PSVI-17 Effect of an insulin bolus during intravenous glucose tolerance tests in horses.

Abstract Intravenous glucose tolerance tests (IVGTT) are important in equine metabolic research as a tool to evaluate insulin resistance and glucose metabolism. The administration of an intravenous bolus of insulin 20 minutes after a dextrose infusion is a typical step in published equine IVGTT protocols. Although commonly used, the justification for the use of this exogenous insulin bolus following dextrose infusion in equine IVGTT protocols is not well documented. It is unclear how exogenous insulin influences circulating blood glucose during the IVGTT. The objective of this study was to determine the effect of a bolus of exogenous insulin on circulating blood glucose concentrations in mares during an IVGTT. We hypothesized that exogenous insulin would increase serum insulin concentrations but not influence glucose concentrations or glucose area under curve (AUC) during an IVGTT in mares. Morgan mares (n = 3) of similar body condition (body condition score = 5.9 ± 0.2; cresty neck score = 3) and age (14.3 ± 1.5 yr) underwent two IVGTTs 1 wk apart, one including an insulin infusion (INS) 20 min after dextrose infusion and one with no exogenous insulin administration (NOINS). Mares were fasted overnight and administered intravenous (i.v.) dextrose (0.3 g/kg BW) during each test. Blood samples were collected -30, -15, 0, 2, 5, 10, 15, 20, 25, 30, 45, 60, and 120 min relative to dextrose infusion. During the INS IVGTT, i.v. insulin (0.3 mU/kg BW) was administered immediately after the 20-minute blood sample was collected. No exogenous insulin was administered during the NOINS IVGTT. Glucose and insulin concentrations were determined by ELISA. Data were analyzed with R Studio with P ≤ 0.05 considered significant and 0.05 < P ≤ 0.10 considered a tendency. The administration of insulin (INS v. NOINS) was included in the model with mare as a random effect. As expected, during the INS test, mares had greater insulin concentrations 5 and 10 minutes after insulin infusion (P < 0.01) and increased insulin AUC (INS: 24,053; NOINS: 3,498; SEM ± 5,090; P < 0.01) after dextrose infusion. However, glucose concentrations were not influenced by insulin infusion (P = 0.70) and there was no significant interaction of time and insulin status (P = 0.97). Glucose AUC after dextrose infusion was also not affected by insulin infusion (INS: 24,468; NOINS: 23,838; SEM ± 1,430; P = 0.49). These data suggest that i.v. infusion of insulin at a dosage consistent with published protocols may not influence circulating blood glucose in horses. Therefore, the addition of exogenous insulin during an equine IVGTT may not provide any benefit in determining equine glucose metabolism and insulin resistance.