Physiology PA3 PDF - Investigation of Glucose Metabolism

Physiology PA 3 Investigation of glucose metabolism: fasting glycemia test, oral glucose tolerance test When glucose concentration in the extracellular fluid decreases, the body uses liver glycogen to release glucose, which can sustain glucose supply for approximatel...

Physiology PA 3 Investigation of glucose metabolism: fasting glycemia test, oral glucose tolerance test When glucose concentration in the extracellular fluid decreases, the body uses liver glycogen to release glucose, which can sustain glucose supply for approximately 16 h During prolonged fasting or extreme exercise the synthesis of glucose from noncarbohydrate compounds gluconeogenesis happen Main substrates are: Lactate (from muscle activity) Alanine (from muscle protein breakdown) Glycerol (from fat breakdown) Glucose homeostasis is controlled by: the anabolic hormone insulin a set of catabolic hormones (glucagon, catecholamines, cortisol, and growth hormone) , also known as counterregulatory hormones. Insulin and glucagon are secreted from the pancreatic islets of Langerhans Insulin is secreted by β cells Glucagon by the α cells The molar ratio of insulin to glucagon is the key determinant of the pattern of fuel metabolism. Function of endocrine system = maintaining normal blood glucose levels The blood glucose level reflects the balance between the main glucose lowering hormone (insulin) and the hormones that rise glycaemia. We do a fasting blood glucose level if an imbalance of glucose homeostasis is suspected. Venous, capillary or arterial blood may be used. Interpreting blood glucose levels depends on several factors: Measurement method: Older chemical methods (Hagedorn-Jensen, Somogyi- Nelson, Benedict) give higher readings than modern enzymatic methods (glucose oxidase, hexokinase), which are more specific. Blood sample source: Glucose levels are 5-15 mg% higher in arterial and capillary blood compared to venous blood. Age: Fasting blood glucose gradually increases with age, rising by about 10 mg% every 10 years in older adults. Recommandation for screening by glucose testing for diabetes: Anyone with a body mass index higher than 25 (23 for Asian-Americans), regardless of age, who has risk factors such as high BP, abnormal cholesterol level, Heart disease,… Older than 45 y.o, every 3 years Woman who had gestational diabetes People with prediabetes, every year Self-monitoring glucose device Glycemic reader (glucose meter) For diabetic people They can adjust and check the effect of their treatment (diet, insulin, exercice) Steps: 1. Wash hands 2. Insert the test strip into glucose meter 3. Insert the lancet into lancing device 4. Prick the end of a finger 5. Squeeze the end of your finger if necessary 6. Apply blood to the test strip 7. Wait a few second 8. Read and write the result or store it in the glucose meter Results A fasting blood sugar level between 80-100 mg/dL (5.6 mmol/L) is normal. A fasting blood sugar level from 100 to 125 mg/dL (5.6 to 6.9 mmol/L) is considered impaired fasting glucose or prediabetes. If it's 126 mg/dL (7 mmol/L) or higher on two separate tests,in two separate days, diagnosis of diabetes should be established. Oral glucose tolerance test (OGTT) OGTT is used because a normal fasting blood glucose level does not exclude the existence of a latent diabetes. It assesses blood glucose response to a carbohydrate load. Such clinical situations are also liable to unmask asymptomatic pre-existing diabetes or in predisposed individuals. Indications Family history of diabetes mellitus. Sudden unexplained loss of weight. Repeated infections, especially in the genitals or urinary tract. Mothers with overweight newborn babies (babies weighing above 4 kg). Premature cataract (below 50 years). Delayed healing of wounds. Must be performed under standard conditions: Patient has to do it in the morning after 10h fast. To avoid stress or exercice related changes in plasma glucose, the patient has to sit Drugs/medications should be avoided if possible Smoking should be avoided = nicotine stimulates catecholamine release which raises blood glucose level Should not be done in patient with recent illness, surgery or major stress Steps: 1. Patient sits 2. Fasting plasma glucose is measured first from venous or capillary blood. 3. The patient is then given a standard quantity of glucose to drink (75 g in 300 mL of water). Anhydrous glucose is dissolved in the water. 4. Plasma glucose concentration is measured again after 120 min. In some protocols, glucose is measured after 20, 60, and 120 min. 5. Urine formed at the time of blood collection is also examined for sugar and ketones. 6. The values are plotted as a graph for comparison with normals. Interpreting Blood Glucose Levels 1. Normal Response: Blood glucose peaks around 60 minutes after eating. It returns to near fasting levels within 120 minutes. 2. Diabetes Diagnosis: If glucose stays above 11.1 mmol/L (200 mg/dL) at 120 minutes, diabetes is diagnosed—even if fasting glucose was normal. 3. Impaired Glucose Tolerance (IGT): If fasting glucose is normal but post-load (after eating) glucose is between 6.1– 7.8 mmol/L (100–140 mg/dL), it indicates IGT, a prediabetes condition. When a normal, fasting person ingests 1 gram of glucose per kilogram of body weight, the blood glucose level rises from about 90 to 120 to 140 mg/100 ml and falls back to below normal in about 2 hours. In a person with diabetes, the fasting blood glucose concentration is almost always above 115 mg/100 ml and often is above 140 mg/100 ml. After ingestion of glucose, these people exhibit a much greater than normal rise in blood glucose level and the glucose level falls back to the control value only after 4 to 6 hours, furthermore, it fails to fall below the control level. If blood glucose levels of the curve fall slowly and don´t return to normal it suggests: The normal increase in insulin secretion after glucose ingestion doesn’t occur The person has decreased sensitivity to insulin Diabetes mellitus can be established based on such a curve and type 1 & 2 can be distinguished by measurement of plasma insulin: Plasma insulin is low or undetectable in type 1 diabetes Increased in type 2 diabetes If there is doubt about the significance of hyperglycaemia, the blood glucose level should be rechecked 30–60 min later and the urine tested for ketones. Insulin Insulin and C-peptide levels are determined with radioimmunological tests. The normal blood level of insulin is above 10 μUI/ml. Patients with marked decrease of insulin level are harder to balance metabolically. The insulin levels are also useful in determining the cause of hypoglycaemia. An acute first phase of insulin secretion occurs in response to an elevated blood glucose, followed by a sustained second phase. The incretin effect means that the body releases more insulin when glucose is taken orally compared to when it’s given intravenously, even if blood sugar levels rise the same amount. This happens because the gut releases peptides that stimule insulin secretion, helping regulate blood sugar more efficiently. C peptide Is a short 31 amino acid polypeptide that connects insulins A chain and B chain in the proinsulin molecule C-peptide and insulin are released from the pancreas at the same time and in about equal amounts Most of the insulin (~60%) that is secreted into the portal blood is removed in a first pass through the liver. In contrast, C peptide is not extracted by the liver at all. C peptide levels can be measured by radioimmunoassay: in insulin-treated diabetic patients to determine how much of their own natural insulin they are still producing in patients with type 1 diabetes who are unable to produce insulin the levels of C peptide will usually be greatly decreased. The glycated hemoglobin (HbA 1c) HbA1c (Glycated Hemoglobin) measures average blood sugar levels over 2-3 months. It forms when glucose attaches to hemoglobin in red blood cells. When glucose is present, it attaches to proteins like hemoglobin in nonenzymatic glycosylation. The higher the blood sugar, the more hemoglobin gets glycated. HbA1c reflects average blood sugar over 2-3 months. Normal range: 4.0%–5.9% HbA 1c forms in the blood erythrocytes at a rate proportional to the prevailing glucose concentration. Because the glycation reaction is irreversible, the formed HbA 1c remains in the circulation for the entire life of an erythrocyte. Thus its concentration reflects the average concentration of plasma glucose over the 8–12 weeks preceding HbA 1c measurement. Because the average lifespan of red blood cells is about 120 days, the HbA1c test is used mainly to assess average blood glucose concentrations for the previous three months. Plasma contains populations of erythrocytes of different age, so the exact period time is difficult to calculate HbA 1c concentration may be affected by anemia and the presence of hemoglobin variants. In United States HbA 1c measurements = percentage of total hemoglobin. In Europe = mmol/mol The new method for measuring HbA1c works by cutting off a small hexapeptide (six-amino-acid segment) from the β-chain of HbA1c using an enzyme (endopeptidase). After this, the pieces are separated and measured using mass spectrometry or capillary electrophoresis, making the test more precise and reliable. HbA 1c is used to diagnose diabetes and to monitor glycemic control Level of 48mmol/L (6.5%) or higher is for diabetes The ADA recommends keeping HbA1c below 7% for most diabetic patients. In young children and the elderly, stricter control may be risky due to hypoglycemia, so treatment goals should be adjusted to balance safety and effectiveness. Glucosuria In healthy individuals, glucose does not appear in urine because the kidneys reabsorb it in the proximal tubule after filtration. However, a small amount may still be present in the final urine. Higher blood glucose → More glucose in urine, but urine glucose levels vary due to fluid intake. In diabetes, when blood glucose exceeds 180 mg/dL, the kidney can’t reabsorb all of it, so glucose spills into the urine—this is called the renal reabsorption threshold. Renal Handling of Glucose Glucose is freely filtered at the glomerulus and is 100% reabsorbed in the proximal tubules by sodium- glucose cotransporters (insulin-independent). However, if blood glucose levels become elevated, as in persons with diabetes, the maximal tubular reabsorption rate (TM) is exceeded, and glucose appears in the urine. The detection of glucose using test strips relies on an enzymatic reaction involving glucose oxidase. This enzyme oxidizes glucose with oxygen from the air, producing lactone and hydrogen peroxide. A second reaction occurs where peroxidase helps combine the hydrogen peroxide with a chromogen (a substance that changes color). The resulting colored compound indicates the concentration of glucose in the sample. Clinical significance: Diabetes Mellitus, gestational diabetes (placental hormones blocking insulin) Hormonal disorders: Hypercortisolism Renal tubular disorders prevent tubular reabsorption of glucose, renal diabetes Ketonuria Ketone bodies are produced by the liver from fatty acids during periods of low food intake or glucose restriction, they serve as energy source for the cells of the body. The level of ketone bodies rises if metabolic imbalance occurs in diabetes mellitus. The ketone bodies in physiological amounts are undetectable in blood by conventional methods. If their blood level rise they appear also in the urine (ketonuria). Normal level: 0,5 mg% Ketones or ketone bodies refers to three intermediate products in the metabolism of fatty acids: Acetone acetoacetic acid beta-hydroxybutyric acid. Elevated concentrations of ketones are not generally found in urine, as all these substances are completely metabolized, producing energy, carbon dioxide and water. In certain condition, there is an increased concentration in blood, they turn positive in urine: disruption of carbohydrate metabolism inability to metabolize carbohydrates (in diabetes) increase in fat metabolism (can be the result of starvation or malabsorption) due to losses from frequent vomiting The test used in the urine test strips is based on the reaction of sodium nitroprusside (nitroferricyanide) In this reaction the acetoacetic acid in an alkali medium reacts with the sodium nitroprusside producing a magenta coloured complex Sodium nitroprusside + Acetoacetic acid + Alkali medium → Pink-magenta complex + Water The test does not measure beta-hydroxybutyric acid and it is only weakly sensitive to acetone when glycine is added to the reaction. Acetone Breath Acetone breath occurs when the body produces high levels of ketones, especially in severe diabetes (Type 1). In diabetes, acetoacetic acid (a type of ketone) increases in the blood and gets converted to acetone. Acetone is volatile, so it enters the lungs and is exhaled, creating a fruity-smelling breath—a common sign of diabetic ketoacidosis (DKA). A ketone meter can measure acetone levels in the breath to help monitor ketosis or diabetes control. Interpretation 0.0 = The person is not in ketosis state 0.1-0.5 = The person is in light state of ketosis 0.5-3.0 = The person is in full state of ketosis, which is optimal ketone 3.0 = The person is in starvation ketosis 5.0 = The person is in too much ketosis and need medical care 7.0 = The person is in ketoacidosis