Insulin-like growth factor binding protein (IGFBP)4 accounts for the connective tissue distribution of endothelial cell IGFBPs perfused through the isolated heart.

Insulin-like growth factor binding protein 4 (IGFBP4) was purified to homogeneity from conditioned media of bovine pulmonary artery endothelial cells and shown to have the N-terminal amino acid sequence DEAIHCPPCS, a sequence unique to IGFBP4. The IGFBP4 was separated into predominantly glycosylated and nonglycosylated fractions, with each fraction separately perfused through isolated, beating rat hearts. Both forms of IGFBP4 crossed the capillary boundary of the heart and distributed primarily in subendothelial connective tissue components with a connective tissue/cardiac muscle distribution ratio of 20:1 for the glycosylated fraction and 27:1 for the nonglycosylated fraction. Perfused IGFBP1, 2, 3, and IGF-I also crossed the capillary boundary but in contrast to IGFBP4, preferentially localized in cardiac muscle with a connective tissue/muscle ratio of approximately 1:3. We conclude that the connective tissue distribution previously reported for IGFBPs in conditioned media of pulmonary artery endothelial cells is due to IGFBP4.