A method of chronic modification of cell barrier properties by exposing a cell to a modification-effective amount of IGF-I for at least about 7 days wherein the modification effective amount is between about 50 .mu.g/kg and less than about 500 .mu.g/kg is disclosed. Further disclosed is a method of chronic amelioration or reversal of insulin resistance as well as a method of diagnosing and screening for rhIGF-I sensitive cell barrier properties.