Binding and growth promoting effects of insulin, insulin analogues modified in the B chain, proinsulin, insulin-like growth factor-I and -II were studied in cultured rat aortic smooth muscle cells. Specific binding of ~2~I-insulin was 0.9 + 0.2% of total ~25I-insulin added, and the ICs0-value was estimated to 8.9 pmol/1. The insulin analogue B10 Asp tended to be more potent than insulin in displacing ~25I-insulin, B28 Asp was equipotent, B9 Asp/B27 Glu was approximately 100 times less potent and insulin-like growth factor-I more than 1000 times less potent than insulin. Specific binding of ~I-insulin-like growth factor-I after 4 h incubation at 10 ~ was five times higher than the specific binding of insulin (4.4_+ 0.4% of total ~25I-insulin-like growth factor-I added), and the ICs0-value was 0.3 nmol/1. Insulin was approximately 500 times less potent than insulin-like growth factor-I in displacing '~I-insulin-like growth factor-I. The insulin analogue B10 Asp was slightly more potent and analogue B28 Asp was equipotent with insulin. Analogue B9 Asp/B27 Glu was ten times less potent and proinsulin was more than ten times less potent than insulin. The order of potency was similar for 3H-thymidine incorporation into DNA: insulin-like growth factor-I > B10 Asp > insulin-like growth factor-II > insulin _> B28 Asp > B9 Asp/B27 Glu > proinsulin. The maximal effect of insulin-like growth factor-I on 3H-thymidine incorporation was 71 _+ 16% higher than the maximal effect of insulin. The maximal effect of insulin-like growth factor-II was at least as high as the effect of insulin-like growth factor-I. Furthermore, the maximal effect of B10 Asp was 62 + 10% higher than the maximal effect of insulin. Insulin-like growth factor-I and B10 Asp tended to increase cell number more than insulin. In conclusion, this study shows that insulin analogues interact with different potencies with receptors for insulin and insulin-like growth factor-I in vascular smooth muscle cells and that insulin-like growth factors and the insulin analogue B10 Asp have more pronounced growth effects than insulin. Substitution of the amino acid Asp for His at position B10 in insulin makes the molecule more similar to insulin-like growth factor-I, chemically and probably also biologically.