Association between hyperinsulinaemia and laminitis severity at the time of pituitary pars intermedia dysfunction diagnosis

BACKGROUND

Hyperinsulinaemia is the suspected component of insulin dysregulation having the strongest association with laminitis and occurs variably in equids with pituitary pars intermedia dysfunction (PPID).

OBJECTIVES

We hypothesised that magnitude of hyperinsulinaemia correlates with laminitis severity in PPID-affected equids. Furthermore, we hypothesised that owners can be unaware of chronic endocrinopathic laminitis.

STUDY DESIGN

Cross-sectional study.

METHODS

Serum insulin concentrations, owner-reported laminitis history and radiographic evidence of laminitis were determined in 38 client-owned horses and ponies with confirmed PPID. Laminitis severity was classified into four categories (normal [nonlaminitic], mild, moderate or severe laminitis) based on degree of distal phalangeal rotation. Animals were also categorised as normoinsulinaemic (<20 μU/ml), mildly hyperinsulinaemic (20-50 μU/ml) and severely hyperinsulinaemic (>50 μU/ml). One-way ANOVA, t tests and Fisher's exact tests were performed.

RESULTS

While owners reported laminitis in 37% of animals, 76% were laminitic based on study criteria (P = 0.01). Owners reported laminitis more frequently in hyperinsulinaemic vs. normoinsulinaemic animals; recognition increased with severity of hyperinsulinaemia (P = 0.03). Mean insulin concentrations were higher in equids with moderate to severe radiographic laminitis (geometric mean 74.1, 95% confidence interval (CI) 38.4-143.1 uU/ml) vs. those classified radiographically as normal to mild (31.9, 95% CI 21.1-48.1 uU/ml P = 0.03).

MAIN LIMITATIONS

Dynamic insulin testing was not performed; some normoinsulinaemic animals might have had subtle insulin dysregulation.

CONCLUSIONS

Although radiographic abnormalities were present in most animals at the time of PPID diagnosis, chronic laminitis remained unrecognised by many owners. Owner awareness of laminitis increased with severity of hyperinsulinaemia and higher insulin concentrations were detected in association with more severe radiographic changes. The Summary is available in Chinese - See Supporting Information.

Effects of age and exercise on inflammatory cytokines, HSP70 and HSP90 gene expression and protein content in Standardbred horses

We hypothesised that the cortisol response to acute exercise, markers of oxidative stress, expression of inflammatory cytokines, heat shock protein (HSP)70 and HSP90 expression in whole blood and skeletal muscle, and HSP70 and HSP90 protein concentrations in skeletal muscle are altered by age and in response to acute submaximal exercise in horses. Young (n=6; 5.5±2.8 year) and aged (n=6; 22.6±2.25 year) unconditioned Standardbred mares underwent an acute submaximal exercise test. Blood samples were collected and analysed for plasma cortisol and malondialdehyde (MDA) concentrations, and for cytokine and HSP gene expression pre- and post-exercise. Gluteus medius biopsies were obtained for analysis of cytokine and HSP gene expression pre- and at 0, 4, 24 and 48 h post-exercise. Data were analysed for main effects using a two-way ANOVA for repeated measures. Post-hoc comparisons of means were conducted using Student-Neuman-Keuls for pair-wise multiple comparisons where appropriate. Acute submaximal exercise increased plasma cortisol concentration in both young and aged mares, and the duration of the post-exercise rise in cortisol was altered in aged horses. Plasma MDA concentration and expression of tumour necrosis factor-α (TNF-α) and interleukin (IL)-6 were unchanged in blood and muscle. Exercise increased IL-1β expression in whole blood of young and aged mares, with young mares having greater exercise-induced expression at 2 (P<0.001) and 4 (P=0.019) h post-exercise. Both young and aged horses had increased HSP70 expression in whole blood following acute exercise, with young horses exhibiting 3-fold greater HSP70 expression than aged mares at 2 h post-exercise. HSP90 expression in whole blood following exercise was increased only in young horses. Both young and aged horses had increased HSP90 expression in skeletal muscle following exercise, but there was no difference due to age. However, the timing of HSP70 expression was different between young and aged horses. The age-related changes in cortisol and IL-1β expression following acute submaximal exercise can have implications for energy homeostasis and the adaption to such disturbances at a cellular and whole animal level. Quantification of HSP expression in whole blood may be a useful biomarker, with implications for cellular adaptation and survival in aged horses.

Effects of age and diet on glucose and insulin dynamics in the horse

BACKGROUND

Age and diet may affect insulin sensitivity (SI) but these factors have received limited investigation in horses.

OBJECTIVES

To measure minimal model parameters during an insulin-modified frequently sampled intravenous glucose tolerance test (FSIGTT) after adaptation to a forage only diet (HAY) or forage supplemented with either starch/sugar-rich (SS) or oil/fibre-rich (FF) concentrate feeds; and to assess glucose and insulin responses to a standardised meal challenge (SMC, 4 g/kg BW of SS) after diet adaptation in adult and aged mares.

STUDY DESIGN

Latin square design with eight adult (5-12 years) and nine aged (>19 years) healthy mares.

METHODS

Diets were fed for 6 weeks, and the FSIGTT and SMC were performed after 31-32 and 41 days on each diet respectively. Data were analysed by a mixed ANOVA for repeated measures.

RESULTS

Acute insulin response to glucose (AIRg) was greater and SI was lower in aged horses, compared with adults, regardless of diet. Both AIRg and SI were greater in aged mares after adaptation to SS, as compared with HAY. Similar trends, although not statistically significant, were observed after adaptation to SS in adult mares. Peak insulin concentration and area under the insulin vs. time curve during the SMC were greater in aged than adult mares with all diets. Furthermore, area under the glucose vs. time curve was lower after adaptation to SS, when compared with other diets, in both groups.

MAIN LIMITATIONS

Transient weight loss occurred at the beginning of the study and only one sex was included. Incomplete ingestion of the SMC by four mares was another limitation.

CONCLUSIONS

Insulin responses to i.v. and enteral nonstructural carbohydrate challenge increase with age in healthy horses, regardless of diet fed.

Effect of age and dietary carbohydrate profiles on glucose and insulin dynamics in horses

BACKGROUND

Glucose and insulin dynamics may be different in adult and aged horses.

OBJECTIVES

To determine the effects of age and dietary carbohydrates on glucose and insulin dynamics in healthy horses.

STUDY DESIGN

Balanced Latin square with four isocaloric diets: CONTROL (hay plus restricted-starch-and-sugar fortified pellets), STARCH (control plus kibbled corn), FIBER (control plus unmolassed sugar beet pulp/soybean hull pellets) and SUGAR (control plus dextrose powder).

METHODS

A total of 16 healthy Thoroughbreds and Standardbreds divided into two age groups: ADULT (8.8 ± 2.9 years; n = 8) and AGED (20.6 ± 2.1 years; n = 8). Following dietary adaptation, horses underwent an insulin-modified frequently sampled intravenous glucose tolerance test (FSIGTT), modified oral sugar test (OST) and dietary meal challenge. Outcome variables included: insulin sensitivity (SI), disposition index (DI), glucose effectiveness (Sg) and acute insulin response to glucose (AIRg) from the FSIGTT; peak glucose, peak insulin, time to peak, area under the curve for glucose (AUCg) and insulin (AUCi) from the OST and dietary meal challenge. Data were analyzed using multivariable linear mixed regression modelling.

RESULTS

AIRg was higher in AGED (mean [95% confidence interval]; 582.0 [455.0-709.0]) vs. ADULT (358.0 [224.0-491.0]; P = 0.03). ADULT and AGED horses had a higher SI on STARCH (adult: 3.3 [2.3-4.2]; aged: 2.8 [1.9-3.7]) and SUGAR (adult: 3.4 [2.5-4.3]; aged: 4.0 [3.1-4.9]) diets compared with CONTROL (adult: 2.0 [1.1-2.9], P = 0.029 (starch), P = 0.009 (sugar); aged: 1.4 [0.5-2.2], P = 0.009 (starch), P < 0.001 (sugar)). Feeding a STARCH (adult: 21581.0 [15029.0-28133.0]; aged: 35205.0 [29194.0-41216.0]) or SUGAR (adult: 26050.0 [19885.0-32215.0]; aged: 25720.0 [19770.0-31670.0]) meal resulted in postprandial hyperinsulinaemia (AUCi).

MAIN LIMITATIONS

Study cohort contained two insulin-sensitive breeds and no insulin-resistant breeds.

CONCLUSIONS

Age and diet should be considered when evaluating glucose and insulin dynamics.

Exercise training, Glut-4 protein abundance and glutamine in skeletal muscle of mature and very old horses

Two groups of unfit Standardbred mares (adult: 9-14 years, 540 kg, n=7) and old (20-25 years, 530 kg, n=5) were used to test two hypotheses, first, that aging and training would alter plasma and muscle glutamine [Gln] and glutamate [Glu] and second, that aging and training would alter Glut-4 expression in skeletal muscle. All animals were housed on pasture with free access to grass and all received hay and supplementation with a commercially prepared supplement (15% crude protein and 3.00 Mcal/kg dry matter) in individual stalls. Mares were fed to meet or exceed NRC (2007) nutrient recommendations for moderate to heavy exercise. The mares were exercise trained in a free-stall motorised circular exercise machine for 30 min/d, 5 d/week, for 8 weeks. Work intensity during training was set at a relative intensity of ~60% of the maximum heart rate, previously determined during an incremental exercise test (GXT). Blood samples and muscle biopsies (gluteus) obtained before and after 8 weeks of training were used for measurement of [Gln], [Glu] and Glut-4 abundance. Samples were collected before the initiation of training and at 24 h after cessation of last bout of exercise in the training period. All samples were immediately frozen in liquid nitrogen and stored at -80 °C until enzymatic analysis for [Gln], [Glu] and Western Blot analysis for Glut-4 protein abundance. Data were analysed by one-way or two-way ANOVA for repeated measures and the Pearson correlation method. Post-hoc differences were identified with the Tukey test. Significance was set at P<0.05. There were no differences (P>0.05) in muscle [Glu] due to aging. Training decreased (P<0.05) muscle [Glu] from 7,561±701 nmol/g of tissue (mean ± standard error) in pre-training samples to 4,491±701 nmol/g of tissue post-training. Plasma [Gln] decreased (P<0.05) with training (368±14 nmol/ml vs 317±14 nmol/ml). There was a trend (P=0.063) towards an effect of aging. There were significant interactions between age and training for plasma [Gln]. Old mares had lower (P<0.05) post-exercise plasma [Gln] (224±21 nmol/ml) when compared with pre-exercise plasma [Gln] (372±21 nmol/ml). Post-training, plasma [Gln] was lower (P<0.05) in the old mares compared to adult mares [Gln] (224±21 nmol/ml vs 410±18 nmol/ml). There was an effect (P<0.05) of age on muscle [Gln] (old = 6,126 ±870 nmol/g of tissue; adult = 3,176±735 nmol/g of tissue); however, there were no changes (P>0.05) due to training. Glut-4 abundance analysis did not differ (P>0.05) between the young adult and old horses; however, there was a trend (P=0.063) towards an effect of training when samples from both groups were pooled. It was concluded that training and aging produce changes in plasma and muscle [Gln], which may affect immune function in athletic horses, but not in Glut-4.