21 THE INJECTION OF CORTISOL TO UTERUS INCREASES THE IMPLANTATION RATE AND LITTER SIZE IN PIG ARTIFICIAL INSEMINATION USING CYROPRESERVED SPERMATOZOA

Abstract

Cryopreserved boar spermatozoa are not routinely available to swine artificial insemination (AI) because conception and farrowing rates, along with litter size, have remained low. We have reported the positive roles of seminal plasma in frozen–thawed sperm functions (Okazaki et al. 2009 Theriogenology 71, 491–498). Moreover, the injection of seminal plasma to uterus with frozen–thawed spermatozoa significantly increased the implantation rate. Thus, the factors in seminal plasma act not only on sperm but also on uterus to induce successful fertilization and implantation in pig AI using cryopreserved spermatozoa. To test this hypothesis, we identified the factors in seminal plasma and then developed novel pig AI method using cryopreserved spermatozoa. The sperm-rich fraction was collected weekly from each boar using the gloved-hand technique. The seminal plasma was removed just after collection by centrifuge and then was frozen as described in our previous study (Okazaki et al. 2009 Theriogenology 71, 491–498). When the frozen–thawed sperm was incubated with Fluo-3/AM to determine the level of intercellular Ca²⁺, the level of Ca²⁺ was increased in a time-dependent manner, and spontaneous capacitation that was judged by tyrosine phosphorylation of sperm protein by Western blotting (Shimada et al. 2008 Development 135, 2001–2011), was also induced in post-thawed sperm. The addition of EGTA to thawing solution significantly suppressed the Ca²⁺-induced capacitation. Moreover, the treatment increased fertilization rate in in vitro fertilization and in vivo in artificial insemination as similar as those in sperm with seminal plasma. The same number of blastocyst was collected from uterus by AI using post-thawed sperm with EGTA. However, the pregnancy rate remained low, and the number of leukocytes in the uterus was increased. In the next experiment, we examined in seminal plasma, the level of cortisol that has been known to play an important role in controlling immune function. The results showed that cortisol (1.0 ng mL^–1) was detected in seminal plasma. When the sows of natural oestrus were twice artificial inseminated with or without cortisol, the injection of cortisol (5 μg/50 mL) to uterus with sperm significantly decreased the number of leukocytes in the uterus or endometrium at 24 to 36 h after AI. The low number of leukocytes in the uterus was similar to that in uterus injected fresh semen. The cortisol injection significantly increased the implantation rate and litter size of sows as compared to AI without cortisol (implantation rate; 83% v. 51%, litter size; 10.6 v. 7.3). From these results, we concluded that the injection of cortisol with frozen–thawed spermatozoa by EGTA-containing solution was a novel method of pig AI using cryopreserved spermatozoa.