COUNTERCURRENT TRANSFER OF I-125 LHRH IN THE PERIHYPOPHYSEAL CAVERNOUS SINUS-CAROTID RETE VASCULAR COMPLEX, DEMONSTRATED ON ISOLATED PIG HEADS PERFUSED WITH AUTOLOGOUS BLOOD

The objective of the study was to determine whether the local permeabi lity of luteinizing hormone-releasing hormone (LHRH) from the venous b lood of the perihypophyseal cavernous sinus into the arterial blood of the carotid rete, supplying the brain and hypophysis in gilts, depend s on the day of the estrous cycle, as well as to determine whether thi s transfer exists when LH concentration in the blood is reduced (the e xperimental short-loop negative feedback for LH secretion after estrad iol injection in ovariectomized gilts). Experiments were conducted on isolated gilt heads with necks, on chosen days of the estrous cycle (n = 40), and on previously ovariectomized gilts treated with estradiol benzoate (EB) (n = 5) or corn oil (n = 3). After exsanguination, the g ilt heads with necks were disarticulated and about 30-45 min later wer e supplied with autologous, oxygenated, and heated blood at a stable b lood flow and pressure through the left carotid artery for 30 min. I-1 25-LHRH was infused into both cavernous sinuses through the cannulated angularis oculi veins for 5 min. After I-125-LHRH infusion, radiolabe led LHRH was found (P < 0.001) in arterial blood taken from the caroti d rete through the open right carotid artery in all animals used in th e experiment: on Days 1-2 (six gilts), on Days 12-14 (seven gilts) of the estrous cycle, and in five ovariectomized gilts during negative fe edback for LH surge (40 hr after EB). No significant radioactivity of I-125-LHRH was found in the arterial blood on Days 3-5 (n = 6), 9-11 ( n = 4), and 15-21 (n = 17) of the estrous cycle. A very low level of r adioactivity was found in the ovariectomized control group after the i njection of corn oil (n = 3). These results provide evidence for the p ermeability of LHRH from the venous to the arterial blood and its retr ograde transport with the arterial blood to the hypophysis and brain, after the ovulation period (Days 1-2) and on Days 12-14 of the estrous cycle. This suggests that a close relationship exists between the day of the estrous cycle and LHRH permeability from the venous to the art erial blood in the perihypophyseal cavernous sinus-the carotid rete co mplex in gilts-and that this mechanism may be included in a short-loop feedback for LHRH secretion. (C) Elsevier Science Inc. 1997.