Insulin action sites and symptoms of insulin resistance – part 1

Insulin action sites and symptoms of insulin resistance – part 1

The effects of insulin, the effects of its deficiency and insulin resistance vary depending on the physiological function of tissues, organs and the extent to which their action is dependent on insulin in metabolic processes. Tissues defined as insulin-dependent, that function is based on intracellular glucose transport, are mainly adipose tissue and muscle. However, it should be emphasized that insulin affects the energy balance throughout the body.

Muscles

Glucose uptake by muscle cells is insulin dependent. The muscles are responsible for about 60-70% of insulin uptake in the whole body. After a meal, insulin induces glycogen synthesis in a way that enables the anaerobic release of energy through glycolysis, e.g. during intense muscle work. In the basic state (i.e. when energy – food is not supplied to the body), the level of insulin is low, and muscle cells obtain energy not from glucose as a result of catabolism (decomposition) of proteins that are the building blocks of muscles. Insulin inhibits protein catabolism, while insulin deficiency promotes it by releasing amino acids for gluconeogenesis. When the body is in a state of starvation, the level of protein synthesis is significantly reduced – up to 50%. In experimental studies, the dose of insulin inducing protein synthesis is much higher than the dose required to inhibit proteolysis (hydrolytic breakdown of the peptide bond by proteases. It leads to the breakdown of proteins into peptides and amino acids). Insulin has an anabolic effect (increases the rate of cell division in individual human tissues) in combination with growth hormone, IGF-1 and appropriate amino acids.

In insulin resistance, muscle glycogen synthesis is impaired, which may be due to limited intracellular glucose transport.

Source: Wilcox G.. Clin Biochem Rev. 2005 May;26(2):19-39. PMID: 16278749; PMCID: PMC1204764.