Ion Association. Vi. Interactions Between Calcium, Magnesium, Inorganic Phosphate, Citrate and Protein in Normal Human Plasma*

Abstract

The total measured concentration of an ionizable constituent of body fluids often fails to reveal either the varied chemical forms in which it may be present, or the portion which is present as the free ionized substance. This truism applies to most organic acids and bases and to inorganic ions which are multivalent. Univalent ions whose corresponding acids and bases are strong may be viewed as exceptions to this generalization. This exclusion is more quantitative than qualitative, since small fractions of these constituents may also be complexed or bound.Plasma and extracellular fluid differ from other body fluids in containing predominantly univalent strong electrolytes. The plasma concentrations of organic acids and bases and of multivalent inorganic ions are relatively low. Nevertheless, the measured concentrations of these substances in plasma may fail to reflect their interactions with one another, and therefore the individual ionic species present.By means of ultrafiltration, a protein-free fluid may be obtained which is probably identical with the protein-free phase in native plasma. This identity is supported by the observation that ultrafiltrate concentrations of calcium (1), magnesium (2) and phosphate (3) do not change during the course of ultrafiltration. In order to quantitate the interactions between nonprotein-bound constituents, one approach is to measure 1) the free ion concentrations of all cations present, 2) the total amount of various anions present, and 3) calculate the amounts of each complex from reported dissociation constants. Undetermined anions will then account for the difference between the total concentration of each cation and the amount accounted for as known complexes. Calcium and magnesium ion concentrations can be * Supported by a United States Public Health Service Grant (A-2306). determined (2). Sodium and potassium are virtually completely ionized. Therefore, this scheme offers promise of analyzing such complex mixtures as plasma, urine and intracellular fluid.In the present report, normal plasma has been analyzed by this procedure with respect to protein, calcium, magnesium, phosphate and citrate. METHODVenous blood was collected without stasis in oiled heparinized syringes from healthy volunteers (medical students, technicians and staff members). Plasma was separated by centrifuging and was stored under oil. Plasma pH was determined promptly at room temperature (230 to 26°C) using a Radiometer pH meter, model 4. No temperature correction was applied. Ultrafiltration was performed as described by Toribara, Terepka and Dewey (1) with two modifications: 1) Visking casing 4 inch in diameter was used, the resulting increase in area for ultrafiltration accelerating the process almost twofold; 2) the strips of casing were soaked in distilled water for 2 days, rinsed, and then stored until used in a chamber saturated with water vapor. Drying in air caused small pinholes to form; use of wet casing, even when thoroughly wiped, led to dilution of ultrafiltrate by water remaining in...

Magnesium buffering in intact human red blood cells measured using the ionophore A23187.

“…It seems likely that the concentration of ionized magnesium inside the red cells under these conditions, which is about 0*4 mM, is the same as that inside the oxygenated red cell when no ionophore is present (see Flatman, 1980). Since the concentration of ionized magnesium in human serum is about 0*5 mm (Walser, 1961;Heaton, 1967), there is both an electrical and a chemical gradient favouring the entry of magnesium into red cells whilst they are in the peripheral circulation. If anything, however, red cells seem to lose magnesium as they age (Bernstein, 1959).…”

Fetal Homeostasis in Maternal Malnutrition. II. Magnesium Deprivation

“…The existence of such a gradient suggests an active transport role for the placenta. The magnesium in plasma exists not only as the free ion, however, but bound to plasma proteins and complexed to organic radicals [11]. In an attempt to evaluate the effect of protein binding, equilibrium dialysis was undertaken between maternal and fetal plasma.…”

Phosphate (Pi)-regulated heterodimerization of the high-affinity sodium-dependent Pi transporters PiT1/Slc20a1 and PiT2/Slc20a2 underlies extracellular Pi sensing independently of Pi uptake

“…About 85% of total phosphate can be found in the skeleton where it is a major constituent of hydroxyapatite crystals deposited on the extracellular organic matrix during the mineralization process. The remaining 15% is found mainly in cells from soft tissues and in extracellular volume where it represents less than 1% of total phosphate (2)(3)(4). In plasma, approximately 16% of circulating phosphate is found as organic phosphate bound to proteins and lipids while the major part (84%) is orthophosphate, or free inorganic phosphate (Pi), that can be filtered by the kidney (1).…”