RECEPTORS FOR INSULIN-LIKE GROWTH FACTOR-II IN THE RAT UTERUS - CHARACTERIZATION AND VARIATION THROUGHOUT THE ESTRUS CYCLE

This study was undertaken to identify uterine insulin-like growth factor II receptors and examine the regulation of these receptor levels throughout the estrus cycle. We have demonstrated IGF-II receptors in crude uterine membranes by binding and cross-linking experiments. IGF-II binding to the rat uterine membranes displayed time and temperature dependence and maximum binding was achieved by 2 h at 22-degrees-C. Uterine IGF-II binding sites were specific for binding IGF-II peptide and demonstrated negligible binding affinity for IGF-I and no affinity for insulin. The specific anti-IGF-II receptor antibody, R-II-PAB 1, blocked the specific [I-125]IGF-II binding to uterine membranes in a dose-dependent manner. The characteristics of uterine IGF-II receptor are similar to those reported for other tissues, with a single class of high-affinity binding sites with an apparent dissociation constant of 1.2 +/- 0.5 nmol/l and beta-max of 2.65 +/- 0.41 pmol/mg protein. Affinity cross-linking experiments indicated that the specific binding of [I-125]IGF-II in the uterus is associated with a single band of protein with a mol wt of 250 kD. In mature cycling rats, the proestrus uterus had the lowest level of [I-125]IGF-II binding per mg membrane protein, without changes in receptor affinity. However, because of greater yield of protein from proestrus uteri, the total [I-125]IGF-II binding capacity of the uterus was similar to the other stages of the estrus cycle. These studies demonstrate the presence of authentic IGF-II receptors in the rat uterus and illustrate variations in the concentration of these receptors in the uterus throughout the estrus cycle.