Two experiments were designed to assess the potential utility of the
propionate challenge test (PCT) as an index of gluconeogenic capacity. In Expt. 1, the dose-response to jugular
propionate infusion was assessed in a duplicated 4 x 4 Latin square experiment with 8 lactating dairy cows.
Sodium propionate (4.5 mol/L, pH 7.4) was infused in an intrajugular bolus at 0 (saline), 0.52, 1.04, or 1.56 mmol/kg
body weight (BW), and jugular blood was sampled over the following 2 h. Peak
propionate concentration in plasma and area under the curve for plasma
glucose both increased linearly with increasing
propionate dose (P < 0.01). Plasma
free fatty acid (FFA) concentration was elevated by all
propionate treatments at 20 min postinfusion (P = 0.03), and plasma
cortisol concentration tended to increase (P < 0.10) after
propionate infusions. Experiment 2 was designed to study the effect of short-term differences in fed state on responses to
propionate infusion. Lactating dairy cows (n = 8) were included in a duplicated 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments.
Sodium propionate (1.04 mmol/kg BW) or saline was infused either before feeding (0900) or 2 h after feeding (1300). Fed cows consumed 4.4 +/- 1.4 kg dry matter before the PCT. Although fed cows had a significantly higher preinfusion plasma
propionate concentration, fed state did not influence postinfusion changes in plasma
propionate,
glucose,
insulin,
glucagon, or FFA concentrations.
Liver glycogen concentration decreased significantly after
propionate, but not saline infusion (P < 0.05). Short-term differences in fed state do not affect the physiological responses to PCT. However,
glucagon release after jugular administration of
propionate is likely supraphysiologic, and postinfusion lipolysis and glycogenolysis suggest that stress responses may alter PCT measurements. Although the PCT may help to diagnose
liver dysfunction, it is not a useful index with which to assess differences in gluconeogenic capacity.