Serum insulin, glucose and lactate concentrations during 18-h fast in female reindeer

We investigated whether secretion of insulin occurred in the absence of feeding in a ruminant. Serum insulin, glucose and lactate concentrations were measured in three adult non-pregnant reindeer at hourly intervals during an 18-h fast (17:30-11:30 h) in October. Mean serum insulin concentration was 39+/-3 micro/ml (range 2-100). The insulin profile of two animals was characterized by a nocturnal rise and an early morning trough, followed by a mid-morning rise. Within the larger peaks, short-term oscillations occurred at 2-3-h intervals. Serum glucose concentrations significantly increased during the fast and exceeded feeding values by 31-45% at 18 h post feeding. Serum lactate concentrations declined significantly in all three animals. Residuals for serum glucose concentrations were significantly negatively correlated to residuals of serum lactate in two animals, but not the third. Serum glucose and lactate concentrations were not related to serum insulin. In conclusion, insulin secretion in reindeer shows a 2-3-h periodicity in the absence of feeding. The periodicity is of similar duration as the inter-meal interval for pen-fed reindeer during winter (2.5 h). Although not necessarily causal, the results are consistent with a hypothesized role for insulin in meal initiation.