Volatile Fatty Acid Tolerance and Effect of Glucose and VFA on Plasma Insulin Levels in Ponies

Volatile fatty acid (VFA) tolerance and plasma insulin response to intravenous VFA (acetate, propionate or butyrate), and glucose injections (3.5 mM/kg) were studied in each of four ponies which were either fed hay ad libitum or fasted for 72 hours. Plasma VFA levels were within preinjection values by 60 minutes in all treatments, and the half-times of VFA disappearance from plasma were not significantly different between fasted or fed animals. Glucose injections produced a biphasic insulin secretory response in both treatments; however, the insulin secretion per unit of glucose load was much greater for the fed animals. In addition, the fed animals exhibited a greater sensitivity to exogenous insulin. The indications suggest that the impaired glucose tolerance previously reported in the fasted ponies appeared to be due to an insulin insensitivity rather than an insulin insufficiency. Of the VFA, only butyrate stimulated a significant insulin response in the fasted ponies, but the response was much less than that from glucose. The nature of the endocrine responses to glucose and VFA in the ponies appeared to be markedly different than that reported for ruminants, but instead resembled the nonruminant pattern.