1. in the present study the effects of acute and chronic insulin treatment on growth factor-induced cell growth were investigated in vascular smooth muscle cells. Cell growth was quantified by the incorporation of [3H]thymidine into cell DNA and by determination of total protein.
2. Insulin in a concentration range of 10−9-10−6 mol/l had no effect on cell DNA synthesis. Insulin-like growth factor-1 in a concentration range of 10−9-10−6 mol/l induced a concentration-dependent increase in total cell protein, whereas no changes in DNA synthesis were observed.
3. Acutely, insulin enhanced the mitogenic effect of epidermal growth factor and angiotensin II. Long-term treatment with 10 and 100 ng/ml insulin over 14 weeks was associated with a time-dependent reduction in this potentiating effect of insulin on growth factor-induced DNA synthesis. To evaluate the mechanism of this effect, receptor binding studies were performed with 125I-labelled insulin-like growth factor-1. No difference in the Kd value (2.1 nmol/l) for insulin-like growth factor-1 was found between untreated and chronically insulin-treated cells, whereas the maximal number of binding sites decreased from 5.9 × 104 (untreated cells) to 4.7 × 104 (10 ng/ml) and to 4.3 × 104 (100 ng/ml) binding sites/cell in chronically insulin-treated cells.
4. We conclude that insulin acutely enhances the mitogenic effect of various growth factors such as epidermal growth factor and angiotensin II on vascular smooth muscle cells. Long-term insulin treatment reduced the number of receptor binding sites for insulin-like growth factor-1. This may contribute to the reduced response of cells to growth stimulation after chronic in contrast with acute insulin treatment.