Oestrous cycle-regulated expression of inositol 1,4,5-trisphosphate receptor type 2 in the pig ovary

The inositol 1,4,5-trisphosphate receptor type 2 (IP3R-2) is an intracellular Ca2+ release channel responsible for mobilizing of Ca2+ from intracellular storage sites and plays a key role in biological processes such as fertilization, cell differentiation, and growth. To study the cell-type-specific IP3R-2 expression in porcine ovaries during different phases of the oestrous cycle, we used reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. A total of 24 ovaries from gilts were collected in early luteal, mid-luteal, and follicular phases of the oestrous cycle. When amplified with the primers common to IP3R-2, a RT-PCR product of the expected size (approximately 388 bp) was clearly detected in the follicular and early luteal phase of the oestrous cycle, but there was no detectable PCR product in the corpus luteum of the mid-luteal phase of the oestrous cycle. Immunohistochemical studies showed that IP3R-2 protein is expressed in granulosa cells and theca cells of growing follicles. IP3R-2 immunostaining was first detected during the late pre-antral stage in granulosa and theca cells. Granulosa cell IP3R-2 expression increased from the pre-antral to mid-antral stage, but was strongly reduced in pre-ovulatory follicles. In the developing corpus luteum, intense IP3R-2 immunostaining was also present in luteal cells, but undetectable in mid-luteal corpora lutea. Furthermore, oocytes, atretic follicles and regressed corpora lutea were negative for IP3R-2. Our results indicate that the expression of the IP3R-2 protein was downregulated in terminally differentiated granulosa cells of pre-ovulatory follicles when granulosa cells lose follicle-stimulating hormone responsiveness. Therefore, we strongly suggest that IP3R-2 may play an important role in the initiation and propagation of intracellular Ca2+ signals during follicular development of the pig.