Effects of insulin on protein metabolism and growth
The way insulin increases protein synthesis is not as well understood as with the mechanism in glucose and fat storage. Here are some results recorded in practice.
Insulin increases protein synthesis and storage
Proteins, carbohydrates and fats are stored in tissues within a few hours after a meal when there is an excess of nutrients in the circulation; insulin is required for this storage to occur. The way insulin increases protein synthesis is not as well understood as with the mechanism in glucose and fat storage. Here are a few results in practice:
1. Insulin stimulates the transport of many amino acids into the cells. Among the amino acids, the strongest transporters are valine, leucine, isoleucine, tyrosine and phenylalanine. Therefore, insulin together with growth hormone increases the uptake of amino acids into the cells. However, the amino acid that is affected independently is not necessarily the same as the other amino acid.
2. Insulin increases translation to make new proteins. In an unknown way, insulin "turns on" the ribosomal machinery. When there is a shortage of insulin, the ribosome simply stops working, almost as if the insulin works on an "on-off" mechanism.
3. Over a long period of time, insulin also increases the transcription rate of selected DNA sequences in the cell nucleus, thereby forming an increase in RNA and protein synthesis, especially increasing many enzymes for carbohydrate storage, fats and proteins.
4. Insulin inhibits protein catabolism, thereby reducing the proportion of amino acids released from cells, especially from muscle cells. It is possible that this results from the ability of insulin to reduce the normal breakdown of cells by lysosomes.
5. In the liver, insulin reduces the rate of gluconeogenesis by reducing the activity of enzymes that increase gluconeogenesis. Since the primary sources of glucose synthesis in gluconeogenesis are plasma amino acids, this inhibition regulates amino acids in the body's storage proteins.
In summary, insulin increases protein formation and prevents protein breakdown.
Insulin deficiency causes protein depletion and elevation of plasma amino acids
In fact, all protein storage stops in the absence of insulin. Protein catabolism increases, protein synthesis stops, and large amounts of amino acids are released into the plasma. Plasma amino acid concentrations are significantly increased, and most of the excess amino acids are used directly for energy or as raw materials for gluconeogenesis. Amino acid degradation also leads to increased urinary excretion of urea. As a result, protein loss is one of the important effects of diabetes. It can lead to severe weight loss and multiple organ failure.
Insulin and growth hormone interact synergistically to promote growth
Since insulin is required for protein synthesis, it is required as a growth hormone for the development of animals. As shown, pancreatectomy, pituitary resection in rats, if left untreated, rarely develop even a little. Furthermore, regulation by either of these two hormones at a time hardly induces growth. However, the combination of these two hormones results in impressive growth, so there appears to be a functional interaction of these two growth-promoting hormones, each with its own specific functional roles. Perhaps the need for both hormones for growth stems from the fact that each selectively enhances cellular uptake of different amino acids, and all of them are necessary for growth.
Figure. Effects of growth hormone, insulin and growth hormone plus insulin on growth in an impaired and hypovolemic rat.