Effect of intravenous glucose and combined glucose-insulin challenges on energy-regulating hormones concentrations in donkeys

Metabolic disorders are highly prevalent in donkeys. Differences in energy regulatory hormones and glucose dynamic testing, including the intravenous glucose tolerance test (IVGTT) and combined glucose-insulin test (CGIT), have been documented between donkeys and horses. The aims of this study were to characterise the insulin:glucagon (IGR) and glucagon:insulin (GIR) molar ratios, at baseline and in response to the IVGTT and CGIT in healthy donkeys, and to determine their correlation with endocrine (leptin, ghrelin and adiponectin) and morphometric variables. Median values and interquartile ranges (IQRs) for IGR and GIR in 49 healthy adult donkeys were 1.5 (IQR, 1.0-1.8) and 0.7 (IQR 0.5-0.9), respectively. IVGTT and CGIT were each performed on eight donkeys, while dynamic testing was performed on six donkeys due to loss of two donkeys from the study. IVGTT induced an increase in IGR (and a decrease in GIR) from 15 to 180 min after the onset of the test, but had no effect on leptin, adiponectin or ghrelin concentrations. CGIT resulted in a significant elevation in IGR (and a decrease in GIR) from 15 to 120 min after the onset of the test. Plasma leptin concentrations increased significantly at 240 min. No correlations were found between ratios, hormones and morphometric measurements. The findings support differences between donkeys and horses, which are likely to be related to proportionally higher glucagon compared to insulin concentrations in donkeys, and may be relevant to disorders related to energy dysregulation in donkeys, including metabolic syndrome and dyslipidaemias.