PROGESTERONE CONCENTRATIONS IN THE PERIPHERAL PLASMA OF THE BLUE FOX (ALOPEX LAGOPUS) DURING PREGNANCY AND THE OESTROUS CYCLE

SUMMARY A competitive protein-binding assay was evaluated and employed in the measurement of progesterone in serial plasma samples obtained from pregnant, non-pregnant and non-receptive blue-fox vixens. At the beginning of the period of pro-oestrous vulval swelling, the plasma progesterone concentration was always found to be below 2 ng/ml. Towards the end of pro-oestrus and during the period of oestrus (sexual receptivity) the concentration increased rather quickly. On the 3rd and the 4th days of oestrus, i.e. when most of the vixens were mated, the plasma progesterone concentrations were about 30 ng/ml (range: 18–36 ng/ml). During gestation (53 or 54 days) the plasma progesterone level rose very steeply and attained a maximum plateau (> 80 ng/ml) between days 5 and 8 post coitum, remained high until about day 20, but thereafter fell rather quickly to below 30 ng/ml between days 35 and 40. Later on the progesterone level fell gradually to below 5 ng/ml at the day of parturition, and, 4 days post partum and later, the levels were below the limit of the sensitivity of the assay used (< 2 ng/ml). The progesterone values and profiles obtained from the non-pregnant vixens (mated vixens which failed to whelp) corresponded clearly to those obtained in the pregnant vixens. Indeed, even in two non-receptive vixens (unwilling to mate during the mating season) typical progesterone profiles were obtained during a period of about 70 days.