DEMONSTRATION OF A RELAXIN RECEPTOR AND RELAXIN-STIMULATED TYROSINE PHOSPHORYLATION IN HUMAN LOWER UTERINE SEGMENT FIBROBLASTS

To elucidate the mechanism of relaxin action, we studied the binding c haracteristics of human relaxin and its effects on intracellular conce ntrations of cAMP and tyrosine phosphorylation of cellular proteins in a model system of human cervix, human lower uterine segment fibroblas ts. Human relaxin labeled with I-125 bound specifically to a single cl ass of high-affinity relaxin binding sites, distinct from insulin rece ptors, with a mean (+/-SEM) dissociation constant (K-d) of 4.36 +/- 1. 7 x 10(-9) M and a mean of 3220 +/- 557 binding sites per cell in huma n lower uterine segment fibroblasts. Relaxin, in quantities that were shown previously to stimulate intracellular levels of cAMP in other ce ll types, had no effect on intracellular levels of cAMP in human lower uterine segment fibroblasts even in the presence of the phosphodieste rase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX). Incubation of the cells with relaxin caused a significant increase in tyrosine phosphory lation of a protein with an apparent M-r of approximately 220 kDa in t hese cells. In concert with results of recent studies that demonstrate d that the M-r of the relaxin receptor is approximately 220 kDa, our d ata suggest that the phosphorylated protein is likely to be the relaxi n receptor.