Growth Hormone, Insulin, and Insulin-like Growth Factor-I: Do They Interact at the Blood-Brain Barrier?

Publisher Summary The interactions of growth hormone (GH), insulin, and IGF-I at the BBB are complex. Insulin-like growth factor-I (IGF-I) mediates the effects of GH in a variety of circumstances. It has been shown that both insulin and IGF-I cross the blood-brain barrier (BBB) by saturable transport systems. However, permeation of GH across the BBB has been suspected and there is no direct evidence whether GH enters the brain by means of a saturable transport system. Experimental results with radioactively labeled GH show that it has significant influx from blood to the brain. This entry is not mediated by a saturable transport system at the BBB, as excess nonradioactively labeled GH had no modulatory effect on the influx transfer constant or the volume of distribution of 125I-labeled GH in the brain. Similarly, excess insulin had no effect on GH entry at the doses tested. However, IGF-I did reduce the influx transfer constant of 125I-labeled insulin, suggesting that IGF-I may share the transport system for insulin. The chapter concludes that GH can cross the BBB independently of the transport of insulin and IGF-I.