5~15-22) HYPERLIPEMIA is a common condition of Hyperlipemia is defined as an increase in plasma triglycerides greater than 500 mg/dl.' Obesity and stressful conditions (such as pregnancy, lactation, anorexia or transportation) have been associated with fat mobilization, hypertriglyceridemia, and clinical signs of the hyperlipemia ~yndrome.*.~,'~' clinical signs of the hyThe perlipemia syndrome are nonspecific. Depression, weakness, anorexia, ataxia, and terminal recumbency may be noted.2 Obese ponies, especially those with a history of laminitis are more commonly insulin resistant than are normal ponies5 In obese humans, tissue resistance to insulin is common. Insulin resistance is defined as a state in which increased amounts of insulin are required to produce a biologic response.' In these people insulin resistance results in abnormal lipoprotein lipase activity From Department of Veterinary Clinical Sciences (Freestone and Wolfsheimer), School of Veterinary Medicine, Department of Experimental Statistics (Church and Bessin), Louisiana State University, Baton Rouge, Louisiana, and Department of Companion Animal and Species Medicine (Ford), School of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina. Supported by School of Veterinary Medicine Organized Research Funds, Louisiana State University, Baton Rouge, Louisiana Reprint requests: J. Freestone, Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803. and increased endogenous triglyceride synthesis by the liver, resulting in hypertriglyceridemia.g-" Adrenocorticotrophic hormone (ACTH) and cortisol influence lipid metabolism both directly and indirectly. Corticosteroids stimulate hormone sensitive lipase. Hormone sensitive lipase increases the mobilization of free fatty acids from adipose tissue and increases plasma triglycerides. Corticosteroids indirectly influence lipid metabolism by antagonizing the action of insulin." When insulin is functioning normally its release will increase lipoprotein lipase activity. Lipoprotein lipase activity clears the plasma of triglycerides. Therefore stress-induced increases in serum cortisol concentrations may cause hypertriglyceridemia and decreased insulin activity." In dogs, humans, and horses hyperadrenocorticism has been associated with hyperglycemia and hyperin~ulinemia.'~~~~~'~ In hyperadrenocorticoid states, the cellular insulin response is affected at the receptor or postreceptor 1 e ~ e l . I ~ Glucocorticoids have also been implicated in the development of laminitis in ponies and horses.15Hyperadrenocorticoid ponies are predisposed to developing laminitis. In one study, ponies with the most severe hyperinsulinemia were those with a history of la mini ti^.^ In this study the ponies were not screened for hyperadrenocortici~m.~ is therefore possible that increased It circulating cortisol concentrations may play a role in the FREESTONE ET AL Journal of Veterinary Internal Medicine TABLE. Physical, Selected Endocrine, and Fasting Triglyceride Characteristics of 8 Ponies I Age (YrS) Pony # Sex Weight (kgs) I07 167 135 134 130 232 200 I35 Body Score Fair Good Good Good Obese Good Fair Good IIG Indices Dex SUPP (t = 24) N N N N N N N N TG mg/d (t = 72) I 2 3 4 5 F F F F F F F M N ABN N ABN ABN ABN N N I ,060* 492 170 293 388 626 235 333 N: Normal; ABN: Abnormal: TG: plasma triglycerides after 72 hours of fasting. * Pony # I died between 48 and 72 hours samples. development of laminitis, insulin resistance, and hyperlipemia. Our objectives in this study were 1) to identify ponies with hyperinsulinemia by monitoring the insulin response following oral glucose; 2) to identify ponies with hyperadrenocorticism by the dexamethasone suppression test; 3 ) to measure total and fractional triglycerides, glucose, cortisol and insulin in normal and hyperinsulinemic ponies during a 72-hour acute feed withdrawal period; and 4) to measure the changes in glucose, insulin, cortisol, and triglycerides following dexamethasone administration at time 72 hours during 120 hours of fasting. Materials and Methods Endocrine Evaluation Oral Glucose Tolerance Test Ponies Eight ponies, consisting of seven females and one male, with a mean age of 5.7 _t 3.5 yrs and weight of 155 -t 41.9 kgs were studied. The studies were done during December and January. All ponies were normal on physical exam. The hemograms were normal, and they were negative for equine infectious anemia. The ponies were wormed with fenbendazole 5 mg/kg orally before the start of the study. The poniesâ body condition was subjectively scored as obese, good, and fair. Five ponies were in good condition, two were fair, and one was obese (Table 1). Ponies were kept on pasture and were supplemented by a high concentrate pelleted rationâ and grass hay. Ponies were removed from pasture before each experiment and kept in stalls. All ponies, except #7, gained weight from the start of the study ( 1 7 1.8 -+ 40.8 kgs) to the fasting trial. Two ponies ( 3 , 7) developed a respiratory infection during the study and were treated with oral trimethoprim-sulfamethazine. They recovered before endocrine evaluations and fasting. One of these ponies (#7) lost weight. A recovery period of 14 days was allowed between each endocrine study and before fasting. * Pure Pride 200 Purina Mills Inc, St Louis, MO. The insulin response to oral glucose administered at 1 g/kg as a 20% solution was measured. Feed was withheld from ponies overnight ( 1 7 hrs) before glucose administration. Serum and EDTA anticoagulated blood was collected from the jugular vein. EDTA tubes were spun immediately and plasma retrieved for glucose determination.? Serum insulin was measured by a commercially available kit$ by methods previously validated in the horse. Samples were collected at 0, 15,30,45,60,90, 120, 150, 180, and 210 minutes post-glucose administration as previously d e ~ c r i b e dNo sedatives were ad.~ ministered. Based on insulin response, ponies were classified into normal or abnormal groups by evaluating 1) the insulin to glucose (I/G) ratio at 90 minutes; 2) summing insulin and glucose concentrations at times 60,90, and 120 minutes and comparing the ratio (I/G sum); 3 ) calculating total insulin secreted (TIS); 4) determining insulin peak response (IPR) and glucose peak response from baseline (GPR); 5) calculating the insulinogenic index; and 6) comparing the total insulin secreted to total glucose ratio (TIS/GT) (Table 2). The IPR was defined as the highest concentration of insulin achieved above the baseline insulin concentration. The insulinogenic index was calculated as the ratio of the IPR to the greatest glucose increment above the fasting value.I6 Total insulin secretion (TIS) and glucose (GT) was determined by calculating the area under the curve from baseline to 2 10 minutes. Dexamethasone Suppression Testing A dexamethasone suppression test was performed on all ponies as previously described.â Ponies were removed from pasture at 9.30 A.M. and not fed until the completion of the study 29 hours later. Water was available ad libitum. Dexamethasone (0.04 mg/kg) was administered t Encore Chemistry System, Baker Instruments, Allentown, PA. $ Coat-a-Count, Diagnostic Products, Los Angeles, CA. Vol. 5 . NO. 1 , 1991 TRIGLYCERIDE AND ENDOCRINE RESPONSE IN PONIES TABLE Insulin Response to Oral Glucose (lg/kg) Administration in 8 Ponies 2. Pony ## Hyperinsulinemic 2 IPR (uIU/ml) 90.0 55.5 149.5 65.0 39.9 17.2 38.0 19.7 GPR (mg/dU I I9 Insulinogenic Index 0.76 0.62 1.5 0.77 0.32 0.22 0.34 0.16 Istl/G~o Iso-tzo/G~o-lzo Ratio 0.60 0.37 0.87 0.33 0.23 0.09 0.24 0.08 Ratio 0.54 0.37 0.79 0.38 0.19 0.09 0.22 0.10 TIS uIU/ml 492.5 304.0 769.9 345.6 284. I 109.7 191.5 128.8 GT (mg/dl) TlS/GT Ratio 5 6 Normal I 3 7 8 89 1 00 84 I26 78 I13 I25 0.45 0.32 0.69 0.36 0.22 0.10 0.20 0.1 I IM at 2.30 P.M., 5 hours later. Blood was collected from the jugular vein before and at 15, 19, and 24 hours after dexamethasone adminjstration. Triglyceride (TG) and cholesterol (CH) and the concentration of the four lipoprotein fractions (total [TI, very low density [VLDL], low density [LDL], and high density [HDL]) were measured on the heparinized plasma.5 Total cholesterol was determined using a standardized modification of the Liebermann-Burchard reaction and read spectrophotometrically at 640 nm. Total cholesterol was considered adequate to illustrate the overall cholesterol trend in this study. Triglycerides were determined by a totally enzymatic method (hydrolysis by microbial lipase) and read spectrophotometrically at 530 nm. Separation of the lipoprotein fractions was performed by ultracentrifugation. Chylomicrons were recovered from the top layer following ultracentrifugation at 100,000 RPM for 10 minutes. Plasma samples were then ultracentrifuged at 100,000 RPM for a further 2.5 hours and the supranatant recovered for VLDLs. High density lipoproteins were assessed after initial precipitation of LDL and VLDL fractions with a nonmetallic polyionic precipitating agent, moderate centrifugation, and supernatant recovery. For chylomicrons, VLDL and HDL purity was verified by single band electrophoresis. Low-density lipoproteins were derived mathematically subtracting HDL from the combined HDL-LDL fraction. Serum cortisol determinations were made by radioimmunoassay using a commercially available kit7 validated for use in the horse. Fasling Studies: Experimental Design All ponies were weighed and kept in stalls on the day At 72 hours the ponies were divided into two groups. Group 1 (4 ponies) received dexamethasone IM (0.04 mg/kg) and group 2 (3 ponies) received 0.9% saline (placebo). Each group contained two hyperinsulinemic ponies. Blood samples were then collected at 80, 88, 96, 104, 112, and 120 hours of fasting. At each sampling time (0, 24, 48, 72, 80, 88, 96, 104, 112, 120 hours) during fasting plasma triglyceride and total cholesterol, plasma glucose, serum insulin, and serum cortisols were determined. In addition cholesterol and triglyceride concentrations in each of three major lipoprotein fractions (VLDL, LDL, HDL) were determined. Statistical Analysis The data of the oral glucose tolerance test were analyzed using a repeated measures one-way analysis of variance and the Huyhn-Feldt test statistic to adjust for sphericity. A hypothesised abnormal group was compared with a normal group. Means were compared univariately using least square In the feed withholding experiment, groups were compared with a two-way repeated measures analysis of variance using a Huyhn-Feldt test statistic to adjust for sphericity.â During the first 72 hours of feed, withdrawal ponies with normal insulin responses (n = 3) were compared with hypothesised abnormal ponies (n = 4). In addition, repeated measures analysis of covariance using the Huyhn-Feldt test statistic to adjust for sphericity was performed on the drug versus placebo group during all 10 experimental times and during the six post-drug administration times (80 h-120 h). A covariate of the insulin to glucose sum was used to remove variation of hyperinsulinemic ponies. Means were compared univariately for groups within time and times within a group using least square Significance was reported at the P 5 0.05 level.â before the start of the study. A complete pelleted feed ration11 was given at midday then withdrawn 3 hours later (3 P.M., and baseline samples were collected. Blood samples were collected 24,48,72 hours postfasting. One pony (# 1 ) died between 48 and 72 hours of fasting, and was not included in the fasting data analysis. ResuIts Oral Glucose Tolerance Test 0 Lipid ultracentrifugation, Beckman Airfuge, Fullerton, CA. 7 Coat-a-Count, Diagnostic Products, Los Angeles, CA. 1) Horse Chow 200 Purina Mills Inc, St Louis, MO. Ponies resting plasma glucose concentrations (range, 66-95 mg/dl) and serum insulin concentrations (range, FREESTONE ET AL. Journal of Veterinary Internal Medicine 3.8- 1 1.2 uIU/ml) were within reference ranges. Oral glucose absorption was normal in all ponies and the plasma glucose concentration doubled over baseline within 90 minutes. Four of eight ponies were hypothesised to be hyperinsulinemic in response to the administration of glucose orally based on calculated indices (Table 2). The I/G sum used to compare the normal versus the hyperinsulinemic ponies was significantly different (P < 0.03). In addition the I/G sum of the normal versus the abnormal ponies changed across time (P < 0.0002) and the groups changed differently over time (P < 0.02). Insulin concentrations in the abnormal ponies were significantly increased versus normal ponies at times 15, 30, 45, 60, 90, 120, 150, 180, and 210 minutes (Fig 1). In the hyperinsulinemic ponies, there was a rise in peak insulin concentration over baseline of 1,18496. In contrast, the normal pony peak insulins increased 522% above fasting concentrations (Fig I). There was no significant difference between the normal and abnormal ponies for glucose concentration (Fig 2). Dexamethasone Suppression Test TIME (rnins) FIG.2. Glucose response to oral glucose administration ( 1 g/kg as a 20% solution) by hyperinsulinemic (n = 4) and normal (n = 4) ponies. Data expressed as mean SD. between the normal and hyperinsulinemic ponies for insulin, glucose, cortisol, cholesterol, or plasma triglycerides. Fasting-0 to 72 Hours: Dexamethasone vs. Placebo Baseline cortisol for all ponies ranged between 4.4-9.8 ug/dl. In all ponies, the exogenous administration of dexamethasone-suppressed serum cortisol concentration and the cortisol concentration was still suppressed by 24 hours postadministration (Table 3). There were no changes in the lipid profile after dexamethasone administration (Table 3). Fasting During 72 Hours: Normal vs. Hyperinsulinemic Ponies Fasting resulted in decreased glucose concentrations across time (P < 0.0001) and increased Total - TG (P < 0.0001), VLDL - TG (P< 0.0005), and Total - CH (P < 0.01) (Table 4). There were no significant differences *** *r : : : 9-9 hyperinsulinemic normal 25t 2 f : : TIME (mins) FIG.I . Insulin response to oral glucose administration ( 1 g/kg, as a 20% solution) by hyperinsulinemic (n = 4) and normal (n = 4)ponies. Data expressed as mean f SD. Significant differences from normal ponies: * (PI 0.05), ** (PI0.00I), *** ( P I0.0001). The withholding of feed followed by administration of either dexamethasone or a placebo at time 72 hour caused significant decreases in glucose concentration (P < 0.04) and increases in TG-T ( P < 0.000 l), VLDL-TG (P < 0.004) and total CH (P < 0.003) during the study. Glucose concentration was significantly different between the groups (P < 0.03), increasing after the administration of dexamethasone. Cortisol showed significant group (P < 0.03), group X time (P < 0.0008) and time (P < 0.04) effects, as dexamethasone decreased cortisol concentrations in the treated group (Table 5). Baseline glucose and insulin concentrations were elevated, associated with feeding 2 hours before the collection of the samples. Feed withdrawal during the first 72 hours resulted in a decline in plasma glucose and serum insulin concentration (Table 5). As glucose decreased serum insulin also decreased, while VLDL-TG was increased by 48 hours (P < 0.05) and remained increased (Table 5). Pony #I developed clinical signs of depression and ataxia between the 48- and 72-hour sampling and was found to have severe hypoglycemia (20 mg/dl) and hypertriglyceridemia (TG-T I060 mg/dl). No treatment was administered. The pony died before the 72-hour sample. Necropsy showed centrolobular congestion, diffuse periportal fatty change, hemosiderin accumulation in the liver, and catarrhal sinusitis. The liver changes were consistent with the hyperlipemia syndrome. The affected pony was in fair body condition and had a normal oral glucose tolerance test and dexamethasone suppression test (Table I). VOl. 5 . No. 1. 1991 TRIGLYCERIDE AND ENDOCRINE RESPONSE IN PONIES TABLE Alteration in Cortisol Concentration and Lipid Profile Following Dexamethasone (0.04 mg/kg, IM) in 8 Ponies Over Time 3. Time (hr after dexamethasone) Np. of Ponies 8 Cortisol (ug/dl) Triglyceride (mg/dl) Total VLDL LDL H DL Cholesterol (mg/dl) Total 6.2 f 1.8* 78.5 f 11 .O 5.5 f 3.1 15.0 f 7.9 58.9 f 8.8 17.9 f 6.1 f sd. 15 0.28 f 0.1 83.1 f 11.8 3.6 f 3.1 17.9 f 3.8 62.0 f 9.4 33.9 +_ 19 0.29 f 0.1 85.9 6.5 17.6 62.4 41.4 f 11.5 f 6.6 f 2.3 f 9.0 24 0.3 f 0.1 86.3 f 9.6 6.9 f 4.9 21.8 f 16.0 57.9 18.6 52.1 f 18.9 * Data expressed as mean Feed Withdrawal Over 120 Hours: Post-Dexamethasone Response (t 80-120) The administration of dexamethasone led to differences between the groups for only cortisol concentration (P < 0,008). Serum cortisol was suppressed after dexamethasone administration and differed between the groups at times 80 ( P < O.Ol), 88 (P < 0.001), 96 ( P < 0.001), and 104 ( P < 0.01) hours. Glucose concentration was increased over time ( P < 0.04). This increase was noted at the first sample collected 8 hours (t = 80 hours) after sample, following the administration of dexamethasone at 72 hours ( P < 0.05). The glucose concentrations for the two groups also changed differently across time ( P <0.0001). There were no differences in insulin, total triglycerides, cholesterol, or tri- glyceride fractions between the dexamethasone-treatekl and placebo group. In all ponies, serum creatinine (0.9-1.8 mg/dl) and serum urea nitrogen ( 12-25 mg/dl) concentrations remained within a reference range for the 120 hours of evaluation. Discussion In ponies, hyperlipemia occurs when stored triglycerides are mobilized from adipose tissue to increase plasma triglycerides.* When plasma triglyceride concentrations are markedly elevated, triglycerides accumulate in liver, spleen, and kidneys, leading to impaired function of these The hyperlipemia syndrome of ponies has been associated with a period of negative energy TABLE Effect of Feed Withdrawal Over 72 Hours on Glucose, Insulin, Cortisol and Lipid Profile Alterations in Ponies With Normal Insulin 4. Response and Hyperinsulinemia Time (hr after feed withdrawal) Group Normal Glucose (mg/dl) Insulin (uU/ml) Cortisol (ug/dl) Triglyceride (mg/dl) Total LDL VLDL HDL Cholesterol (mg/dl) Total H yperinsulinemic Glucose (mg/dl) Insulin (uU/ml) Cortisol (ug/dl) Triglyceride (mg/dl) Total LDL VLDL HDL Cholesterol (mg/dl) Total No. of Ponies 3 177.7 f 48.8* 34.8 f 6.0 4.3 f 0.4 20.0 zk 11.0 f 5.3 f 8.0 f 4 7.5 9.5 9.2 6.0 63.3 f 11.6 2.8 f 0.5 6.9 f 1.7 65.1 f 45.6 18.0 f 12.1 35.1 f 40.7 14.0 f 7.5 86.7 f 72.3 f 5.9 f 8.7 f 8.4 3.8 1.2 4.6 65.0 f 23.3 3.0 f 1.0 6.1 f 2.7 417.3 f 179.0 76.3 f 66.2 283.7 f 172.3 57.3 f 22.1 114.3 f 20.6 66.8 f 11.1 7.0 f 76.7 f 25.4 3.8 t 0.3 5.8 f 1.7 446.0 t 284.8 22.7 t 21.6 388.3 f 293.7 138.3 f 191.1 103.3f 27.5 68.8 f 9.9 t 7.0 t 6.6 7.2 1.2 0 24 48 72 76.0 f 9.5 115.0 f 17.3 67.3 f 71.8 6.4 f 1.0 48.0 f 27.4 26.3 f 20.4 2.0 f 1.6 21.5 f 4.8 97.3 f 7.0 204.8 f 197.1 80.8 f 97.4 54.3 f 19.2 70.3 f 96.0 104.0 & 16.2 321.5 f 112.6 67.8 f 39.2 216.3 f 57.4 37.8 f 22.2 122.8 f 15.6 449.8 t 142.9 50.8 f 69.2 367.3 f 142.8 32.0 ?c 5.4 126.3 ?c 31.6 * Data are expressed as mean f sd. N o significant differences noted between normal and hyperinsulinemia ponies. FREESTONE E l AL Journal of Veterinary Internal Medicine TABLE Sequential Glucose, Insulin, Cortisol and Lipid Profile Alterations Following Feed Withdrawal Over 120 Hours, With 5. Dexamethasone and Placebo Administration at 72 Hours in Ponies Time (hr after feed withdrawal) Group Placebo Glucose (mg/dl) Insulin (uU/ml) Cortisol (ug/dl) Triglyceride (mg/dl) Total LDL VLDL HDL Cholesterol (mg/dl) Total Dexamethasone Glucose (mg/dl) Insulin (uU/ml) Cortisol (ug/dl) No. of Ponies I33 f47* 29.8 f 2.7 24 71 f211 4.2 t 2.311 6.3 f2.3 70 f47 17 f 10 48 67 t 2311 3.6 f 1.911 5.1 f0.8 72 77 f2511 4. I fO.4// 6.3 f0.9 464 f2757 17 t-23 413 f2831 I37 f 192 88 73 t 14$11 6.2 2.411 6.4 f1.3$ 409 f 1747 I04 74 1411 5.2 f 4,111 f 1611 I20 85 t 177 10.6 5,711 f 14tll f 3.611 4.7 t 2.011 5.5 f0.7$ 525 f23311 16 t-22 157 f2071 I47 f212 I14 f20 95 247 10.2 f 6.211 f 2,711 4.4 f0.7 33 f23 I7 f 17 6.0 f1.2$ 402 f 1957 5.9 t0.1$ 47 I f2001 4.6 f0.3 480 f 1591 6. I f1.1 510 f 15511 f 1555 44 f44 229 f 1715 15 f22 347 f 1927 15 19 359 t 1687 I56 f231 25 f25 413 f 1507 32 f25 45 1 f 16911 27 t 6 122 f205 86 f611 9.5 f 7.811 6 f 9 13 f8 f 19 40 f40 15 f8 f 1807 35 f27 I09 16 f 74 141 f 183 I17 f 19 105 t 148 122 f 125 f 15 I06 f 9 115 f45 13.1 f 81 1 r 17 I00 4 I48 f 50 65 f611 f 5.011 69 f611 9.7 f 7.411 f87 f 0.211 98 f61 13.8 f 6.211 90 f41) 9.5 f 5.011 f 69.2 9. I f 4.2 202 +200 82 f96 51 f24 70 t-96 I04 f 16 f 2.0 f 1.8 1.1 f 1.411 f 2.49 4. I f 2.2 Triglyceride (mg/dl) Total LDL VLDL HDL Cholesterol (mg/dl) Total f 30 i 1347 436 1547 55 f66 349 f 1507 i 1921 368 399 t 2 1 6 ~ f 1841 15 t20 22 f22 * is81 19 t24 330 t 1607 46 f 18 41 I 15 f20 35 1 f 1467 55 f20 I26 f465 f 1 3 8 ~ t iion 22 f20 12 f 2 92 f44 258 f765 55 f 18 f 50 f 16 f 1967 314 34 1 ~ 2 1 2 1 f 1767 41 f8 I28 +42Â§ 44 t 16 126 f425 f 1021 f8 93 t8 33 f 4 135 t 305 43 f20 124 f465 54 f22 132 f405 f 125 131 f44g * Data expressed as mean f sd. Significantly different between dexamethasone and placebo groups: t ( P < 0.01), $(P < 0.000 I). Means significantly different from baseline s(P < 0.05),7 ( P < 0.01), II(P < 0.0001). balance and s t r e s ~ . Glucocorticoids (exogenous and ~~'~~~ endogenous) have been shown to produce hypertriglyceridemia by stimulating hormone-sensitive lipase and antagonizing the action of insulin.".20 In this study we tried to reproduce these predisposing factors by fasting the ponies for 120 hours and then administering dexamethasone after 72 hours of fasting. Both placebo and dexamethasone-treated groups became hypertriglyceridemia but the dexamethasone-treated ponies failed to have increased triglyceride concentrations. A single injection of dexamethasone used in this model to simulate stress was probably inadequate. It may be necessary to prolong the administration of glucocorticoids to more accurately reproduce this response. The administration of dexamethasone did, however, suppress serum cortisol concentrations and increase plasma glucose concentrations as previously reported." The principle triglyceride fraction in the ponies prefasting were LDLs in five ponies and HDLs in three ponies. Within 48 hours of fasting there was a marked Vol. 5 . NO. 1, 1991 TRIGLYCERIDE AND ENDOCRINE RESPONSE IN PONIES increase in total triglycerides due to the VLDL fraction; this has been noted in other s t ~ d i e s .The VLDL re~,~ sponse varied among individual ponies ranging from eightfold to 100-fold increases from baseline. Once increased, the VLDL-TG fraction remained increased throughout the fasting period. Total cholesterol was also increased during fasting, while LDL and HDL triglyceride fractions were unchanged. In this study, four ponies were identified as hyperinsulinemic. They had abnormally increased serum insulin concentrations following an oral glucose load. The ponies were hypothesized to by hyperinsulinemic based on I/G ratios at 90 minutes, I/G ratios from the sum of the insulin, and glucose concentrations at 60, 90, 120 minutes (I/G sum), insulinogenic index, and the total insulin and glucose secreted (Table 2). These indices were selected in an attempt to establish what is the normal insulin response to a glucose load in the pony. These mathematical indices have previously been used in the dog.â6In the four hyperinsulinemic ponies in this study fasting glucose and insulin concentrations were within normal limits, making it necessary to measure the insulin response to exogenous glucose for diagnosis. The use of these mathematical calculations is cumbersome and costly because the calculations require multiple insulin and glucose determinations. In this study, blood glucose was nearly doubled by 90 minutes in all ponies receiving an oral glucose load demonstrating normal small intestinal absorptive It would appear that in ponies with normal gastrointestinal tract absorptive function, a single insulin to glucose ratio made at 90 minutes would be a useful aid for identification of hyperinsulinemia (Table 2). No significant differences were noted between the normal and hyperinsulinemic ponies for glucose concentrations. The hyperinsulinemic ponies could control glucose concentration by increasing serum insulin concentrations. Most cases of diabetes mellitus in the horse are type S, associated with a pituitary adenoma. 12.24 In ponies with a pituitary tumor secreting excessive adrenocorticotrophic hormone (ACTH), hypercortisolemia antagonizes the action of insulin leading to hyperin~ulinernia.~~ All ponies had normal cortisol suppression after dexamethasone administration. After fasting, glucose and insulin concentrations decreased, while total triglycerides significantly increased by 48 hours. The rapid increase in plasma triglycerides demonstrated by these ponies differs from the fasting response in the adult horse, where plasma triglycerides fail to increase to high concentration^.^^ It has been proposed that the rapid development of hypertriglyceridemia in ponies is due to insulin r e s i ~ t a n c e . ~ , â ~ Insulin resistance has been documented as a cause of hypertriglyceridemia in humans.â All ponies developed hypertriglyceridemia during fasting but only four were considered to have a normal insulin response to oral glucose. It should be noted that the four hyperinsulinemic ponies in this study were not as severely hyperinsulinemic as the pony group classified as obese and laminitic ponies in Jeffcottâs s t ~ d yIt ~ possible that the four . is ponies in the hyperinsulinemic group may have been inaccurately classified even though they had statistically significant increases in insulin concentrations compared to the four normal ponies. The evaluation of a larger population of ponies is needed to definitively establish guidelines for the classification of the normal insulin response after an oral glucose load. If ponies with more severe hyperinsulinemia are evaluated, a relationship between insulin resistance and hypertriglyceridemia may be appreciated. It is of interest to note that the pony (#1) who died from the hyperlipemia syndrome was neither obese nor hyperinsulinemic. Therefore alternate possibilities for the development of hypertriglyceridemia potentially exist. Consideration should be given to the possibility that ponies may have a defect in their ability to clear the VLDL triglyceride fraction from plasma. Further studies on lipoprotein lipase function in the pathogenesis of fasting-induced hypertriglyceridemia may be enlightening. In humans a genetic hypertriglyceridemia (type IV hyperlipidemia) has been described, which bears some similarities to the hypertriglyceridemia of ponies. Type IV hyperlipidemia in humans has an adult onset and has increases in the VLDL fraction due to increased synthesis and decreased catabolism of trigly~erides.â.~~ Commonly, people with type IV hyperlipidemia are obese and diabetic.26 The possibility exists that hyperlipemia could occur in the pony from several syndromes: 1) primary diabetes and secondary hypertriglyceridemia, 2) primary hypertriglyceridemia and secondary diabetes, and 3 ) primary diabetes and primary hypertriglyceridemia, as type IV hyperlipidemia and diabetes are inherited independently in humans.*â We attempted to characterize the normal insulin response to an oral glucose load in ponies using multiple mathematical indices (Table 2). Additional work is required in the pony to further evaluate the complexities of insulin action following a glucose load and ascertain whether there is a correlation between the degree of obesity and insulin resistance in the pony.
Journal of Veterinary Internal Medicine – Wiley
Published: Jan 1, 1991
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