Effect of short-term dopamine reduction on insulin sensitivity and post-prandial insulin and glucose responses in Standardbred horses.

The role of dopamine in the regulation of insulin secretion in horses is poorly understood and requires further investigation. Pituitary pars intermedia dysfunction (PPID) is associated with decreased activity of dopaminergic neurons which normally suppress peptide hormone secretion from the pituitary pars intermedia. A high proportion of horses with PPID also have insulin dysregulation (ID), characterised by post-prandial hyperinsulinaemia and/or tissue insulin resistance, which are risk factors for the development of laminitis.

Physiological and metabolic effects of short-term dopamine reduction in healthy horses using a tyrosine hydroxylase inhibitor (alpha-methyl-para-tyrosine).

Alpha-methyl-para-tyrosine (AMPT) is a reversible inhibitor of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine synthesis. This study aimed to determine whether AMPT could reduce dopamine concentrations in horses. Six healthy adult Standardbred geldings were administered AMPT (40 mg/kg BW, orally) or placebo in a randomised crossover study design.

Studies in vitro of equine intestinal glucagon-like peptide-2 secretion.

Equine insulin dysregulation (ID) is a significant metabolic problem because the hyperinsulinaemia that develops increases the animal's risk of developing laminitis, a debilitating foot condition. The role of gastrointestinal factors, such as incretin hormones, in the pathogenesis of ID and hyperinsulinaemia in horses is poorly understood, particularly in comparison to other species. Glucagon-like peptide-2 (GLP-2) is an intestinotrophic peptide released from L cells in the gastrointestinal tract and is implicated in metabolic dysfunction in other species.

The detection of ACTH and insulin in equine plasma by solid-phase extraction and micro-flow LC/MSMS.

Previous liquid chromatography/mass spectrometry (LC/MS) methods for the detection of insulin and other similar peptide hormones in equine plasma relied on the use of antibody affinity extraction. As a result, these methods were not suitable for routine high-throughput analysis. A solid-phase extraction (SPE) method incorporating size exclusion as well as reversed-phase interactions allows the selective extraction of peptide hormones such as adrenocorticotropic hormone (ACTH), insulin and their synthetic analogues from equine plasma with approximately 80% extraction efficiencies.

Evidence for dopamine production and distribution of dopamine D2 receptors in the equine gastrointestinal mucosa and pancreas.

Insulin dysregulation in horses is characterised by hyperinsulinaemia and/or tissue insulin resistance and is associated with increased risk of laminitis. There is growing evidence in other species that dopamine attenuates insulin release from the pancreas; however, this has yet to be examined in horses. The present study aimed to identify whether there are cells capable of producing or responding to dopamine within the equine gastrointestinal mucosa and pancreas.

Expression of the GCG gene and secretion of active glucagon-like peptide-1 varies along the length of intestinal tract in horses.

Background: Active glucagon-like peptide-1 (aGLP-1) has been implicated in the pathogenesis of equine insulin dysregulation (ID), but its role is unclear. Cleavage of proglucagon (coded by the GCG gene) produces aGLP-1 in enteral L cells.

Objectives: The aim in vivo was to examine the sequence of the exons of GCG in horses with and without ID, where aGLP-1 was higher in the group with ID.

Diagnosis of equine endocrinopathies: The value of measuring blood glucose during an oral glucose test.

Blood glucose concentration is often measured during an oral glucose test (OGT), but is not thought to aid in diagnosing insulin dysregulation (ID) or pituitary pars intermedia dysfunction (PPID). The aim of this retrospective study was to investigate whether the change in blood glucose concentration during an OGT aligned with indicators of equine metabolic syndrome or PPID, including serum insulin and plasma ACTH concentrations, clinical observations, age, sex, breed type and the test dose. The cohort included 149 horses, miniature horses, and ponies that had undergone an in-feed OGT and clinical examination between 2015 and 2021.

Associations between feeding and glucagon-like peptide-2 in healthy ponies.

Background: Gastrointestinal peptides, such as glucagon-like peptide-2 (GLP-2), could play a direct role in the development of equine hyperinsulinaemia.

Objectives: To describe the secretory pattern of endogenous GLP-2 over 24 h in healthy ponies and determine whether oral administration of a synthetic GLP-2 peptide increases blood glucose or insulin responses to feeding.

Study Design: A cohort study followed by a randomised, controlled, cross-over study.

Carbohydrate pellets to assess insulin dysregulation in horses.

Background: A glycemic challenge test is used for the diagnosis of insulin dysregulation (ID) in horses and ponies. Different forms of the test exist where the administrative route and dose of glucose vary, which makes interpretation of results challenging.

Hypothesis/objectives: To evaluate the palatability of, and blood glucose and insulin responses to, carbohydrate pellets fed as an oral glucose test (OGT), and to establish the diagnostic threshold for ID when using the pellets.

A starch-rich treat affects enteroinsular responses in ponies.

Objective: To determine the effect of a starch-rich treat, added to the daily diet of ponies for 10 days, on enteroinsular responses to meal consumption.

Animals: 10 mixed-breed adult ponies owned by Queensland University of Technology were used in the study. Six ponies were metabolically healthy, and 4 were insulin dysregulated at the start of the study, according to the results of an in-feed oral glucose test.

Demographic, morphologic, hormonal and metabolic factors associated with the rate of improvement from equine hyperinsulinaemia-associated laminitis.

Background: Although several studies have investigated factors associated with the onset and occurrence of hyperinsulinaemia-associated laminitis (HAL), few have examined the factors associated with the rate of improvement during recovery from an acute bout of the disease. This observational study sought to discover if a range of demographic, morphologic, hormonal and metabolic variables are associated with the improvement rate from HAL in 37 naturally-occurring cases identified by 16 clinics across Germany. Each case was evaluated for laminitis severity on the day of inclusion in the trial (d 0), then after 4, 9, 14, 25 and 42 d.

Metabolic profile distinguishes laminitis-susceptible and -resistant ponies before and after feeding a high sugar diet.

Background: Insulin dysregulation (ID) is a key risk factor for equine endocrinopathic laminitis, but in many cases ID can only be assessed accurately using dynamic tests. The identification of other biomarkers could provide an alternative or adjunct diagnostic method, to allow early intervention before laminitis develops. The present study characterised the metabolome of ponies with varying degrees of ID using basal and postprandial plasma samples obtained during a previous study, which examined the predictive power of blood insulin levels for the development of laminitis, in ponies fed a high-sugar diet.

The application of a new laminitis scoring method to model the rate and pattern of improvement from equine endocrinopathic laminitis in a clinical setting.

Background: Endocrinopathic, or hyperinsulinaemia-associated laminitis (HAL) is a common and debilitating equine foot disease, and although no pharmacological treatments are registered, several are under development. To evaluate the effect of such treatments, an accurate and consistent method is needed to track the clinical signs of laminitis over time, and the natural history of the disease, in terms of a 'normal' pattern of improvement, needs to be understood. This study examined the improvement pattern in clinical cases of naturally-occurring HAL subjected to a range of best-practice interventions, using two different scoring methods.

Effects of an anti-IGF-1 receptor monoclonal antibody on laminitis induced by prolonged hyperinsulinaemia in Standardbred horses.

Currently, there are no registered veterinary drugs for the treatment of endocrinopathic equine laminitis, and although this form of the disease is known to be caused by prolonged hyperinsulinaemia, the mechanism of insulin toxicity is unclear. One possibility is that high concentrations of insulin activate IGF-1 receptors (IGF-1R) in lamellar tissue, leading to uncontrolled cell proliferation and epidermal lamellar dysregulation. An equinized version of a human anti-IGF-1R therapeutic monoclonal antibody (mAb11) was generated to test this theory, using a modification of the prolonged euglycaemic-hyperinsulinaemic clamp technique.

Effects of insulin on IGF-1 receptors in equine lamellar tissue in vitro.

Although it is understood that equine endocrinopathic laminitis can be triggered by high concentrations of insulin, it is unclear whether this represents a direct action on lamellar tissue via insulin receptors (InsR), an interaction with IGF-1 receptors (IGF-1R), or some other, indirect action. This uncertainty is because of the reported scarcity of InsR in lamellar tissue and the low affinity of insulin for equine IGF-1R. In the present study, the effects of insulin and IGF-1 (as a positive control) were examined using lamellar explants isolated from the hooves of healthy horses and incubated in cell culture medium for between 2 min and 48 h.

Identification of monoclonal antibodies suitable for blocking IGF-1 receptors in the horse.

Prolonged hyperinsulinemia is thought to be the cause of equine endocrinopathic laminitis, a common and crippling disease of the foot, for which there are no pharmacologic treatments other than pain relief. It has been suggested that insulin causes its effects on the lamellae by activating IGF-1 receptors (IGF-1R), as insulin receptors (InsR) are scarce in this tissue, whereas IGF-1R are abundant and become downregulated after prolonged insulin infusion. As a first step toward confirming this mechanism and beginning to develop a therapeutic anti-IGF-1R monoclonal antibody (mAb) for horses, it was necessary to identify available human IGF-1R mAbs that would recognize equine receptors.

The effect of diet change and insulin dysregulation on the faecal microbiome of ponies.

The equine microbiome can change in response to dietary alteration and may play a role in insulin dysregulation. The aim of this study was to determine the effect of adding pasture to a hay diet on the faecal bacterial microbiome of both healthy and insulin-dysregulated ponies. Faecal samples were collected from 16 ponies before and after dietary change to enable bacterial 16S rRNA sequencing of the V3-V4 region.

Glucagon-like peptide-1, insulin-like growth factor-1, and adiponectin in insulin-dysregulated ponies: effects of feeding a high nonstructural carbohydrate diet and association with prospective laminitis.

Endocrinopathic laminitis, related to equine metabolic syndrome and insulin dysregulation, causes marked pain and suffering in horses and represents a substantial cost to the horse industry. This study investigated the effect of feeding a diet high in nonstructural carbohydrates on concentrations of active glucagon-like peptide-1 (aGLP-1), total insulin-like growth factor-1 (IGF-1), and high-molecular-weight (HMW) adiponectin, in insulin-dysregulated ponies. Thirty-seven ponies were challenged with this diet for up to 18 d to induce hyperinsulinemia.

An investigation of the equine epidermal growth factor system during hyperinsulinemic laminitis.

Equine laminitis is a disease of the digital epidermal lamellae typified by epidermal cell proliferation and structural collapse. Most commonly the disease is caused by hyperinsulinemia, although the pathogenesis is incompletely understood. Insulin can activate the epidermal growth factor (EGF) system in other species and the present study tested the hypothesis that upregulation of EGF receptor (EGFR) signalling is a key factor in laminitis pathophysiology.

The effect of different grazing conditions on the insulin and incretin response to the oral glucose test in ponies.

Background: The oral glucose test (OGT) is a useful tool for diagnosing insulin dysregulation (ID) and is somewhat repeatable in ponies under consistent management. This study aimed to determine whether the insulin and incretin responses to an OGT in ponies differed after short-term access to fertilised pasture, compared to unfertilised pasture, by using a randomised, repeated measures study design. Sixteen mixed-breed ponies were classified as severely insulin-dysregulated (SD; post-prandial insulin ≥80 μIU/mL) or not severely insulin-dysregulated (NSD; post-prandial insulin < 80 μIU/mL) using an OGT prior to the study.

The cresty neck score is an independent predictor of insulin dysregulation in ponies.

Generalized obesity, regional adiposity, hyperinsulinemia and hypertriglyceridemia are all potential indicators of equine metabolic syndrome (EMS). This study aimed to assess the relationship between morphometric measurements of body condition and metabolic hormone concentrations in ponies, with and without a neck crest or generalised obesity. Twenty-six ponies were assigned a body condition score (BCS) and cresty neck score (CNS).

A "modified Obel" method for the severity scoring of (endocrinopathic) equine laminitis.

Background: Laminitis is a common equine disease characterized by foot pain, and is commonly diagnosed using a five-grade Obel system developed in 1948 using sepsis-related cases. However, endocrinopathic laminitis is now the most common form of the disease and clinical signs may be mild, or spread across two Obel grades. This paper describes a modified method which assigns scores to discreet clinical signs, providing a wider scale suitable for use in a research setting.

Incidence and risk factors for recurrence of endocrinopathic laminitis in horses.

Background: Endocrinopathic laminitis is common in horses and ponies, but the recurrence rate of the disease is poorly defined.

Objectives: To determine the incidence of, and risk factors for, the recurrence of endocrinopathic laminitis.

Animals: Privately owned horses and ponies with acute laminitis (n = 317, of which 276 cases with endocrinopathic laminitis were followed up to study completion).