Effect of Insulin In Vivo and In Vitro on Incorporation of Amino Acid into Protein in Normal and Diabetic Rat Aorta

Summary

The influence of insulin treatment in vivo and of insulin in vitro on the incorporation of amino acid into protein was studied in aorta of normal and streptozotocin-diabetic rats. The incorporation of amino acid into protein was measured by incubating aorta in vitro for 2-3 h in the presence of ¹⁴C-labelled L-leucine or L-phenylalanine and then determining the amount of radioactivity incorporated into alkali-soluble protein.

After insulin treatment of diabetic rats for 48 h the incorporation of leucine-¹⁴C into aortic protein was increased, while insulin treatment for 12 or 24 h had no effect. In non-diabetic rats insulin treatment for 24-72 h did not affect the incorporation of leucine-¹⁴C or phenylalanine-¹⁴C into aortic protein.

Incubation of normal or diabetic rat aorta with insulin (0.1 U/ml) in vitro without serum for up to 48 h did not influence the leucine-¹⁴C incorporation. Addition of 5 % foetal bovine serum increased the incorporation of leucine-¹⁴C into aortic protein, determined after 48 h. When the incubation medium was supplemented with serum the leucine incorporation into protein in diabetic aorta was significantly increased after 48 h by insulin added in a high concentration (0.1 U/ml), while insulin at a physiological concentration had no demonstrable effect.

The absence of a direct effect of insulin in a physiological concentration in vitro on the amino acid incorporation in normal and diabetic aorta and the finding that insulin treatment in vivo only stimulated the leucine-¹⁴C incorporation in diabetic aorta suggests that the effect of insulin administration on the amino acid incorporation in diabetic aorta is due to the correction of the diabetic state and not to a direct action on the aortic protein metabolism.

Key-Words

Rat Aorta - Vascular Smooth Muscle - Protein Metabolism - Diabetes Mellitus - Insulin - Streptozotocin