Electrolytes Part 2

Bicarbonate 2nd most abundant anion of ECF. Major component of the HCO3- /H2CO3 buffering system. Serves as a transport form for CO2 produced from metabolic processes in tissues HCO3- accounting for more than 90% of the total CO2 at physiologic pH Total CO2 measurement is indicative of HCO3- measurement M. Zaharna Clin. Chem. 2009

Bicarbonate Carbonic anhydrase in RBCs converts CO2 and H2O to carbonic acid, which dissociates into H+ and HCO3-.  HCO3- diffuses out of the cell in exchange for Cl- to maintain ionic charge neutrality within the cell (chloride shift) This process converts potentially toxic CO2 in the plasma to an effective buffer: HCO3- HCO3- buffers excess H+ by combining with acid, then eventually dissociating into H2O and CO2 in the lungs where the acidic gas CO2 is eliminated. M. Zaharna Clin. Chem. 2009

Regulation Bicarbonate conc. is regulated both in: Kidneys through increased or decreased tubular reabsorption And in lungs through exhalation of gaseous CO2 and H2O Decreased levels of HCO3- in plasma result in acid-base disorder (acidosis) Increased levels result in alkalosis M. Zaharna Clin. Chem. 2009

Alkalosis & Acidosis In alkalosis, In acidosis a relative increase in HCO3- compared to CO2, the kidneys increase excretion of HCO3- into the urine, carrying along a cation such as Na+. This loss of HCO3- from the body helps correct pH. In acidosis The body increases excretion of H+ into the urine. In addition, HCO3- reabsorption is virtually complete, with 90% of the filtered HCO3- reabsorbed in the proximal tubule and the remainder in the distal tubule. M. Zaharna Clin. Chem. 2009

Assay Two common methods: Ion selective electrode Enzymatic: converts all forms of CO2 to HCO3; HCO3 is used to carboxylate phosphoenolpyruvate. Coupled enzyme reaction that measures the amount of NADH consumed. The rate of absorbance change is proportional to amount of CO2 present. M. Zaharna Clin. Chem. 2009

M. Zaharna Clin. Chem. 2009

Magnesium 4th most abundant cation in the body and 2nd most abundant intracellular cation. 53 % of Mg found in the bone, 46 % in muscle and tissue, <1% is present in the serum & RBCs. The Mg circulating in serum one third-bound to albumin, of the remaining two thirds- 61% is in the free or ionized form, 5 % bound to phosphate and citrate. Free form is physiologically active. M. Zaharna Clin. Chem. 2009

Regulation of Magnesium Regulated by dietary intake, intestine may absorb 20-65 % of dietary intake and body needs. Regulation of body Mg2+ is controlled largely by the kidney, which can: reabsorb Mg2+ in deficiency states or readily excrete excess Mg2+ in overload states  2%-5% is reabsorbed in the DCT 25%-30% is reabsorbed by PCT  50%-60% of filtered Mg2+ is reabsorbed in Henle's loop  M. Zaharna Clin. Chem. 2009

Regulation of Magnesium Mg2+ regulation is related to that of Ca2+ and Na+. Parathyroid hormone (PTH) increases the renal reabsorption of Mg2+ and enhances the absorption of Mg2+ in the intestine. Aldosterone and thyroxine apparently have the opposite effect of PTH in the kidney, increasing the renal excretion of Mg2+ M. Zaharna Clin. Chem. 2009

Clinical Significance Roles in the body: Myocardial rhythm and contraction It is an essential cofactor of more than 300 enzymes, Regulation of ATPase ion pump Abnormal levels related to cardiovascular, metabolic, and neuromuscular disorders. M. Zaharna Clin. Chem. 2009

Hypo- & Hypermagnesaemia Hypomagnesaemia: Reduce intake Decreased absorption (GI disorders, Malabsorption syndromes) Increased excretion (as a result of various renal and endocrine disorders) Hypermagnesaemia: Decreased excretion (Adrenal insufficiency ) Increased intake M. Zaharna Clin. Chem. 2009

Assay Methods (colormetric) Calmagite Formazan dye Methylthymol blue Mg2+ binds with calmagite to form a reddish-violet complex Formazan dye Mg2+ binds with the dye to form a colored complex Methylthymol blue Mg2+ binds with the chromogen to form a colored complex Reference Range : 0.63-1.0 mmol/L Naphthol sulphonic acid M. Zaharna Clin. Chem. 2009

Calcium 99 % of calcium is associated with bone tissue Only 1 % of body calcium is in the plasma 45 % ionized (active form) 40 % protein bound 15 % bound to other compounds Critical component of cardiac function Decreased ionized Ca2+ concentrations in blood can cause neuromuscular irritability Tetany is a medical sign, the involuntary contraction of muscles M. Zaharna Clin. Chem. 2009

Regulation Three hormones, PTH, vitamin D, and calcitonin, are known to regulate serum Ca2+ by altering their secretion rate in response to changes in ionized Ca2+. Decreased plasma ionized Ca stimulates release of PTH PTH activates a process known as bone resorption PTH increases renal reabsorption of Calcium PTH stimulates Vitamin D activation Vitamin D increases GI absorption of Calcium Calcitonin exerts its Ca2+-lowering effect by inhibiting the actions of both PTH and vitamin D M. Zaharna Clin. Chem. 2009

Regulation osteoclasts M. Zaharna Clin. Chem. 2009

Clinical Applications Both total Ca2+ and ionized Ca2+ measurements are available in many laboratories, Ionized Ca2+ is usually a more sensitive and specific marker for Ca2+ disorders. Causes of hypocalcemia Hypoparathyroidism Vitamin D deficiency Renal disease Hypoalbuminemia (total calcium only, ionized not affected) 2.15 – 2.55 mmol/l M. Zaharna Clin. Chem. 2009

Clinical Applications Causes of hypercalcemia Hyperparathyroidism Malignancy (many tumors produce PTH-related peptide (PTH-rP), which binds to normal PTH receptors hyperthyroidism can sometimes cause hyperparathyroidism because of the proximity M. Zaharna Clin. Chem. 2009

Assay Methods The two commonly used methods for total Ca2+ analysis which form a complex with Ca2+ use either: ortho-cresolphthalein complexone (CPC) or arsenzo III dye Adult reference range: 8.6-10.0 mg/dL M. Zaharna Clin. Chem. 2009

Phosphate Element found everywhere in living cells Participates in various biochemical processes Most significant: ATP, Creatine Phosphate, phosphoenolpyruvate reactions. Important compound in the release of O2 from Hb (2,3-diphosphoglycerate) (DNA) and ribonucleic acid (RNA) are complex phosphodiesters M. Zaharna Clin. Chem. 2009

Phosphate The concentration of all phosphate compounds in blood is about 12 mg/dL most of that is organic phosphate and only about 3 to 4 mg/dL is inorganic phosphate. Phosphate is the predominant intracellular anion, with intracellular concentrations varying, depending on the type of cell less than 1% is active in the serum/plasma M. Zaharna Clin. Chem. 2009

Regulation Phosphate in blood may be: absorbed in the intestine from dietary sources, released from cells into blood, and lost from bone. In healthy individuals, all these processes are relatively constant and easily regulated by: renal excretion or reabsorption of phosphate. M. Zaharna Clin. Chem. 2009

Regulation Renal regulation is effected by factors such as: Vitamin D, increases both phosphate absorption in the intestine and phosphate reabsorption in the kidney PTH, lowers blood concentrations by increasing renal excretion. M. Zaharna Clin. Chem. 2009

Clinical Application Hypophosphatemia: Hyperphosphatemia: Intracellular shift Hyperparathyroidism Renal tubular defect Hyperphosphatemia: Increase intake Decrease excretion Cell lysis M. Zaharna Clin. Chem. 2009

Assay Methods Most of the current methods involve the formation of an ammonium phosphomolybdate complex. This colorless complex can be measured by ultraviolet absorption at 340 nm or can be reduced to form molybdenum blue, a stable blue chromophore, which is read between 600 and 700 nm. Adult reference range: 2.4-4.4 mg/dL M. Zaharna Clin. Chem. 2009

[Na]+ [K] + [other cations] = [Cl] + [HCO3] + [other anions] Anion Gap Body water compartments exist in a state of electroneutrality (anions=cations) Routine measurements: Na, K, Cl & HCO3 levels Anion Gap is the difference between unmeasured anions and unmeasured cations. Formula: AG=(Na + K)- (Cl + HCO3) The "real" balance is given by the equation:     [Na]+ [K] + [other cations] = [Cl] + [HCO3] + [other anions] ([Na]+ [K]) - ([Cl] + [HCO3])= [other anions] - [other cations] = "Anion Gap“ 8-16 or 12-20 mmol/l M. Zaharna Clin. Chem. 2009

M. Zaharna Clin. Chem. 2009

Anion Gap Some of the unmeasured cations (~7mmol/L) include calcium, magnesium, and most other minerals. Unmeasured anions (~24 mmol/L) include proteins like albumin, and phosphates, sulfates, etc. There are always more unmeasured anions than cations, and thus the "anion gap" equation is always greater than zero. It has a reference range of 10-20 mmol/L M. Zaharna Clin. Chem. 2009

Clinical Uses of the Anion Gap An elevated anion gap may be caused by: uremia/renal failure, which leads to PO4- and SO42- retention; ketoacidosis, as seen in cases of starvation or diabetes; methanol, ethanol poisoning and instrument error. Low anion gap values are rare but may be seen with: hypoalbuminemia (decrease in unmeasured anions) or severe hypercalcemia (increase in unmeasured cations). *** M. Zaharna Clin. Chem. 2009