Use of proxy measurements of insulin sensitivity and insulin secretory response to distinguish between normal and previously laminitic ponies

Reasons for performing study: Insulin resistance may be a risk factor for pasture‐associated laminitis. Diagnosis of insulin resistance could help identify individuals at increased risk of laminitis. Objective: To calculate proxy measurements of insulin sensitivity (reciprocal of the square root of insulin: RISQI and quantitative insulin sensitivity check index: QUICKI) and insulin secretory response (modified insulin‐to‐glucose ratio: MIRG) based on basal glucose and insulin concentrations in normal (NP) and previously laminitic (PLP) ponies. Methods: Proxies were calculated in 7 NP and 5 PLP from 20 separate measurements of insulin and glucose taken in spring, summer and winter when ponies were adapted to eating either pasture or hay. Proxies were RISQI: Insulin^‐0.5, QUICKI: 1/(log[fasting Insulin]+ log[fasting Glucose]) and MIRG: (800?0.3×[Insulin‐50]²)/[Glucose‐30]. A modified insulin‐to‐glucose ratio for ponies (MIGRP) was investigated using: (3000?0.012 ×[Insulin‐500]²)/[Glucose‐30]. Statistical analysis used linear mixed models. Results: Diet did not significantly affect measurements, so values were pooled for further analysis. RISQI (mean ± s.d.) was lower in PLP (0.26 ± 0.15 [mu/l]^‐0.5) than NP (0.29 ± 0.12 [mu/l]^‐0.5; P = 0.05). QUICKI was lower in PLP (0.31 ± 0.05) than NP (0.33 ± 0.04; P = 0.047). There was no difference in MIRG between NP and PLP. MIGRP (median [interquartile range]) was greater in PLP (4.0 [7.9][mu_ins]²/10·l·mg_gluc) than NP (2.6 [3.2][mu_ins]²/10·l·mg_gluc; P = 0.022). In spring, NP had higher RISQI and QUICKI and lower MIGRP than PLP (P<0.001). In PLP, RISQI and QUICKI were higher in summer than spring (P<0.02) and MIGRP was lower in summer than other seasons (P<0.01). In NP, RISQI, QUICKI and MIGRP were each different between seasons (P<0.017). MIRG did not vary with season. Conclusions: RISQI, QUICKI and MIGRP, but not MIRG, differentiated between NP and PLP. None of the proxies accurately identified individual PLP. Seasonal changes in insulin sensitivity and insulin secretory response were apparent. Potential relevance: Current proxy measurements cannot determine an individual's laminitis susceptibility. MIGRP may be useful in hyperinsulinaemic animals.     

The effect of metformin on measurements of insulin sensitivity and beta cell response in 18 horses and ponies with insulin resistance

REASONS FOR PERFORMING STUDY: Laminitis in equids is a very common debilitating disease, and insulin resistance (IR) and hyperinsulinaemia are increasingly recognised as important predisposing factors. Pharmacological modification of IR and hyperinsulinaemia might reduce the risk of laminitis. HYPOTHESIS: Metformin, a drug commonly prescribed for treatment of human IR, may also decrease IR in equids. METHODS: Eighteen horses and ponies with IR and recurrent laminitis were treated with 15 mg/kg bwt metformin per os q. 12 h. Each animal served as its own control by comparing pre- and post treatment proxies for IR, insulin sensitivity (IS) and pancreatic beta cell function while controlling for possible dietary and managemental influences on IR. RESULTS: Evidence of significantly improved IS and decreased pancreatic beta cell secretion was found following metformin treatment. The magnitude of effect was greater at earlier resampling (6-14 days) than at later times (23-220 days). Apparent subjective clinical benefits were good but less favourable than effects on IR. CONCLUSIONS: Metformin is safe and appears to increase IS in equids. POTENTIAL RELEVANCE: Metformin may be indicated as a treatment for IR in equids. Further studies are required to define appropriate selection of subjects warranting therapy, dosing schedule and pharmacokinetics.     

Prediction of incipient pasture‐associated laminitis from hyperinsulinaemia,hyperleptinaemia and generalised and localised obesity in a cohort of ponies

Reasons for performing study: The ability to predict ponies at increased risk of laminitic episodes, when exposed to nutrient dense pasture, would facilitate management to avoid disease. Objectives: To identify variables and clinically useful cut‐off values with reproducible diagnostic accuracy for the prediction of ponies that subsequently developed laminitis when exposed to nutrient dense pasture. Methods: A cohort of predominantly Welsh and Dartmoor ponies from a closed herd was evaluated in March 2006 (n = 74) and March 2007 (n = 57). Ponies were categorised as never laminitic or previously laminitic according to reported laminitic history and as clinically laminitic (CL) if laminitis was observed within 3 months following evaluation. Body condition score (BCS), cresty neck score (CNS), girth and neck circumferences (NC), withers height, blood pressure and hoof surface temperature, and plasma insulin, glucose, triglyceride, leptin, cortisol, ACTH, uric acid and TNF‐α concentrations were measured. Analysis of sensitivity, specificity and receiver operating characteristic curves was used to evaluate the diagnostic accuracy for a variable to predict CL ponies. Results: Variables with diagnostic accuracy for the prediction of CL ponies included insulin, leptin, BCS, CNS, and NC:height ratio. Specific cut‐off values of insulin (>32 mu/l), leptin (>7.3 ng/ml), BCS (≥7), CNS (≥4) and NC:height ratio (>0.71) had reproducible diagnostic accuracy for the prediction of laminitis. Combining tests did not result in higher diagnostic accuracy than individual tests of insulin or leptin during either evaluation. Conclusions: Tests of insulin and leptin concentrations and measures of generalised (BCS) and localised (CNS or NC:height ratio) obesity were beneficial in the prediction of laminitic episodes. Potential relevance: These results highlight the importance of monitoring and reducing insulin concentration, and generalised and regional obesity in ponies to reduce risk of laminitis.     

Hypertension and insulin resistance in a mixed-breed population of ponies predisposed to laminitis

OBJECTIVE: To determine the metabolic phenotype of a group of laminitis-prone ponies when at pasture in summer, compared with when at pasture in winter. ANIMALS: 40 ponies of various breeds predisposed to recurrent pasture-associated laminitis and 40 unaffected control ponies. PROCEDURES: Body condition score and size of the crest of the neck were assessed, blood samples obtained, and blood pressure measured by use of an indirect oscillometric technique, while ponies were kept on winter pasture (last week of November or beginning of December) and again on summer pasture (June). Serum insulin concentration and plasma glucose, triglyceride, uric acid, and ACTH concentrations were measured. Insulin sensitivity was calculated with proxies derived from basal serum insulin and plasma glucose concentrations. RESULTS: No significant differences were apparent between ponies predisposed to laminitis and control ponies during winter. However, in June, laminitis-prone ponies had increased serum insulin concentration and plasma triglyceride and uric acid concentrations, compared with control ponies. Also, laminitis-prone ponies were relatively insulin resistant, compared with control ponies. Mean blood pressure was significantly higher during summer in laminitis-prone ponies (median [interquartile range], 89.6 mm Hg [78.3 to 96.9 mm Hg]), compared with control ponies (76.8 mm Hg [69.4 to 85.2 mm Hg]). CONCLUSIONS AND CLINICAL RELEVANCE: Summer pastures appear to induce metabolic responses in some ponies, leading to expression of the prelaminitic phenotype, which includes hypertension as well as insulin resistance. Signs of this metabolic syndrome may not be apparent in affected ponies during periods of grazing winter pasture. Understanding this syndrome may enable improved countermeasures to be devised to prevent laminitis.     

Histopathology of insulin-induced laminitis in ponies

Reasons for performing study: Ponies with laminitis associated with insulin resistance and hyperinsulinaemia lack systemic and/or intestinal inflammatory signs, suggesting a different pathogenesis potentially reflected in differing histopathology. Objectives: To describe the histological appearance and quantify morphological changes in primary and secondary epidermal lamellae (PEL and SEL) of laminitis lesions from ponies with insulin‐induced laminitis. Methods: Equine hoof lamellar tissue was obtained from 4 control ponies and 5 ponies with laminitis induced following infusion of insulin (1036 ± 55 µU/ml) while maintaining euglycaemia for 55.4 ± 5.5 h. Sections from all 4 hooves were stained and examined by a veterinary pathologist. Measurements of lamellar length (PEL and SEL) were made in mid‐dorsal sections of the right forefeet by 2 blinded observers. Immunolabelling for calprotectin was performed using a monoclonal antibody. Results: No lesions were detected in normal ponies. Lesions detected in ponies with laminitis were variable in severity between ponies. Within ponies, SEL lesions were more severe along the axial region of PEL. Lesions included swelling, disorganisation and abnormal keratinisation of epidermal cells, increased mitotic activity and apoptosis. Separation of basement membranes was minimal. Immunostaining revealed inflammatory cells within the lamellar dermis. SEL were significantly elongated in laminitic hooves relative to controls, with the greatest elongation in those attached to abaxial and middle regions of PEL. Conclusions: Laminitis induced by prolonged infusion of insulin lacked widespread basement membrane disintegration, and increases in epidermal cellular proliferation at axial aspects were marked for this acute stage of disease. Potential relevance: Defining equine laminitis entirely in terms of separation of the basement membrane may not be appropriate for laminitis associated with hyperinsulinaemia.     

Increased plasma fructosamine concentrations in laminitic horses

Reasons for performing study: The use of plasma fructosamine concentration ([fructosamine]) as a marker of abnormal glucose homeostasis in laminitic horses has not been investigated. Hypothesis: Plasma fructosamine concentration may be higher amongst laminitic horses than normal horses; this might relate to underlying insulin resistance. Objectives: 1) To compare [fructosamine] between laminitic and normal horses. 2) To investigate associations between [fructosamine] at presentation in laminitic horses with a) single sample markers of insulin resistance and b) outcome. Methods: Plasma fructosamine concentration, fasting serum insulin concentration (insulin) and fasting plasma glucose concentration (glucose) were measured in 30 horses that presented with laminitis. Clinical details and follow‐up data were recorded. Plasma fructosamine concentration was also measured in 19 nonlaminitic control horses. Results: Laminitic horses had significantly higher mean [fructosamine] than normal horses (P<0.001). Thirteen of 30 laminitic horses had fasting hyperinsulinaemia, 2/30 had fasting hyperglycaemia. Statistically significant univariable correlations were identified between [fructosamine] and [glucose], [insulin] and the proxies RISQI and MIRG. Trends for association between [fructosamine] and negative outcome did not reach statistical significance. Conclusions and potential relevance: Increased mean [fructosamine] in laminitic horses may represent abnormal glycaemic control and [fructosamine] may become a clinically useful marker.     

Effects of a supplement containing chromium and magnesium on morphometric measurements, resting glucose, insulin concentrations and insulin sensitivity in laminitic obese horses

Reasons for performing study: Obesity and insulin resistance are risk factors for laminitis in equids and supplements containing chromium and magnesium might improve insulin sensitivity. Hypothesis: A supplement containing chromium, magnesium and other nutraceuticals would alter morphometric measurements, blood variables, and insulin sensitivity in laminitic obese horses. Methods: Twelve previously laminitic obese (body condition score ≥ 7/9) horses were randomly allocated to treatment (n = 6) and control (n = 6) groups and 2 obese horses with clinical laminitis were included in the treatment group. Treated animals received 56 g supplement with 0.25 kg oats once daily for 16 weeks. The supplement contained chromium (5 mg/day as yeast), magnesium (8.8 g/day as oxide/proteinate), and other nutraceuticals. Insulin‐modified frequently sampled i.v. glucose tolerance tests were performed with hay provided at 0, 8 and 16 weeks, and insulin sensitivity was estimated by minimal model analysis. Physical measurements were collected at the same points. Horses were not exercised. Results: Hyperinsulinaemia (>30 µu/ml) was detected in 12 of 14 horses prior to treatment. Glucose and insulin data from one mare with clinical laminitis were excluded because of persistent pain. Mean ± s.d. insulin sensitivity was 0.64 ± 0.62 × 10^?4 l/min/mu prior to treatment for the remaining 13 horses. Time and treatment × time effects were not significant for any of the variables examined, with the exception of resting insulin concentrations, which significantly increased over time (P = 0.018). Health status remained the same. Conclusions: The supplement containing chromium and magnesium evaluated in this study did not alter morphometric measurements, blood variables, resting insulin concentrations or insulin sensitivity in laminitic obese horses. Potential relevance: Additional research is required to determine the appropriate use of chromium and magnesium supplements in horses.