Effects of experimentally induced LH surge on local regulation of apelin/APJ in follicle during ovulation and corpus luteum formation in the cow

The purpose of the study was to characterize the expression patterns of apelin and its receptor APJ (apelin/APJ) in superovulation induced follicles and corpus luteum (CL) tissue in the cow. Ovaries containing different classes of preovulatory follicles and early CL were timely defined during GnRH (gonadotropin hormone-releasing hormone) induced superovulation as follows: Before GnRH application, follicles group I: 0 h (control group) and after GnRH; group II: 4 h (corresponds to LH peak); group III:10 h; group IV: 20 h; group V: 25 h (corresponds close to ovulation) and group VI: 60 h (after ovulation - early CL). This experiment has made it possible for the direct comparison of the apelin/APJ system members in follicle tissue (theca interna plus granulosa cells) of periovulatory follicle groups with the subsequent CL tissue after ovulation (early CL, day 2-3). Relative gene expression levels were measured by RT-qPCR. The low level of apelin mRNA in the follicle group before the GnRH application (Group I), remained unchanged even in the follicle group II and III (4 h and 10 h after GnRH respectively). Apelin expression levels increased significantly by only 20 h after GnRH (group IV follicles: just before ovulation), remaining high during follicle ovulation (Group V: 20 h) and after ovulation (Group VI: early CL). In contrast, the low APJ mRNA level increased significantly in follicle group II (during LH peak) remaining high in all follicle groups before ovulation, as well as after ovulation (early CL). Our results indicate the possible involvement of apelin system (apelin/APJ) as a possible key regulator in the local mechanisms supporting final follicle maturation and ovulation, as well as the follicular-luteal transition and CL formation in cows.