Transport of uterine PGF2α to the ovaries by systemic circulation and local lymphovenous-arterial diffusion during luteolysis in sheep

The theory of countercurrent vascular transfer of PGF 2α during luteolysis was examined. In the first experiment, pulmonary clearance of PGF 2α was determined to re-examine whether the total amount of PGF 2α was degraded in the lungs after one passage. Cardiac output was measured by the Fick method and PGF 2α by radioimmunoassay before and after vascular lung supply, using pulmonary catheterization and the interventional radiology method in ten anaesthetized ewes on day 16 of the oestrous cycle. Cardiac output remained stable (7156 ± 439 ml min -1 ). Infusion of 5 iu oxytocin resulted in an increase in plasma PGF 2α concentrations at 30 min in the uterine vein and the pulmonary and femoral arteries (3811 ± 806, 224 ± 55 and 18 ± 4 pg ml -1 , respectively). The PGF 2α concentrations decreased exponentially and the half-time decreases were 27 (r = 0.99), 16 (r = 0.99) and 18 (r = 0.98) min, respectively. Pulmonary clearance of PGF 2α was estimated at 6338 ± 451 ml min -1 . In a second experiment, an arterio-arterial gradient of plasma PGF 2α concentrations was analysed between the proximal and distal segments of the ovarian artery to verify whether the total amount of PGF 2α flowing to the ovary was from the local venous-arterial countercurrent pathway. Surgical catheterization techniques were performed on 11 ewes on day 16 of the oestrous cycle. The ovarian arterial blood flow was measured by the implantable Doppler method (8 ± 1 ml min -1 ). The maximum plasma PGF 2α concentrations in the femoral and distal ovarian arteries were 23 ± 6 and 42 ± 11 pg ml -1 (P < 0.05), respectively. Plasma PGF 2α decreased exponentially in the femoral artery and the half-time decrease was 26 min (r = 0.98), and in the distal ovarian artery close to the ovary PGF 2α decreased linearly and the half-time decrease was 108min (r = 0.96). Consequently, the arterio-arterial diffusion gradient of PGF 2α concentrations was extended to 3 h. These experiments showed that the PGF 2α flow rate in the pulmonary artery was 42.275 ± 10.793 μg per 150 min (n = 10) and the systemic arterial PGF 2α flow rate was 5.359 ± 1.658 μg per 150 min (n = 10). Therefore, 12% of the PGF 2α was not oxidized by the lungs. The proximal ovarian PGF 2α flow rate was 6.909 ± 2.341 ng per 150 min, while the distal flow rate was 21.003 ± 5.703 ng per 150 min (n = 11). Thus, 33% of the PGF 2α was transported rapidly to the ovary via the systemic route, while 67% was transported by slow local countercurrent diffusion, which extended the duration of luteolytic activity to four times that of the PGF 2α surge. These results indicate both rapid systemic transport of PGF 2α to the ovaries and a slower buffer mechanism involving a local diffusion pathway, rather than a direct countercurrent system.