Potassium
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Potassium (K+)
• major intracellular cation, • In cells and RBC average concentration is 150mmol/L and 105mmol/L respectively
Increased intracellular K+ are maintained by:
• Na+ and K+ (atp) pump
What happens to K+ ions when pump activity decreases
• K+ diffuse out of the cell into the ECF and plasma
Excess potassium is excreted by which organ
• kidneys
What hormone is responsible for excretion and reabsorption of K+ in exchange for Na+
• aldosterone
Specimen requirements for K+ determination
• Serum or plasma must be collected with minimal hemolysis to avoid contamination from RBC intracellular K • Whole blood samples should be kept at 4 degrees b4 separation • Severe leukocytes • Skeletal muscle activity causes K+ efflux from muscle cells into plasma causing potassium level elevation
Why should blood samples be kept at 4 degrees Celsius b4 separation?
• to prevent glycolysis • The energy-dependent Na+, K+- ATP can’t maintain the Na+/K+ gradient
Clinical significance or conditions
• Hypokalemia associated with gastrointestinal loss thru vomiting, diarrhea, gastric suction, intestinal tumor, malabsorption, cancer therapy • Renal loss through diuretics the, nephritis, renal acidosis and hyperaldosteronism • Cellular shift due to acidosis, chemotherapy, muscle/cellular injury • Artifactual due to sample hemolysis
Reference intervals of potassium
• Serum= 3.5-5.1 mmol/L • Plasma= males 3.5-4.5mmol/L Females 3.4- 4.4mmol/L • Urine ( 24hr)= 25-125mmol/L
Determination of Na+ and K+
• Flame emission spectrophometry • Spectrophometry essays • Ion selective electrodes
Flame emission spectrophometry principle
• Samples are diluted in a diluent containing known amounts of Lithium or cesium and are aspirated into a propane air flame • Li and Cs ions when excited they emit spectra with sharp bright lines at 671 and 852 nm respectively • Lights emitted from the excited ions is directed through separate photo detectors • The Li+ or Cs+ emission signal is used as an internal standard against which the Na+ and K+ signals are compared
categories of Spectrophometric methods
• Based on enzyme activation • Kinetic Spectrophometric essays • Those that detect the spectral shift produced when Na+bor K+ binds to a macrocyclic chromophore
Kinetic Spectrophometric essays are based on what?
• Activation of the enzyme B-galactosidase by Na+
What does the enzyme B-galactosidase hydrolyze?
• O-nitrophenol-B-d_galactosidase
ion selective electrodes
• ISE method uses a semipermeable membrane to develop an electric potential produced by having different ion concentrations on either side of the membrane. • Two electrodes are used; one electrode has a constant potential, making it the reference electrode. • The difference in potential between the reference and measuring electrodes is used to calculate the “concentration” of the ion in solution • Analyzers fitted with ISEs usually contain Na+ electrodes with glass membranes and K+ electrodes with liquid ion-exchange membranes that incorporate valinomycin • The change in electromotive force in a circuit between a measurement electrode (the ISE) and a reference electrode is measured • Most blood gas analyzers are capable of measuring sodium and potassium • Indirect ISE methods, uses diluted samples • Direct ISE methods, uses undiluted samples • Errors are observed due to lack of selectivity, repeated protein coating and contamination of the membrane with competing ions