Expression of insulin-like growth factor-I (IGF-I) in the rat fallopian tube: possible autocrine and paracrine action of fallopian tube-derived IGF-I on the fallopian tube and on the preimplantation embryo.

Recent studies have indicated that growth factors such as insulin-like growth factors (IGFs) increase the growth rate of cultured preimplantation embryos. We therefore hypothesized that the fallopian tube may produce IGFs which in turn participate in the regulation of preimplantation embryo development in vivo. In the present study we examined the expression of IGF-I in the fallopian tube. We demonstrated that IGF-I transcripts (7.0, 1.7, and 1.2-0.8 kilobases) were abundant in the fallopian tube. Immunoreactive IGF-I was most abundant in the epithelial cells in the fallopian tube, and IGF-I messenger RNA (mRNA) was detected in the luminal region of the fallopian tube. A solution hybridization assay was used to examine the regulation of IGF-I mRNA. The abundance of IGF-I transcripts changed markedly during the 4-day estrous cycle with the highest levels on the day of proestrus. The increase in IGF-I mRNA between the day of diestrus II and the day of proestrus was 4-fold (P < 0.01). The pattern of IGF-I mRNA expression in the fallopian tube resembled the pattern of ovarian estrogen production during the estrous cycle. The level of IGF-I mRNA decreased after hypophysectomy. The expression of IGF-I mRNA in the fallopian tube was dose-dependently regulated by estradiol, and a single sc injection of estradiol [5 micrograms/100 g body wt (BW)] increased the IGF-I mRNA in a time-dependent manner with a significant increase after 3 h (P < 0.01). The lowest estradiol dose tested (0.1 microgram/100 g BW) increased the expression after 6 h, whereas progesterone (5 micrograms/100 g BW) was ineffective. The presence of embryos in the fallopian tube did not statistically significantly influence the abundance of IGF-I transcripts as measured with a solution hybridization assay on RNA extracted from whole fallopian tubes. In order to determine possible targets for fallopian tube-derived IGF-I we examined the expression of IGF-I receptor mRNA. Northern blot analysis revealed that an 11-kilobase IGF-I receptor transcript was expressed in the fallopian tube. Using a reverse transcriptase-polymerase chain reaction, IGF-I receptor mRNA was also detected in the eight-cell but not two-cell preimplantation embryo. The present study demonstrates that IGF-I is produced in the fallopian tube and its expression is regulated by estradiol. Both the fallopian tube and the eight-cell preimplantation embryo express IGF-I receptors and are therefore potential target tissues.