Temporal relationships between hormonal concentrations and the electrical resistance of the vaginal tract of blue foxes (Alopex lagopus) at pro-oestrus and oestrus.

During pro-oestrus, baseline LH concentrations for 9 vixens (pooled data) ranged from 0.8 to 5.3 ng/ml. In each vixen, baseline levels were interrupted by elevations of LH ranging from 3.1 to 10.4 ng/ml. A major preovulatory LH surge was detected in all the vixens. The LH peak ranged from 13.5 to 73.0 ng/ml with an average of 27.8 +/- 18.8 (s.d.) ng/ml. Plasma LH concentrations declined to a basal level of 1.3 +/- 1.0 ng/ml within 48 h of the peak value. The duration of the LH surge was 1-3 days. The LH peak occurred 1 or 2 days before any sexual receptivity was observed. All the vixens were mated twice 2-5 days after the LH peak; 8 conceived. Plasma concentrations of oestradiol-17 beta increased gradually during the last 6-7 days before oestrus and reached maximum values (124-373 pg/ml) at the time of the preovulatory LH peak. The first significant increase in plasma progesterone concentration occurred simultaneously with the LH peak. During oestrus (normally 3-5 days), progesterone levels rose steeply, attaining a mean concentration of 57.0 +/- 17.5 ng/ml when the vixens went out of heat. Androstenedione and testosterone values changed similarly, both increasing at the beginning of pro-oestrus and reaching maximum values (805-1879 pg/ml and 328-501 pg/ml respectively) 1 day before to 1 day after the oestradiol-17 beta peak. The electrical resistance of the vaginal tract increased rapidly during the last 2-3 days of pro-oestrus, reaching a maximum value (300-640 omega) approximately 2 days after the oestradiol-17 beta peak that corresponded with the onset of sexual receptivity. Towards the end of oestrus, the values fell to 100-200 omega.