Estrogen associated changes in uterine blood flow and maternal recognition of pregnancy in Chinese (Meishan) pigs

Our laboratory has recently demonstrated a reduced embryonic growth rate and estradiol-17β (E₂β) content of preimplantation Meishan embryos compared with embryos of European breeds. Estrogen secretion by the preimplantation conceptus has been linked to both the transient increase in uterine blood flow (UBF), and maternal recognition of pregnancy (MRP) on Days 11-13 of gestation in European swine breeds. The objective of these experiments was to evaluate the temporal association between estrogen secretion by the preimplantation conceptus, increases in UBF and the onset of MRP in Meishan pigs. In Experiment 1, multiparous Meishan sows (N = 3) were observed for estrus once daily, and hand-mated at first observed estrus (Day 0) and 24 h later with a Meishan boar. On Day 2 postmating, sows were bilaterally instrumented with blood flow transducers and uterine venous catheters. UBF (ml min^-1) was monitored and blood samples collected once daily from Days 6 to 30 postmating. UBF increased on Day 11 (P = 0.06) to peak on Day 13, then decreased on Days 14-17, before increasing progressively thereafter, coincident with increasing uterine E₂β secretion. In Experiment 2, second cycle Meishan (N = 20) and Yorkshire (N = 18) gilts were hand-mated at first observed estrus (Day 0), and 24 h later, to boars of their own breed. Embryos were flushed from the gilts on Day 9, 11, 13 or 15 postmating. Gilts were again observed for estrus to determine treatment estrous cycle length (ECL). Treatment ECL was similar (P > 0.1) to presurgical ECL when embryos were collected from either Meishan or Yorkshire gilts on Days 9 and 11. ECL was extended in both breeds (P < 0.05) when embryos were recovered on Days 13 and 15.