Distribution and Biotransformation
Upon infusion, glucose is first distributed intravascularly and then is taken up into the intracellular compartment.
Metabolism
Glucose, as a natural substrate of the cells in the organism, is ubiquitously metabolised. Under physiological conditions it is the most important energy-supplying carbohydrate with a caloric value of approximately 17kJ/g or 4kcal/g. Central nervous system tissues, erythrocytes and renal medulla are among others the obligatory glucose users. The normal fasting concentration of blood glucose range is 50–95mg/100ml or 2.8– 5.3mmol/l.
Glucose serves the formation of glycogen as the body’s carbohydrate reserve and undergoes glycolysis to pyruvate or lactate for energy production in the cells. Glucose also enables the maintenance of the blood glucose level and the biosynthesis of important components of the organism. Insulin, glucocorticoids, and catecholamines are mainly involved in hormonal regulation of the blood glucose level.
By glycolysis, glucose is metabolised to pyruvate or lactate. Lactate can be partially re-introduced into the glucose metabolism (CORI cycle). Under aerobic conditions pyruvate is completely oxidised to carbon dioxide and water. The final products of this complete oxidation of glucose are eliminated via the lungs (carbon dioxide) and the kidneys (water).
A precondition for optimal utilisation of supplied glucose is a normal electrolyte and acid-base status. Particularly, acidosis can be a sign of impaired oxidative metabolism.
There is a strong correlation between electrolyte and carbohydrate metabolism which particularly affects potassium. Utilisation of glucose is associated with increased potassium requirements. If this relationship is not taken into account considerable disturbances of potassium metabolism may occur which can lead to massive cardiac arrhythmias amongst other conditions.
Under pathological metabolic conditions impairment of glucose utilisation (glucose intolerances) may occur. These include primarily diabetes mellitus as well as the hormonally-induced decrease of glucose tolerance resulting from metabolic stress (e.g. intra and post-surgery, serious illnesses, injuries) that may cause hyperglycaemia even without exogenous substrate supply. Hyperglycaemia – depending on the severity – may lead to osmotically based loss of fluid via the kidneys resulting in hypertonic dehydration, hyperosmolar disturbances and even hyperosmolar coma.
Excessive supply of glucose, in particular, in the course of a post-aggression syndrome, can considerably increase the disturbance of glucose utilisation and contribute to enhanced conversion of glucose to fat resulting from impaired oxidative glucose utilisation. This in turn may be associated with an increased carbon dioxide burden of the organism (problems with weaning from respirator) and additional fat infiltration in the tissues, specifically in the liver. Patients with intracranial injury and brain oedema possess a particular risk to disturbances of glucose homeostasis. In these patients, even slight disturbances of the glucose concentration and the associated increase in plasma (serum) osmolality can contribute to a significant increase of cerebral injury.
Upon infusion acetate is first distributed intravascularly and then is taken up into the interstitial compartment. Under physiological conditions acetate is converted to bicarbonate and carbon dioxide. The plasma bicarbonate and acetate concentrations are regulated by the kidneys; the carbon dioxide plasma concentration by the lungs.
Elimination
In healthy subjects, there is practically no renal elimination of glucose. In pathological metabolic situations (e.g. diabetes mellitus, post-aggression symptom) that are accompanied by hyperglycaemia (blood glucose concentrations above 120mg/100ml or 6.7 mmol/l) glucose is also eliminated via the kidneys (glucosuria) when the maximum tubular transport capacity (180mg/100ml or 10mmol/l) is exceeded.