Serum Insulin, Free Fatty Acids and Blood Sugar during the Estrous Cycle of Dogs

Summary

Changes in blood sugar and both serum, immunoreactive insulin and free fatty acid responses to glucose-induced hyperglycemia and insulin-induced hypoglycemia in normal bitches over different phases of their spontaneous estrous cycles were studied. The phases were carfully checked through series of vaginal smears, taken over several months.

The “sex seasons” did not affect both mean, basal blood sugar and serum immunoreactive insulin levels in these animals, while they increased their mean, basal circulating free fatty acids. Mean blood sugar during the intravenous glucose tolerance test was higher (34-37 %) in the bitches “in season” than that in anestrous; no differences at every phase of the cycle were found. Mean glucose space was smaller (22 %) in the former, and glucose “k” value was not affected by the season. Mean blood sugar during the insulin tolerance test in the bitches in anestrous, proestrous and estrous coincided, but it was slightly higher in those in metadiestrous. Mean serum immunoreactive insulin after exogenous insulin administration was similar in the bitches in anestrous (42 μU/ml) and proestrous (49 μU/ml), it significantly grew during estrous (81 μU/ml) and fell over metadiestrous (57 μU/ml), still above the anestrous value. Insulin space, 73 % of body weight during anestrous, significantly fell just over estrous to 46 %. Insulin “k” constants per group were not affected by the “season”. Mean endogenous insulin response to hyperglycemia in bitches in anestrous was weak and non-significant; it was hardly more intense but significant at estrous and metadiestrous, and was the highest in those at proestrous. Therefore, the mean pancreatic insulin release was increased at proestrous and metadiestrous, but the hyperinsulinemia following glucose load over estrous was chiefly mediated by a reduction in the insulin space. The mean serum free fatty acid curve in the bitches at anestrous during the intravenous glucose tolerance test was definitely flat, while a mean intense initial fall and a mean following rebound in those “in season” was observed. During the insulin tolerance test, positively flat mean serum free fatty acid curves at the mean basal level in all groups were found.

The present paper shows that “sex seasons” in bitches do develop several major alterations in the three parameters studied by us, which are obviously due to the activation of the hypothalamic-pituitary-ovarian axis whose metabolic, combined actions at physiological level still remain unknown.

Key-Words:

Serum Insulin - Free Fatty Acids - Glucose - Estrous Cycle - Dogs