What are the mechanisms that regulate the secretion of PGF2a in the bovine estrous cycle?
a. The exposure of the endometrium to P4 during the early and middle phases of the estrous cycle is essential to initiate endometrial production of PGF2a and, therefore, luteolysis.
b. The concentration of PGF2a in the uterine vein increases during luteolysis associated with pulsatile secretion of PGF2a by the endometrium.
c. The low amplitude of uterine PGF2a pulses stimulates large luteal cells to release OT from secretory granules every 6-8 hours, inducing pulsatile release of the luteolytic PGF2a.
d. The secretion of OT luteal can be stimulated by an agonist of PGF2a.

a) A, B, and C are correct.
b) B and C are correct.
c) A and D are correct.
d) B, C, and D are correct.