The effect of unilateral progesterone infusion into the ovarian artery during the middle luteal phase on progesterone secretion in gilts.

The aim of the study was to determine, in an experiment performed on conscious gilts, whether an increased amount of progesterone (P4) supplied to the porcine corpus luteum (CL), maintained within a physiological systemic concentration would influence its own secretion. On day 9 of the estrous cycle the jugular veins as well as both ovarian arteries and both ovarian veins were cannulated. In the experimental gilts (n=5), P4 was infused into the right ovarian arteries on days 10, 11 and 12 of the estrous cycle at a rate adequate for physiological retrograde transfer found during the middle luteal phase. The left ovarian arteries of these gilts were infused with saline. Both ovarian arteries of the control gilts (n=5) were infused with saline. The P4 infusion rate was 0.62 microg/min (10 day), 2 x 0.62 microg/min ( 11 day) and 3 x 0.62 microg/min (12 day) and physiological levels of the steroid were maintained. Blood samples were collected from the jugular vein and both ovarian veins in the experimental and control animals on days 10, 11 and 12 of the estrous cycle during two periods on each day: before and after P4 or saline infusion. The mean plasma P4 level in the ovarian vein ipsilateral to the P4-infused ovary was significantly (p<0.001) higher on days 10-12 of the estrous cycle than those found in contralateral ovarian vein of the experimental gilts and in the ovarian vein of the control gilts. This was also true for day 12 of the estrous cycle (p<0.001). However, on days 10 and 11 plasma P4 in the vein from the P4-infused ovary tended (p<0.061) to be higher than those in veins from the saline-treated ovaries. The mean P4 concentration in the ovarian vein ipsilateral to the P4-infused ovary did not vary significantly (p>0.05) among the particular days of the experiment. In contrast, mean P4 levels in the contralateral ovarian vein of the experimental gilts as well as in the ovarian vein of the control gilts tended to decrease (p<0.06) between day 10 and day 12. The results of the present paper indicate that during the middle luteal phase of the porcine estrous cycle (days 10-12), ovarian P4 secretion remained unaltered due to the elevation of P4 concentration in blood supplying the steroid-infused ovary, while a decrease in P4 concentration was observed in ovarian veins of the saline-infused ovaries. The influence of the progestagen on its own secretion suggests that on days 10-12 of the porcine estrous cycle there is a short regulatory loop of positive feedback between P4 being retrograde transferred into the ovary and P4 ovarian secretion.