CONSTITUTIVE EXPRESSION OF UTERINE RECEPTORS FOR INSULIN-LIKE GROWTH FACTOR-I DURING THE PERIIMPLANTATION PERIOD IN THE PIG

Insulin-like growth factor-I (IGF-I), synthesized by the uterine endometrium of cyclic and early pregnant gilts, accumulates in the uterine luminal fluid, where it comes in contact with the developing conceptus and the rapidly growing uterus. The uterus and the conceptus thus represent potential target sites for the biological effects of IGF-I, provided high-affinity Type I receptors are present. This study was undertaken to evaluate the expression of functional IGF-I receptors in the endometrium and myometrium of pregnant (Day 10, 12, and 15) gilts and in the endometrium of cyclic (Day 15) and pseudopregnant (Day 15) gilts and to correlate levels of these receptors with temporally regulated uterine production of IGF-I. Specific binding of I-125-IGF-I to endometrial membranes pretreated with MgCl2 (4 M) at 4-degrees-C for 16 h, was saturable and membrane concentration-dependent. Competition of I-125-IGF-I binding to endometrial membranes was highest with unlabeled IGF-I > IGF-II >> insulin, whereas porcine relaxin was noncompetitive. Affinity cross-linking of endometrial membranes with I-125-IGF-I followed by SDS-PAGE and autoradiography revealed two labeled bands of M(r) > 200 000 and M(r) 135 000, with the major band being the M(r) 135 000 species. Scatchard analysis of I-125-IGF-I binding to endometrial membranes from Day 12 pregnant gilts revealed a single class of binding sites with a dissociation constant (K(d)) = 4.08 +/- 0.09 nM. Membranes prepared from endometrium of Day 10, 12, and 15 pregnant gilts exhibited comparable I-125-IGF-I binding (p > 0.05) that was higher (p < 0.001) than that for the corresponding myometrial membranes. In contrast, endometrial and myometrial tissue content of IGF-I decreased (p < 0.01) between Days 10 and 15 of pregnancy, although no difference (p > 0.05) was detected between tissues within the same day. Endometrial membranes from Day 15 pregnant, cyclic, and pseudopregnant gilts bound similar (p > 0.05) amounts of I-125-IGF-I. Specific binding of I-125-IGF-I to endometrium from ovariectomized gilts was not affected by progesterone or estrogen + progesterone treatment and was comparable to binding to endometrium from early pregnant gilts. These results demonstrate expression of IGF-I receptors in pig endometrial and myometrial tissues. However, unlike tissue IGF-I content, which changes with hormonal or pregnancy status, uterine IGF-I receptors in vivo appear to be constitutively expressed.