Characterization of the intravenous glucose tolerance test and the combined glucose–insulin test in donkeys

• This is the first study to characterize the combined glucose–insulin test (CGIT) in healthy adult donkeys.
• It also characterizes for the first time the intravenous glucose tolerance test (IVGTT) in healthy adult donkeys.
• The results are compared with those reported in the literature for horses.
• This work establishes normal glucose–insulin proxies in healthy donkeys.

Glucose–insulin dynamic challenges such as the intravenous glucose tolerance test (IVGTT) and combined glucose–insulin test (CGIT) have not been described in donkeys. The objectives of this study were (1) to characterize the IVGTT and CGIT in healthy adult donkeys, and (2) to establish normal glucose–insulin proxies. Sixteen donkeys were used and body morphometric variables obtained each. For the IVGTT, glucose (300 mg/kg) was given IV. For the CGIT, glucose (150 mg/kg) followed by recombinant insulin (0.1 IU/kg) were administered IV. Blood samples for glucose and insulin determinations were collected over 300 min.In the IVGTT the positive phase lasted 160.9 ± 13.3 min, glucose concentration peaked at 323.1 ± 9.2 mg/dL and declined at a rate of 1.28 ± 0.15 mg/dL/min. The glucose area under the curve (AUC) was 21.4 ± 1.9 × 103 mg/dL/min and the insulin AUC was 7.2 ± 0.9 × 103 µIU/mL/min. The positive phase of the CGIT curve lasted 44 ± 3 min, with a glucose clearance rate of 2.01 ± 0.18 mg/dL/min. The negative phase lasted 255.9 ± 3 min, decreasing glucose concentration at rate of −0.63 ± 0.06 mg/dL/min, and reaching a nadir (33.1 ± 3.6 mg/dL) at 118.3 ± 6.3 min. The glucose and insulin AUC values were 15.2 ± 0.9 × 103 mg/dL/min and 13.2 ± 0.9 × 103 µIU/mL/min. This is the first study characterizing CGIT and IVGTT, and glucose–insulin proxies in healthy adult donkeys. Distinct glucose dynamics, when compared with horses, support the use of species-specific protocols to assess endocrine function.