Glucose Tolerance and Plasma Insulin of the Rat in Relation to the Oestrous Cycle and Sex Hormones

Abstract

Glucose tolerance and plasma insulin levels were determined at different stages of the oestrous cycle of Wistar rats having regular 4-day vaginal cycles. Compatible changes of glucose tolerance and plasma insulin were observed. At oestrus glucose tolerance was greatest and plasma insulin levels were highest. The pro-oestrus and metoestrus values were not significantly different, but at dioestrus glucose tolerance was impaired and plasma insulin levels were lowered. As circulating sex hormones might be contributing to these changes, the effects of individual sex hormones on glucose tolerance and plasma insulin were investigated.

Age and weight matched groups of spayed rats of both sexes were given daily intra-muscular injections of 17β- oestradiol-3-benzoate (5 µgm), progesterone (5 µgm), human menopausal gonadotrophin (5 i.u. follicle-stimulating hormone and 5 i.u. luteinizing hormone) and methyl testosterone (5 µgm) respectively for 14 days. Oestradiol, progesterone and gonadotrophin improved glucose tolerance and elevated plasma insulin levels in the female rat, while testosterone impaired glucose tolerance but did not affect the plasma insulin levels. In the male rat, testosterone and progesterone tended to impair glucose tolerance and lowered plasma insulin levels; oestradiol and gonadotrophin showed little effect. It was noted that the shape of the glucose tolerance curve of the spayed and untreated (control) male rats was distinctly different to that of the control females, and that the plasma insulin levels were considerably lower in the control male.

Key words

Glucose Tolerance - Plasma Insulin - Oestradiol - Progesterone - Gonadotrophin - Testosterone