A serum prolactin-binding protein: implications for growth hormone

Many membrane receptors have truncated soluble forms that circulate in the blood, and a protein in serum with characteristics of the extracellular domain of the human prolactin receptor has recently been identified. Because the extracellular domain of the prolactin receptor binds human growth hormone, does the prolactin-binding protein in serum affect the amount of bound circulating growth hormone? A more general question is whether this serum protein has a minor or a major effect on the biological activities of growth hormone and prolactin.