulinaemia is detected in horses with insulin resistance (IR) and has previously been attributed to increased pancreatic insulin secretion. Connecting peptide (C-peptide) can be measured to assess pancreatic function because it is secreted in equimolar amounts with insulin and does not undergo hepatic clearance. HYPOTHESIS A human double antibody radioimmunoassay (RIA) detects C-peptide in equine serum and concentrations would reflect responses to different stimuli and conditions. METHODS A validation procedure was performed to assess the RIA. Six mature mares were selected and somatostatin administered i.v. as a primed continuous rate infusion, followed by 50 nmol human C-peptide i.v. Insulin and C-peptide concentrations were measured in horses (n = 6) undergoing an insulin-modified frequently sampled i.v. glucose tolerance test, and in horses with insulin resistance (n = 10) or normal insulin sensitivity (n = 20). RESULTS A human RIA was validated for use with equine sera. Endogenous C-peptide secretion was suppressed by somatostatin and median (range) clearance rate was 0.83 (0.15-1.61) ml/min/kg bwt. Mean + or - s.d. C-peptide-to-insulin ratio significantly (P = 0.004) decreased during the glucose tolerance test from 3.60 + or - 1.95 prior to infusion to 1.03 + or - 0.18 during the first 20 min following dextrose administration. Median C-peptide and insulin concentrations were 1.5- and 9.5-fold higher, respectively in horses with IR, compared with healthy horses. CONCLUSIONS Endogenous C-peptide secretion decreases in response to somatostatin and increases after dextrose infusion. Results suggest that relative insulin clearance decreases as pancreatic secretion increases in response to dextrose infusion. Hyperinsulinaemia in insulin resistant horses may be associated with both increased insulin secretion and decreased insulin clearance. POTENTIAL RELEVANCE Both C-peptide and insulin concentrations should be measured to assess pancreatic secretion and insulin clearance in horses.

Effects of pretreatment with dexamethasone or levothyroxine sodium on endotoxin-induced alterations in glucose and insulin dynamics in horses.

OBJECTIVE To investigate the effects of dexamethasone or levothyroxine sodium on endotoxin-induced alterations in glucose and insulin dynamics. ANIMALS 24 horses. PROCEDURES Horses were randomly allocated to 3 treatment groups and received 48 mg of levothyroxine mixed with 200 g of oats, 20 mg of dexamethasone plus oats, or oats alone (control) for 15 days, followed by IV infusion of lipopolysaccharide (20 ng/kg) while individually housed in stalls. Frequently sampled IV glucose tolerance tests were performed prior to pretreatment, after pretreatment, and 20 hours after lipopolysaccharide administration. Area under the curve for plasma glucose and serum insulin concentrations was calculated, and minimal model analyses were performed. RESULTS Significant treatment-by-time effects were detected for insulin sensitivity (SI) and area under the curve for glucose and insulin in the 15-day pretreatment period. Insulin sensitivity significantly decreased over time in all treatment groups, with the largest decrease detected in the dexamethasone group. Administration of lipopolysaccharide further decreased mean SI by 71% and 63% in the dexamethasone and control groups, respectively, but did not affect horses in the levothyroxine group. Mean SI was the lowest in the dexamethasone group, but percentage reduction was the same for dexamethasone and control groups. CONCLUSIONS AND CLINICAL RELEVANCE Insulin sensitivity decreased during the pretreatment period in all 3 groups, indicating that hospitalization affected glucose and insulin dynamics. Dexamethasone significantly lowered SI, and endotoxemia further exacerbated insulin resistance. In contrast, there was no additional effect of endotoxemia on SI in horses pretreated with levothyroxine, suggesting that this treatment prevented endotoxemia-induced insulin resistance.

Effects of endotoxaemia and carbohydrate overload on glucose and insulin dynamics and the development of laminitis in horses

REASONS FOR PERFORMING STUDY Insulin resistance (IR) is a risk factor for pasture-associated laminitis in equids and alimentary carbohydrate overload may trigger laminitis. Whether glucose metabolism responses to carbohydrate overload are more pronounced in insulin-resistant horses requires further study. HYPOTHESIS Horses pretreated with endotoxin to alter insulin sensitivity differ significantly in their glucose and insulin responses to carbohydrate overload. METHODS Horses (n=24) were divided into 3 groups. A lipopolysaccharide (LPS; n=8) group that received endotoxin as an 8 h 7.5 ng/kg bwt/h i.v. continuous rate infusion, an oligofructose (OF; n=8) group that received an infusion of saline followed by 5 g/kg bwt OF via nasogastric intubation, and a LPS/OF (n=8) group that received LPS followed 16 h later by OF. Glucose and insulin dynamics were evaluated at -24 h and 48 h using the frequently sampled i.v. glucose tolerance test and minimal model analysis. Physical examinations and haematology were performed and the severity of laminitis assessed. RESULTS Horses receiving LPS developed leucopenia and both LPS and OF induced clinical signs consistent with systemic inflammation. Insulin sensitivity significantly decreased (P < 0.001) over time, but responses did not differ significantly among groups. Time (P < 0.001) and treatment x time (P = 0.038) effects were detected for the acute insulin response to glucose, with mean values significantly increasing in LPS and LPS/OF groups, but not the OF group. Five horses in the LPS/OF group developed clinical laminitis compared with 0 and 2 horses in the LPS and OF groups, respectively. CONCLUSIONS Endotoxaemia and carbohydrate overload reduce insulin sensitivity in horses. Endotoxin pretreatment does not affect the alterations in glucose metabolism induced by carbohydrate overload. POTENTIAL RELEVANCE Insulin sensitivity decreases after carbohydrate overload in horses, which may be relevant to the development of pasture-associated laminitis.

Optimisation of the frequently sampled intravenous glucose tolerance test to reduce urinary glucose spilling in horses

REASONS FOR PERFORMING STUDY The frequently sampled i.v. glucose tolerance test (FSIGTT) is used to evaluate glucose and insulin dynamics in horses, but it has not been determined whether urinary glucose spilling (UGS) affects results. HYPOTHESIS UGS occurs in horses during the FSIGTT and this problem can be minimised by adjusting the dextrose and insulin dosages used. METHODS Six mature mares were included in this study. In the first phase, 6 FSIGTT procedures were performed in each horse to evaluate 6 different dextrose dosages. Six different insulin dosages were evaluated during the second phase of the study after administration of 300 mg/kg bwt dextrose. Area under the glucose (AUCg) and insulin (AUCi) curves were calculated and minimal model analyses performed. UGS was measured in the third and fourth phases of the study during the combined glucose insulin test and established FSIGTT. A new FSIGTT was developed and evaluated. RESULTS Positive linear effects of dextrose dosage on AUCg, AUCi and acute insulin response to glucose were detected, with AUCg reaching a plateau at doses > or =200 mg/kg bwt. Insulin dosage had an inverse linear effect on AUCg, but other values remained unaffected. UGS occurred during all 3 tests and was the highest for the established FSIGTT and the lowest for the new FSIGTT. The type of FSIGTT performed did not affect minimal model results. CONCLUSIONS Results indicate that the dextrose dosage of 300 mg/kg bwt used in the established FSIGTT is too high. UGS can be reduced by lowering the dextrose dosage to 100 mg/kg bwt. POTENTIAL RELEVANCE A new FSIGTT involving the administration of 100 mg/kg bwt dextrose followed by 20 mu/kg bwt insulin 20 min later is recommended for use in horses because this test provides adequate data for minimal model analysis while minimising UGS.

Effects of continuous or intermittent lipopolysaccharide administration for 48 hours on the systemic inflammatory response in horses

OBJECTIVE To determine whether the method of lipopolysaccharide (LPS) administration (intermittent vs continuous) affects the magnitude and duration of the systemic inflammatory response in horses and whether prolonged (48 hours) endotoxemia induces laminitis. ANIMALS 12 healthy adult horses (10 mares and 2 geldings). PROCEDURES Horses were randomly assigned to receive LPS (total dose, 80 μg; n = 4) or saline (0.9% NaCl) solution (80 mL/h; 4) via constant rate infusion or 8 bolus IV injections of LPS (10 μg, q 6 h;4) during a 48-hour period. Physical examinations were performed every 4 hours, inflammatory cytokine gene expression was determined for blood samples obtained every 8 hours, and IV glucose tolerance tests were performed. RESULTS All LPS-treated horses had signs of depression and mild colic; those signs abated as the study progressed. Administration of LPS increased expression of interleukin-1β, interleukin-6, and interleukin-8, but results were not significantly different between LPS treatment groups. Cytokine expression was significantly higher on the first day versus the second day of LPS treatment. Interleukin-1β expression was positively correlated with rectal temperature and expression of other cytokines. Glucose and insulin dynamics for both LPS groups combined did not differ significantly from those of the saline solution group. Signs of laminitis were not detected in any of the horses. CONCLUSIONS AND CLINICAL RELEVANCE Horses developed LPS tolerance within approximately 24 hours after administration was started, and the method of LPS administration did not affect the magnitude or duration of systemic inflammation. Laminitis was not induced in horses.