LONG-TERM INSULIN-TREATMENT OF VASCULAR SMOOTH-MUSCLE CELLS FROM RAT AORTA ATTENUATES THE SYNERGISTIC EFFECT OF INSULIN ON ANGIOTENSIN-II-INDUCED AND EPIDERMAL GROWTH FACTOR-INDUCED DNA-SYNTHESIS

1. In the present study the effects of acute and chronic insulin treat ment on growth factor-induced cell growth were investigated in vascula r smooth muscle cells. Cell growth was quantified by the incorporation of [H-3]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 ef fect on cell DNA synthesis. Insulin-like growth factor-1 in a concentr ation range of 10(-9)-10(-6) mol/l induced a concentration-dependent i ncrease in total cell protein, whereas no changes in DNA synthesis wer e observed. 3. Acutely, insulin enhanced the mitogenic effect of epide rmal 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-indu ced DNA synthesis. To evaluate the mechanism of this effect, receptor binding studies were performed with I-125-labelled insulin-like growth factor-1. No difference in the K(d) value (2.1 nmol/l) for insulin-li ke growth factor-I was found between untreated and chronically insulin -treated cells, whereas the maximal number of binding sites decreased from 5.9 x 10(4) (untreated cells) to 4.7 x 10(4) (10 ng/ml) and to 4. 3 x 10(4) (100 ng/ml) binding sites/cell in chronically insulin-treate d cells. 4. We conclude that insulin acutely enhances the mitogenic ef fect of various growth factors such as epidermal growth factor and ang iotensin II on vascular smooth muscle cells. Long-term insulin treatme nt reduced the number of receptor binding sites for insulin-like growt h factor-1. This may contribute to the reduced response of cells to gr owth stimulation after chronic in contrast with acute insulin treatmen t.