Insulin-like growth factor I enhances proenkephalin synthesis and dopamine beta-hydroxylase activity in adrenal chromaffin cells.

Insulin-like growth factor I (IGF-I) increased both the contents of proenkephalin-derived enkephalin-containing peptides and the activity of dopamine beta-hydroxylase in bovine adrenal chromaffin cells. These increases in dopamine beta-hydroxylase and enkephalin-containing peptides continued for at least 8 days. The half-maximal IGF-I concentration for these effects was approximately 1 nM, with maximal effects observed at 10-30 nM. In contrast, insulin was 1000-fold less potent. Pretreatment of chromaffin cells with IGF-I increased the rate of [35S]proenkephalin synthesis 4-fold compared to untreated cells. Total protein synthesis increased only 1.5-fold under these conditions. These results suggest that IGF-I may be a normal regulator of chromaffin cell function.