Insulin

Biosynthesis

Insulin is a 51-amino acid peptide hormone secreted from beta cells of the islets of Langerhans in the pancreas. In normal physiology, glucose concentrations of 2 to 4 mM are required to trigger the biosynthesis of insulin; however, insulin is released from cells only when glucose concentrations rise to levels greater than 5 mM. ^[1]

Insulin is initially synthesized in the endoplasmic reticulum and Golgi apparatus as proinsulin; it is then cleaved to insulin and C-peptide. Although insulin and C-peptide are co-secreted in equal molar proportions, the ratio of serum insulin to C-peptide is 1:5-15. Fifty to sixty percent of insulin is extracted by the liver before it reaches the systemic circulation, and it has a half-life of only 4 minutes. In contrast, C-peptide and proinsulin are excreted via the kidney. ^[2]

Function

Insulin is an anabolic hormone that promotes glucose uptake, glycogenesis, lipogenesis, and protein synthesis in skeletal muscle and fat tissue through the tyrosine kinase receptor pathway. Through these functions, insulin serves as the most important factor in maintaining plasma glucose homeostasis, counteracting glucagon and other catabolic hormones such as epinephrine, glucocorticoid, and growth hormone. In addition, the literature suggests that insulin plays a pivotal role in maintaining normal neurologic function, bone development, and hair growth and in preventing aging. ^{[3, 4]}

Secretion

When plasma glucose levels are elevated, glucose diffuses to beta cells through glucose transporter 2 (GLUT2) and activates the glycolysis pathway, leading to elevated adenosine triphosphate (ATP) levels. Increasing ATP levels induce ATP-sensitive K⁺ channels to shut down, subsequently stimulating depolarization of the beta-cell membrane. ^[5] Then, voltage-gated Ca²⁺ channels are opened to increase cytosolic Ca²⁺ and trigger insulin exocytosis. ^[2] However, high insulin levels in the hypoglycemic state have been found; an example is insulinoma, in which insulin is secreted at a high rate independent of the plasma glucose level.

Interestingly, oral administration of glucose is more effective at increasing insulin secretion than is intravenous glucose administration; this is called the "incretin effect." Carbohydrate meals potentiate insulin secretion through multiple gastrointestinal hormones (incretin hormones), including cholecystokinin, glucagonlike peptide-1 (GLP-1), and gastric-inhibiting polypeptide (GIP). ^{[2, 6]}

Collection and laboratory measurement

Methods: Radioimmunoassay; chemiluminescent assay; enzyme-linked immunosorbent assay (ELISA)

Specifics for collection and panels are as follows:

• Specimen type - Blood; an 8-hour fasting specimen is required for fasting insulin measurement
• Container - Vacutainer, red top
• Collection method - Venipuncture
• Specimen volume - 1 mL

Indications/applications

A standard insulin test measures endogenous and exogenous insulin. In patients not receiving exogenous insulin, serum insulin levels reflect the quantitative functioning of pancreatic beta cells. Calculations based on plasma insulin and glucose concentrations can be used to determine insulin sensitivity and resistance. Measurement of insulin levels is also helpful in differentiating between type 1 and type 2 diabetes and can assist in identifying causes of hypoglycemia.

Normal insulin levels vary widely and depend on age, gender, and race. In addition, insulin tests demonstrate a minimal cross-reaction with proinsulin and insulin-like growth factors 1 and 2, with the degree of variability depending on the brand of the testing toolkit and technique used. 

The pattern of insulin secretion in response to a glucose load depends on the route of glucose administration and, in cases of oral administration, the meal composition. ^[7] The numbers mentioned in table 1 (below) provide a broad estimate of normal values in adults; however, standardized reference values are yet to be determined due to the significant variability among immunoassays used today. ^[8]

Table 1. Reference Range of Insulin Levels ^[7] (Open Table in a new window)

Considerations

Insulin levels may be falsely elevated by the following:

• Amino acids - Leucine, arginine, lysine
• Steroids
• Insulin secretagogues - Sulfonylureas, glinides
• Estrogen ^[9]
• Beta-2 agonists

Insulin levels may be falsely decreased by the following:

• Acarbose, metformin, octreotide, and beta-blocker use
• Hemolysis - Results in release of an insulin-degrading enzyme found in red blood cells ^[10]
• Hemodialysis

Conditions associated with elevated insulin levels

Conditions associated with increased insulin resistance , wherein beta cells compensate via hypersecretion of insulin, include the following ^[10] 5: 

• Obesity
• Steroid administration
• Acromegaly
• Cushing syndrome
• Insulin receptor mutation
• Type 2 diabetes (early stage)

Research suggests that mitochondrial reactive oxygen species are linked to the development of insulin resistance in adipose tissue and skeletal muscle. ^[11]

A study by Gabay et al indicated that insulin resistance in female adolescents with obesity may have a negative impact on brain structure and function. The report found, for example, that in the study population, the insular cortices in such patients tended to be thinner, a phenomenon that was not seen in male adolescents with obesity and insulin resistance. ^[12]

Conditions associated with increased insulin secretion include the following:

• Insulinomas (insulin- or proinsulin-secreting tumors)
• Administration of insulin secretagogues

Autoimmunity to insulin or insulin receptors is also associated with elevated insulin levels.

Conditions associated with decreased insulin levels

Conditions associated with beta-cell destruction include the following:

• Post pancreatectomy
• Chronic pancreatitis
• Autoimmune destruction and type 1 diabetes

Role of insulin measurement in determining the etiology of hypoglycemia

Insulin testing is used to assist in identifying causes of hypoglycemia (plasma glucose levels < 55 mg/dL). When spontaneous hypoglycemia does not recur, a 72-hour fasting test is performed to provoke hypoglycemia. ^[13]  Insulin levels in different conditions are described below:

• Insulinoma - High insulin and C-peptide levels
• Non–beta cell tumors - Low insulin and C-peptide levels and high insulin-like growth factor 2 level ^[14]
• Excessive insulin administration - High insulin levels and low C-peptide levels
• Insulin secretagogue administration (sulfonylureas or glinides) - High insulin and C-peptide levels
• Congenital hyperinsulinism (mutation in insulin-secreting gene) - High insulin and C-peptide levels
• Autoimmunity to insulin or insulin receptor (common in patients receiving insulin or those who have autoimmune diseases such as systemic lupus erythematosus [SLE] or Hashimoto thyroiditis) - Postprandial insulin is bound to antibodies and dissociated 1 hour later, resulting in an extremely elevated insulin level and high insulin–to–C-peptide ratio ^[10]

Table 2. Interpretation of 72-hour Fasting Test Results ^[13] (Open Table in a new window)

Insulin levels can be used to assess insulin resistance versus sensitivity. In insulin resistance, the ability of cells to respond to the action of insulin in transporting glucose into tissues is diminished; consequently, the resistant individual begins secreting above-normal amounts of insulin to obtain a quantitatively normal response.

Insulin resistance develops long before the appearance of disease signs. A study by Kraft found borderline diabetes in 14% of subjects with normal oral glucose tolerance tests who had been randomly referred for such evaluation. ^[15]

There are multiple methods available to assess insulin resistance, including the following:

• Hyperinsulinemic-euglycemic glucose clamp
• Fasting insulin
• Homeostasis model assessment (HOMA)
• Quantitative insulin sensitivity check index (QUICKI)
• Kraft insulin patterns/protocol
• Hayashi protocol

Each of these methods has its own limitations. The lack of standardization of the insulin assay procedures prevents the comparison of results between studies; consequently, studies can be compared only qualitatively. The American Diabetes Association (ADA) organized a task force to standardize insulin assays. ^[16]

HOMA equations have been one of the tools widely used in research to estimate insulin resistance. The two equations (which use fasting blood levels) are as follows, with HOMA-IR used to assess insulin resistance and HOMA-B used to assess pancreatic beta cell function ^{[17, 18]} :

• HOMA-IR = (glucose in mmol/L x insulin in mIU/mL)/22.5
• HOMA-B = (20 x insulin in mIU/mL)/(glucose in mmol/L - 3.5)

Fasting insulin levels can serve as a tool to help guide the choice of therapy in patients newly diagnosed with type 2 diabetes. A study by Saxena et al found that such patients with normal to low initial fasting serum insulin levels responded better to glipizide than to metformin. On the other hand, those with high fasting serum insulin levels responded significantly better to metformin than to glipizide. ^[19]

Using the HOMA index, a pediatric study by Genovesi et al indicated that, just as a relationship between degree of insulin resistance and risk of hypertension exists in children with excess weight, the same holds true in youngsters of normal weight. The investigators noted that while children with higher body mass index (BMI) z-scores had greater HOMA index values, the HOMA index was linked to the systolic blood pressure z-score regardless of weight class. ^[20]