Comparison of endocrine and metabolic responses to oral glucose test and combined glucose-insulin tests in horses

Abstract

Different diagnostic tests to determine the insulin sensitivity in horses are commonly used in veterinary practice. However, endocrine and metabolic responses provoked by physiological processes during the respective test procedures are not well described. In the present study, oral glucose tests (OGTs) and combined iv glucose-insulin tests (CGITs) were employed under standardized conditions. The OGTs and CGITs were performed in twelve healthy warmblood horses of different sex, age (15 ± 6.5 years), weight (567± 81 kg) and body condition score (4.8 ± 1.6). Horses were tested under fasting conditions. The OGT was performed with 1 g/kg BW glucose administered via nasogastric intubation and CGIT was performed by injection of 150mg/kg BW glucose solution and 0.1IU/kgBW porcine zinc-insulin. Blood samples were taken for three hours at intervals of 15 minutes and were analysed for insulin, glucose, triglyceride, non-esterified fatty acids (NEFA), fructosamine and Cortisol concentrations. Glucose concentrations increased in the OGTs and CGITs directly after administration. Insulin concentrations increased significantly in OGTs within 30 minutes and stayed elevated for three hours. Peak concentrations of 493.98 ± 86.84μIU/mL were measured in the CGITs, followed by a continuous decline. Baseline NEFA concentrations varied between individual horses and declined in a comparable manner to similar minimum concentrations of 93.82 ± 53.22/L/mol/L in OGTs and 91.97±56.89μmol/L in CGITs. Regarding the stress response of the test procedure, Cortisol concentrations remained unaffected during CGITs, while the OGT procedure was accompanied by a significant initial rise in Cortisol concentrations. To conclude, OGT and CGIT mirror different facets of the metabolic response to a glycemic stimulus, highlighting different aspects of glucose homeostasis and insulin regulation. During the CGIT, insulin dynamics with porcine zinc-insulin differ from insulin dynamics described in reports published previously using short-acting insulins. Furthermore, the antilipolytic effects of insulin during OGTs and CGITs via endogenous secretion or exogenous injection resulted in similar reduction of NEFA concentrations and unaffected triglyceride concentrations. This indicates a saturation of the suppression of lipolysis by insulin with already low concentrations and no induction of re-esterification in liver tissue.