Characterization of Glucose Response Curves after Insulin Injection in Sensitive versus Insensitive Mares

The glucose responses to intravenous injections of a range of doses of recombinant human insulin were determined for six mares known to be insulin sensitive and six mares known to be insulin insensitive, with the goal of better characterizing the regression lines resulting from the two categories of mares. Insulin doses between 8 and 198 mU of insulin per kg of body weight (mU/kg BW) were administered intravenously between September 13 and 26, 2010, starting with 50 mU/kg BW on the first day. Higher and lower doses were administered on alternate days to obtain percentages of decreases in blood glucose concentrations between 10% and 70%. Linear regression analysis revealed that insulin-insensitive mares have glucose response curves with higher y intercepts (P = .066), less steep slopes (P = .0003), and less goodness of fit (P = .053) in addition to the expected greater dose required to produce a 50% reduction in blood glucose concentrations (ED50; P = .006), despite the similarities between their body weights and those of insulin-sensitive mares. Linear and nonlinear regression of responses to the 32, 50, and 79 mU/kg BW insulin doses with the overall estimates of ED50 and the natural log of ED50 indicated that the 50 mU/kg BW dose had the greatest coefficient of determination (>0.95). Generally, it appears that estimates of insulin sensitivity based on a single injection of insulin or on multiple injections of insulin are least variable for insulin-sensitive mares.     

Comparison of Insulin Sensitivity of Horses Adapted to Different Exercise Intensities

Diets high in concentrates and soluble carbohydrates are associated with reduced insulin sensitivity in horses. Exercise training could protect against diet-induced insulin resistance. The objective of this study was to determine the intensity of exercise training required to affect insulin sensitivity in stabled horses fed a diet high in concentrates but moderate in soluble carbohydrates. In all, 31 stabled horses underwent three different exercise regimens: turnout, light exercise, and moderate exercise, while being fed a diet containing 60% concentrate. Blood was sampled monthly and analyzed for insulin. Insulin sensitivity was assessed using basal insulin concentrations and calculated insulin sensitivity (reciprocal of the square root of insulin) and compared across months by analysis of variance with repeated measures. Insulin sensitivity (reciprocal of the square root of insulin) was higher during periods of moderate and light physical activity as compared with turnout. These results indicate that turnout alone may not be adequate to improve insulin sensitivity in horses fed high amounts of concentrate.     

Assessment of Owner and Veterinarian Awareness of Equine Insulin Dysregulation and Available Treatments in Southeastern United States

Insulin dysregulation (ID) refers collectively to horses afflicted with fasting hyperinsulinemia and insulin resistance. Although there is no current data indicating the percentage of equines afflicted with ID, it appears to be a common subject among equine enthusiasts. In order to gauge public awareness of ID in horses and its available treatments, 122 horse owners and 18 veterinarians, representing 25 states, participated in a self-administered online survey. Questions were postulated to ascertain participant's awareness of the symptoms, management techniques, current treatments and medications, and knowledge acquisition associated with ID. The study revealed more than half of participants were “moderately aware” (34%) to “very aware” (25%) of ID in horses; however, when identifying symptoms of ID, some reported symptoms of pituitary pars intermedia dysfunction as those of ID. Many owners recognized current management strategies for ID; however, actual implementation strategies did not reflect standard management methods. Treatments mentioned for ID include supplements (14% of owners), thyroid supplement Thyro-L (13% and 72%), and pergolide/Prascend (21% and 61%) of owners and veterinarians, respectively. Veterinarians recommended diet and exercise as their main management strategy. Both owners and veterinarians reported using published research and veterinarians as a main source of information. The findings of this survey suggest that although awareness and recognition of the condition is high, proper identification, treatment, and management of ID varies. Moreover, research addressing the efficacy of pergolide as a treatment for ID is lacking. Additional education and research is needed to address these shortcomings.     

Equine Metabolic Syndrome

Equine metabolic syndrome (EMS) is important because of its association with laminitis. Obesity and insulin resistance are two important components of EMS, and the underlying cause of this syndrome is likely to be enhanced metabolic efficiency. Affected horses are often referred to as “easy keepers” because they require fewer calories to maintain body condition, and enhanced metabolic efficiency is an inherent risk factor for EMS that may be genetically determined. Pony breeds, Morgan horses, and Paso Finos are predisposed to EMS, but this problem can be prevented through effective management. Overfeeding, abundant pasture grass, and inadequate exercise are risk factors that relate to modern management practices. Obesity and adiposity induce insulin resistance, and recent research suggests that this is the determinant of laminitis susceptibility in ponies. Increased plasma insulin concentrations are detected in most affected horses and ponies, so this serves as a useful screening test for EMS. Physical characteristics also should be examined because horses with EMS exhibit regional adiposity in the form of a cresty neck or abnormal adipose tissue deposits close to the tailhead. All horses with enhanced metabolic efficiency, obesity, or regional adiposity should be screened for EMS. The combined intravenous glucose−insulin test can be performed to diagnose insulin resistance in mildly affected horses and quantify insulin sensitivity. Most horses with EMS can be effectively managed by reducing caloric intake, decreasing the starch and sugar content of the diet, increasing exercise, and limiting or eliminating access to pasture, but medical therapy is warranted in select cases.     

Oral glucose leads to a differential response in glucose,insulin, and GLP-1 in lean versus obese cats

The response to oral glucose was examined in 10 obese and 9 lean age-matched, neutered cats. In all cats, oral administration of 2 g/kg glucose was followed by a prompt increase in glucose, insulin, and glucagon-like peptide (GLP)-1. There were significant differences between lean and obese cats in the areas under the curve for glucose, insulin, and GLP-1. However, the responses were variable, and a clear distinction between individual lean and obese cats was not possible. Therefore, this test cannot be recommended as a routine test to examine insulin resistance in individual cats as it is used in people. A further disadvantage for routine use is also the fact that this test requires gastric tubing for the correct administration of the glucose and associated tranquilization to minimize stress and that it was associated with development of diarrhea in 25% of the cats. GLP-1 concentrations were much lower in obese than lean cats. The low GLP-1 concentrations in obese cats might indicate a contribution of GLP-1 to the lower insulin sensitivity of obese cats, but this hypothesis needs to be further investigated.     

Periparturient insulin secretion and whole-body insulin responsiveness in dairy cows showing various forms of ketone pattern with or without puerperal metritis

To study the effect of time and different forms of hyperketonemia, with or without puerperal metritis, on insulin and glucose responses, 31 Holstein cows were subjected to glucose (GTT) and insulin tolerance tests (ITT) between 18 and 22 d before, and on days 7 and 60-70 after calving. Plasma concentrations of beta-hydroxybutyrate (BHB), nonesterified fatty acids, glucose, insulin, insulin-like growth factor I and leptin were measured from 18 d before until 70 d after calving. The revised quick insulin sensitivity index (RQUICKI) was calculated at each time point. First postpartum (PP) ovulation was monitored by milk progesterone. Based on BHB patterns and clinical findings, animals were classified as 1) Normoketonemic (NK, n = 9); 2) Transiently hyperketonemic (tHK, n = 7); 3) Continuously HK (cHK, n = 7); and 4) Continuously HK, with signs of puerperal metritis (cHK + PM, n = 6). Insulin area under the curve (AUC) and insulin response to glucose were significantly lower in the early PP period than in late-pregnancy (P < 0.001), and on day 7 after calving in cHK and cHK + PM groups compared to NK and tHK groups (P < 0.001). On day 7, insulin stimulated a decrease in plasma glucose in cHK, cHK + PMthan NK, and tHK groups. Normoketonemic cows (group 1) ovulated earlier than all other groups (P = 0.002). There was no correlation between GTT and ITT variables and the RQUICKI. Time had a significant effect on RQUICKI. Long-term hyperketonemia, especially combined with puerperal metritis, interacts with secretion of insulin and whole-body IR, and results in a significant delay in PP ovarian activity in dairy cows.     

Comparison of a 2-step insulin-response test to conventional insulin-sensitivity testing in horses

Equine insulin resistance is important because of its association with laminitis. The insulin-response test is described to diagnose insulin resistance in clinical settings. Practitioners may be reluctant to perform this test because of the time needed for the test and the fear of inducing hypoglycemia. The objective of the study was to compare a 2-step insulin-response test with a complete insulin-response test. A complete insulin-response test was performed on 6 insulin-resistant horses and 6 controls. A 2-step insulin-response test consisting of an intravenous injection of 0.1 IU/kg human insulin and blood glucose determination at 0 and 30 min after injection was performed on the same horses. Times to reach a 50% reduction of glucose baseline were compared between tests and horses. All the horses tolerated both tests well. No significant difference was observed between baseline glucose concentrations of insulin-resistant horses and controls (P = 0.09). Time to reach 50% reduction of glucose baseline for controls was not significantly different with the use of the complete insulin-response test or the 2-step test (P = 0.98). For insulin-resistant horses, the time to reach 50% reduction of glucose baseline with the use of the 2-step test was significantly longer than for controls (P = 0.004). With a cut-off time of 30 min, the 2-step test had the same characteristics as the complete test. The 2-step test provided a safe, rapid, and low-cost method to diagnose insulin resistance in horses in a clinical setting.     

Compensation for obesity-induced insulin resistance in dogs: assessment of the effects of leptin, adiponectin, and glucagon-like peptide-1 using path analysis

The hormonal mediators of obesity-induced insulin resistance and compensatory hyperinsulinemia in dogs have not been identified. Plasma samples were obtained after a 24-h fast from 104 client-owned lean, overweight, and obese dogs. Plasma glucose and insulin concentrations were used to calculate insulin sensitivity and β-cell function with the use of the homeostasis model assessment (HOMA_{insulin sensitivity} and HOMA_{β-cell function}, respectively). Path analysis with multivariable linear regression was used to identify whether fasting plasma leptin, adiponectin, or glucagon-like peptide-1 concentrations were associated with adiposity, insulin sensitivity, and basal insulin secretion. None of the dogs were hyperglycemic. In the final path model, adiposity was positively associated with leptin (P < 0.01) and glucagon-like peptide-1 (P = 0.04) concentrations. No significant total effect of adiposity on adiponectin in dogs (P = 0.24) was observed. If there is a direct effect of leptin on adiponectin, then our results indicate that this is a positive relationship, which at least partly counters a negative direct relationship between adiposity and adiponectin. Fasting plasma leptin concentration was directly negatively associated with fasting insulin sensitivity (P = 0.01) and positively associated with β-cell function (P < 0.01), but no direct association was observed between adiponectin concentration and either insulin sensitivity or β-cell function (P = 0.42 and 0.11, respectively). We conclude that dogs compensate effectively for obesity-induced insulin resistance. Fasting plasma leptin concentrations appear to be associated with obesity-associated changes in insulin sensitivity and compensatory hyperinsulinemia in naturally occurring obese dogs. Adiponectin does not appear to be involved in the pathophysiology of obesity-associated changes in insulin sensitivity.     

Influence of glucose dosage on interpretation of intravenous glucose tolerance tests in lean and obese cats

Intravenous glucose tolerance tests (IVGTTs) are used in cats and other species to assess insulin sensitivity. Several dosages have been reported but the dosage that maximally stimulates insulin secretion in cats has not been determined nor has it been compared in lean and obese animals. IVGTTs were performed in 4 lean and 4 obese spayed female cats with 5 glucose dosages: 0.3 (A), 0.5 (B), 0.8 (C), 1.0 (D). and 1.3 (E) g/kg body weight (BW). Each cat received each dosage in a random design. The glucose disposal rate was significantly different only between lean and obese cats at the highest glucose dosage. The area under the curve for insulin increased significantly among A, B, C, and D in lean and among A, B, and C in obese cats but not between D and E in lean and among C, D, and E in obese cats. Baseline insulin secretion was significantly higher (P = .03) and 1st peak insulin secretion was approximately 50% lower in obese as compared to lean cats (P = .03). Lean but not obese cats reached baseline insulin concentrations at all dosages at 120 minutes. We conclude that the glucose dosage for maximal insulin secretion is 1.0 g/ kg BW in lean and 0.8 g/kg BW in obese cats, supporting routine use of 1 g/kg BW to maximally stimulate insulin secretion regardless of body composition. Obese cats showed an abnormal insulin secretion pattern, indicating a defect in insulin secretion with obesity and insulin resistance.     

The Effect of Feeding Two or Three Meals Per Day of Either Low or High Nonstructural Carbohydrate Concentrates on Postprandial Glucose and Insulin Concentrations in Horses

Eight mature idle gelding horses (mean body weight [BW], 558 ± 45 kg) were used in a replicated 2 × 2 Latin square design study. Horses received either two or three meals per day (MPD) for 7 days, of either a high (H; 43%; 215 g/100 kg BW) or low (L; 18%; 90 g/100 kg BW) nonstructural carbohydrate (NSC) concentrate feed to achieve four treatment groups: low NSC in two MPD (L2), low NSC in three MPD (L3), high NSC in two MPD (H2), and high NSC in three MPD (H3). On day 7 of the treatments, blood was collected before (baseline) and for 5 hours after feeding the morning meal (10, 20, 30, 40, 50, 60, 90, 120, 150, 180, 210, 240, 270, and 300 minutes after feeding). Baseline insulin concentrations tended (P = .093) to be higher for horses fed high NSC than low NSC, and horses fed two MPD tended (P = .092) to have higher baseline insulin concentrations than horses fed three MPD. In addition, baseline glucose-to-insulin ratio (GIR) was higher in horses fed high NSC compared with low NSC (P < .001). Horses fed high NSC had higher area under the curve of insulin and higher peak insulin after feeding than those fed low NSC. These findings suggest that NSC content of a concentrate feed has an impact on baseline insulin and GIRs and on postprandial insulin concentrations. Meanwhile, the number (and therefore size) of MPD had fewer impacts on glucose metabolism.     

Gestational diabetes mellitus in 13 dogs

Background: There are few reports on the clinical appearance, prognosis, and risk factors for gestational diabetes mellitus (GDM) in dogs. Objective: To describe the clinical characteristics of GDM in dogs. Animals: Thirteen dogs with GDM. Methods: Retrospective study. Medical records were reviewed and owners and referring veterinarians were contacted for follow‐up information. Results: Nordic Spitz breeds (11/13 dogs) were overrepresented in the case material. Diagnosis was established at a median of 50 days after mating (range, 32–64). Median glucose concentration at diagnosis was 340 mg/dL (18.9 mmol/L) (range, 203–587). One dog was euthanized at diagnosis, 5 bitches were treated with insulin until whelping, and in 7 dogs, pregnancy was terminated within 4 days of diagnosis. One dog died after surgery. Tight glycemic control was not achieved in any of the insulin‐treated dogs during pregnancy. Diabetes mellitus (DM) resolved in 7 dogs at a median of 9 days after the end of their pregnancies and DM was permanent in 4 dogs. Puppy mortality was increased compared with offspring of healthy dams. Conclusion: This report suggests that GDM affects mainly middle‐aged bitches in the 2nd half of pregnancy with a breed predisposition toward Nordic Spitz breeds. GDM may resolve within days to weeks after pregnancy has ended. Further research is needed to investigate optimal treatment regimens for dogs with GDM and risk factors for unsuccessful outcome.     

Clinical significance of plasma mannose concentrations in healthy and diabetic dogs

Circulating levels of monosaccharides can act as a reflection of systemic glucose/ energy metabolism. Characteristic changes observed in these levels can be seen in patients with diabetes and other metabolic disorders. There have been a few reports describing the significance of mannose metabolism as an energy source under physiological and pathological conditions. However, the relationship between circulating levels of mannose and the pathophysiology of diabetes mellitus are unknown in dogs. This study examined circulating levels of mannose between healthy control and diabetic dogs and evaluated the clinical significance of mannose levels in dogs. Diabetic dogs demonstrated a higher circulating level of mannose in comparison to normal healthy control dogs. Plasma mannose was positively correlated with plasma glucose and fructosamine, respectively. Interestingly, plasma mannose levels were affected by plasma insulin levels. In the context of feeding and glucose tolerance tests, plasma mannose levels responded to changes in circulating insulin levels. Circulating plasma mannose levels decreased after feeding in both control and diabetic animals in spite of observed insulin level differences. However, when glucose tolerance tests were given, a positive correlation between mannose levels and insulin levels was observed. Therefore, plasma mannose levels obtained via glucose tolerance testing may be used as a new diagnostic method for evaluating insulin resistance or deficiency in diabetic dogs.     

Remission of diabetes mellitus in cats cannot be predicted by the arginine stimulation test

Background: Cats with diabetes mellitus frequently achieve clinical remission, suggesting residual β‐cell function. Responsiveness of β‐cells to arginine persists the longest during diabetes progression, making the intravenous arginine stimulation test (IVAST) a useful tool to assess residual insulin and glucagon secretion. Hypothesis: Diabetic cats with and without remission will have different arginine‐induced insulin or glucagon response. Animals: Seventeen cats with diabetes, 7 healthy cats. Methods: Blood samples collected on admission and during subsequent IVAST. Glucose, insulin, and glucagon were measured. Response to IVAST was assessed by calculating the insulin and glucagon area under the curve (AUC) and the AUC glucagon‐to‐insulin ratio. Diabetic cats were treated with insulin and were followed for 18 weeks. Remission was defined as normoglycemia and disappearance of clinical signs of diabetes for ≥4 weeks, without requiring insulin. Results: Seven diabetic cats (41%) achieved remission. On admission, blood glucose concentration was significantly lower in cats with remission (median, 389 mg/dL; range, 342–536 mg/dL) than in those without remission (median, 506 mg/dL; range, 266–738 mg/dL). After IVAST, diabetic cats with remission had higher AUC glucagon‐to‐insulin ratios (median, 61; range, 34–852) than did cats without remission (median, 26; range, 20–498); glucose, insulin, and glucagon AUCs were not different. Diabetic cats had lower insulin AUC than did healthy cats but comparable glucagon AUC. Conclusions and Clinical Importance: Diabetic cats with and without remission have similar arginine‐stimulated insulin secretion on admission. Although cats with remission had lower blood glucose concentrations and higher AUC glucagon‐to‐insulin ratios, large overlap between groups prevents use of these parameters in clinical practice.