Kidney Wet Weight Research Articles

Zinc, cadmium, mercury and selenium in minke whales, belugas and narwhals from West Greenland

Samples of muscle, liver and kidney from 24 minke whales (Balaenoptera acutorostrata), 43 belugas (Delphinapterus leucas), and 98 narwhals (Monodon monoceros) were analyzed for zinc, cadmium, mercury, and selenium. Highly significant age accumulation of mercury was found. A lower level of significance of age accumulation of cadmium in belugas and narwhals is probably due to the fact that some of the highest cadmium concentrations are in subadults and young adults. The maximum concentrations of cadmium and mercury are very high: 1.68, 73.7, and 125 μg cadmium, and 9.88, 42.8, and 4.61 μg mercury per g wet weight of narwhal muscle, liver and kidney, respectively. The cadmium concentrations are correlated in the three organs, as are mercury and to a lesser extent selenium concentrations. The concentrations of mercury and selenium in liver are highly correlated.

Kidney IGF-I mRNA in initial renal hypertrophy in experimental diabetes in rats

It has recently been demonstrated that immunoassayable kidney insulin-like growth factor I concentration increases 24-48 h after induction of diabetes, preceding the initial renal hypertrophy. To elucidate whether this increase is due to increased local production we studied rat kidney insulin-like growth factor I gene expression during the first four days after induction of streptozotocin diabetes. Eighteen hours after injection with streptozotocin the diabetic animals were divided into two groups, one of which was treated with insulin, and daily for four days animals from each group were taken out for investigation. After four days the wet kidney weight had increased from baseline by 20% (from 687 +/- 23 to 827 +/- 6 mg (mean +/- SEM), p less than 0.01) in the untreated diabetic group, while no significant increase occurred in the insulin-treated group (687 +/- 23 vs 732 +/- 21 mg, NS). Kidney insulin-like growth factor I increased rapidly from baseline, the rise amounting to 52% after 48 h (from 271 +/- 11 to 411 +/- 32 ng/g, p less than 0.01) with a decline to control level on day four in the untreated diabetic group. Kidney insulin-like growth factor I remained unchanged in the insulin-treated diabetic group. Insulin-like growth factor I mRNA was measured by solution-hybridization assay. No differences were found in kidney insulin-like growth factor I mRNA between the two diabetic groups over the study period, while in liver, insulin-like growth factor I mRNA tended to be lower on day four in diabetic rats when compared to insulin-treated rats (p = 0.07).(ABSTRACT TRUNCATED AT 250 WORDS)

PRESERVATION OF RENAL COMPENSATORY GROWTH IN UNINEPHRECTOMIZED RATS BY LOW-DOSE CYCLOSPORINE

The growth-inhibitory effect of prednisone is a serious drawback to its use in kidney graft recipients. At high doses, cyclosporine also inhibits somatic growth in animals. Furthermore, cyclosporine, in addition to being nephrotoxic in patients, is reported to inhibit compensatory renal growth in uninephrectomized rats. It is unclear whether the latter effect is specific to the kidney. In the present study, we have tested, in the uninephrectomized rat model, the effect of a low dose cyclosporine that is immunosuppressive but does not inhibit somatic growth. Compared with sham operation, uninephrectomy resulted in a significantly greater kidney-to-body weight ratio 2 and 7 days postsurgery. In uninephrectomized rats treated daily with 10 mg/kg cyclosporine s.c., compensatory renal growth was intact. Kidney wet weight, dry weight, and kidney-to-body weight ratios were similar in vehicle and cyclosporine treated, uninephrectomized animals. Similarly, body weight, food consumption, and size of other organs were not adversely affected by the 10 mg/kg/day cyclosporine treatment. Thus, by reducing cyclosporine to levels that are not generally toxic, renal growth is preserved. Our results suggest that the present trend toward using lower doses of cyclosporine in kidney transplant recipients will not only reduce nephrotoxicity, but will also be of benefit in terms of somatic and renal growth, which are important considerations in pediatric transplant patients.

Heavy metal concentrations in the tissues of seabirds from Gough Island, South Atlantic Ocean

Concentrations of mercury, cadmium, copper, and zinc were measured in liver and kidney tissues of adult seabirds collected at breeding colonies on Gough Island, South Atlantic Ocean. Copper levels were mostly from 3–8 μg g −1 wet weight of liver and kidney in all species. Zinc levels were generally 30–70 μg g −1 in liver and kidney. Cadmium levels, always greater in kidney than in liver, were highly variable between individuals and between species; levels in kidney exceeding 100 μg g −1 wet weight were found in some apparently healthy Rockhopper Penguins, Wandering Albatrosses, and Atlantic Petrels. Mercury levels in liver were highest in Wandering and Sooty Albatrosses, exceeding 200 μg g −1 wet weight in some. The levels of mercury and cadmium appear likely to be natural rather than due to pollution, but are greatly in excess of the concentrations of these elements known to have severe toxic effects in rats and man, and associated with kidney lesions in seabirds.

Priming with low doses of methyl-CCNU reduce the toxicity of high doses of methyl-CCNU and melphalan, and increase the lifespan of mice implanted with Lewis lung carcinoma.

Pretreatment of mice with low doses of methyl-CCNU was shown to reduce the toxicity of lethal doses of methyl-CCNU or melphalan administered one or two days following the low dose. There was an increase in survival rate, body weight, thymus and kidney wet weight. Tissue morphology was less affected in the primed mice as compared to mice receiving the high dose or a high-low dose combination. In mice implanted s.c. with Lewis lung carcinoma, priming with 5 mg kg-1 methyl-CCNU 2 days before injection of a very high (35 mg kg-1) dose significantly increased the lifespan as compared to treatment with the high dose alone or with high-low dose combination. When the dose of methyl-CCNU was further increased to 40 mg kg-1 toxic death occurred, which was, however, significantly reduced by 'priming' with the low dose given. When low-high dose combination was used twice (the high dose was given on day 7 or 9, and 18 or 20 after tumour inoculation), priming with 5 mg kg-1 (but not with 10 mg kg-1) two days prior to the high dose was beneficial in reducing toxic death (in two experiments) and either increasing lifespan or not significantly increasing it. In no case was there protection of the tumour by the low-high dose combinations.

Determination of renal molar concentrations of phosphorus-containing metabolites in vivo using 31P NMR.

A technique to determine absolute metabolite concentrations of the kidney in vivo using 31P NMR is described. The technique is based on the use of methylphosphonic acid (MPA), which gives rise to a well-resolved peak upfield from in vivo phosphorous metabolite resonances, as an "internal standard." The method involves acquisition of a fully relaxed kidney spectrum with an implanted coil followed by intravenous infusion of MPA (4 ml of 150 mM) for a period of 30 min. The animal is then sacrificed to insure a steady state level of renal MPA and another spectrum is obtained. From these two spectra the ratio of intensities of MPA to beta-ATP was derived. In the method used here, no significant contribution from tissues outside the kidney was observed. In addition, a relatively homogeneous distribution of MPA throughout the kidney was achieved. The amount of MPA per gram wet weight of kidney was also obtained through NMR methods by placing the excised organ in a phosphate-calibrated solenoidal coil. The calibration spectra along with the ratio of intensities for MPA/beta-ATP were used to calculate the number of micromoles of ATP per gram wet weight of kidney. Infusion of a higher concentration of MPA (1.25 M) produced a visible MPA resonance in other organs besides the kidney. Thus, MPA could be useful in determining phosphate metabolite concentrations in other tissues.

Experimental folic acid nephropathy

In rats, single intravenous doses of folic acid induce damage to renal tubular epithelium, deposition of folic acid in tubular lumens, increase in wet kidney weight, oliguria and interstitial connective tissue proliferation. Separation of the nephrotoxic and obstructive effects of folic acid was attempted by pretreatment with NH4Cl or NaHCO3. These effects of folic acid were unaltered by pretreatment with NH4Cl and there was, in addition, accumulation of eosinophilic droplets in papillary collecting duct epithelium. After pretreatment with NaHCO3, folic acid deposition is decreased or absent; there is a smaller increase in wet kidney weight; the rats are polyuric rather than oliguric; interstitial connective tissue proliferation is reduced; and no droplets form in papillary collecting ducts, but lesions are still present in proximal convoluted tubule epithelial cells. These findings indicate that folic acid has direct nephrotoxic effects independent of intraluminal folic acid deposition, and that damage to renal epithelium, unlike that induced by many nephrotoxins, occurs at several levels of the nephron.

Kidney growth and collagen content in rat pups from uninephrectomized mothers.

The effect of uninephrectomy (UNX) or sham-operation (SO) in pregnant rats at the 5th-7th day of gestation upon pup kidney development was studied. In newborns, kidney wet weight was low in pups of the SO and UNX groups, kidney/body weight was low in UNX only; collagen was highest in UNX. At 6 weeks, wet weight was least in UNX; dry weight was low and %H2O high in SO and UNX; collagen/dry weight was high in SO and UNX. Maternal surgery (SO or UNX) appears to impose a non specific development deficit upon pups, although kidney collagen growth is sustained; no specific renotropic activity was evident in pups of the UNX groups.

Osmoregulation in Desiccated Dormant Snails (Helix aspersa; Gastropoda, Pulmonata)

Helix aspersa were subjected to 100 days of desiccation at 19-25 C in summer and in winter; total weight declined 33%. After desiccation, winter animals showed little significant change in hemolymph protein, Na⁺ and K⁺ concentrations, or osmolality. In summer snails, desiccation caused significant increases in all of these hemolymph constituents. Seasonal differences in hemolymph components were limited, except for Na±; in both hydrated and desiccated snails, Na⁺ concentrations were significantly higher in summer than in winter. In snail kidney, desiccation did not change significantly the concentrations of Na⁺, urate, water, or kidney wet weight. In both seasons, kidney K⁺ concentration rose significantly after desiccation. Seasonal differences were found among kidney contents-especially, winter snails (both hydrated and desiccated groups) had much higher Na⁺ and K⁺ contents. The kidney urate content of desiccated winter snails was higher than that of desiccated summer snails. It is hypothesized that the...

N-Acetylneuraminic acid biosynthesis in rat liver and kidney

Rat liver and kidney tissue slices incubated withN-acetyl [3H]mannosamine incorporated radioactivity into free and boundN-acetylneuraminic acid and CMP-N-acetylneuraminic acid (CMP-NeuAc). Liver and kidney also incorporated radioactivity from intravenously injected [3H]ManNAc intoN-acetylneuraminic acid and CMP-NeuAc. From the decrease in the specific radioactivity of CMP-NeuAc after a single injection ofN-acetyl[3H]mannosamine the half-life of CMP-NeuAc was determined. From this half-life and the pool size of CMP-NeuAc a synthesis rate of CMP-NeuAc was calculated, being 1.2 nmol/min/g wet weight of kidney. In previous experiments a value of 1.0 nmol/min/g wet weight was determined for liver [Ferwerdaet al. (1983) Biochem J 216: 87–92]. The synthesis rate of CMP-NeuAcin vivo was in the same range as the synthesis rate calculated from the turnover of boundN-acetylneuraminic acid, which was 2.7 and 0.4 nmol/min/g wet weight for liver and kidney respectively. The assay conditions for UDP-N-acetylglucosamine 2-epimerase andN-acetylmannosamine kinase were adapted to measure low activitiesin vitro. It appeared that the kinase activity detected in kidney can synthesizeN-acetylmannosamine6-phosphate at a rate sufficient for the observed production ofN-acetylneuraminic acidin vivo. Also a low, but measurable activity of UDP-N-acetylglucosamine 2-epimerase was detected in kidneyin vitro, suggesting that the biosynthetic pathway ofN-acetylneuraminic acid in kidney is the same as in liver. The synthesis rate ofN-acetylneuraminic acid in liver determinedin vivo is approximately 12 times slower than the maximal potential rate calculated from the activities of theN-acetylneuraminic acid (precursor-) forming enzymes as detectedin vitro. This indicates that in liverin vivo the enzymes are working far below their maximal capacity.

Studies on antinephritic effect of mizoribine (p-INN, Bredinin), a new immunosuppressive agent, and azathioprine. (2) Effect on crescentic-type anti-GBM nephritis in rats.

Crescentic-type nephritis was induced in rats by immunizing with rabbit gamma-globulin following i.v. injection of rabbit anti-rat glomerular basement membrane (anti-GBM) serum. The antinephritic effect of mizoribine was compared to that of azathioprine by administration from the 2nd day after the injection of anti-GBM serum to the 23rd day (Experiment I) and from the 15th to the 38th day (Experiment II). The experiment I assessment on the 24th day revealed that mizoribine at a dose of 7.5 mg/kg/day p.o. remarkably reduced plasma cholesterol level, wet kidney weight and glomerular histopathological changes (i.e., crescent formation and fibrinoid deposition). In addition, mizoribine at this dose also tended to reduce urinary protein excretion and the adhesion of capillary walls to Bowman's capsule. At a dose of 5 mg/kg/day p.o., mizoribine significantly reduced kidney weight and crescent formation. On the other hand, azathioprine at a dose of 20 mg/kg/day p.o. had a tendency to reduce these biochemical and histopathological parameters. In the experiment II assessment on the 39th day, the effects of both drugs were somewhat diminished compared to those in experiment I. Mizoribine strikingly inhibited the crescent formation with 5 and 7.5 mg/kg/day p.o. and inhibited the fibrinoid deposition with a dose of 7.5 mg/kg/day p.o. Azathioprine at a dose of 20 mg/kg/day p.o. was prone to reduce histopathological parameters. The above data indicate that mizoribine may be a useful new immunosuppressive agent for rapidly progressive glomerulonephritis, which is characterized by severe glomerular lesion with the extensive formation of crescents.

Studies on Antinephritic Effect of Mizoribine (p-INN, Bredinin®), a New Immunosuppressive Agent, and Azathioprine (2) Effect on Crescentic-Type Anti-GBM Nephritis in Rats

Crescentic-type nephritis was induced in rats by immunizing with rabbit r-globulin following i.v. injection of rabbit anti-rat glomerular basement membrane (anti-GBM) serum. The antinephritic effect of mizoribine was compared to that of azathioprine by administration from the 2nd day after the injection of anti-GBM serum to the 23rd day (Experiment I) and from the 1 5th to the 38th day (Experiment II). The experiment I assessment on the 24th day revealed that mizoribine at a dose of 7.5 mg/kg/day p.o. remarkably reduced plasma cholesterol level, wet kidney weight and glomerular histopathological changes (i.e., crescent formation and fibrinoid deposition). In addition, mizoribine at this dose also tended to reduce urinary protein excretion and the adhesion of capillary walls to Bowman’s capsule. At a dose of 5 mg/kg/day p.o., mizoribine significantly reduced kidney weight and crescent formation. On the other hand, azathioprine at a dose of 20 mg/kg/day p.o. had a tendency to reduce these biochemical and histopathological parameters. In the experiment II assessment on the 39th day, the effects of both drugs were somewhat diminished compared to those in experiment I. Mizoribine strikingly inhibited the crescent formation with 5 and 7.5 mg/kg/day p.o. and inhibited the fibrinoid deposition with a dose of 7.5 mg/kg/day p.o. Azathioprine at a dose of 20 mg/kg/ day p.o. was prone to reduce histopathological parameters. The above data indicate that mizoribine may be a useful new immunosuppressive agent for rapidly progressive glomerulonephritis, which is characterized by severe glomerular lesions with the extensive formation of crescents.

Effects of renal capsulectomy on kidney growth in the rat.

There are theoretical and experimental indications that the renal capsule may limit compensatory renal growth, the limitation increasing with age. The experiments reported here were designed to investigate the effects of renal capsulectomy in young and old rats both on the normal kidney and on that undergoing compensatory growth following unilateral nephrectomy. Capsulectomy had no effects on kidney wet or dry weight in control animals, nor during compensatory growth. In old rats but not in young, capsulectomy increased tubular mitotic index in sham-operated and unilaterally nephrectomized rats. Capsulectomy stimulated hyperplasia in fibroblasts remaining on the kidney surface in all groups. In the young adult rats, renal capsulectomy initiates repair mechanisms in the remaining connective tissue overlying the cortex, but does not itself induce the tubular cells to divide and does not influence compensatory renal growth in the 1st week after unilateral nephrectomy. The capsulectomy-induced increase in tubular mitotic index seen in the old rats may be due to subtle injury to the renal parenchyma resulting from the relative difficulty of removing the older, thicker and more adherent capsules, or to a concomitant increase in platelet-derived growth factor generated during rupture of perforating blood vessels.

Premature Appearance of Hepatic Phosphoenolpyruvate Carboxykinase in Fetal Rats, Not Mediated by Adenosine 3′: 5′‐Monophosphate

Injection of streptozotocin in utero to fetuses elicited a premature appearance of cytosolic hepatic activity of phosphoenol pyruvate carboxykinase. This was due to a precocious initiation of the synthesis of the enzyme. The streptozotocin-induced appearance of enzyme activity was not mediated by adenosine 3':5'-monophosphate since the concentration of the cyclic nucleotide in the liver was unaffected by the antibiotic, the administration of dibutyryladenosine 3':5'-monophosphate to streptozotocin-treated fetuses elicited an additive increase in enzyme activity, and insulin administration in utero repressed the streptozotocin effect while the effect due to dibutyryladenosine 3':5'-monophosphate was not inhibited by simultaneous insulin injection. Streptozotocin treatment also caused a small but consistent retardation of fetal growth and a marked reduction of liver wet weight. Histological analysis of the liver demonstrated a premature loss of some hematopoietic elements, while hepatocytes appeared normal. Hepatic protein synthesis was unaffected by the streptozotocin treatment. Streptozotocin treatment had no effect on fetal renal phosphoenol pyruvate carboxykinase activity or kidney wet weight.

1,1-Dichloroethylene nephrotoxicity in the rat

The acute, oral nephrotoxicity of 1,1-dichloroethylene (1,1-DCE, vinylidene chloride) as demonstrated by increases in kidney weight, plasma urea nitrogen, plasma creatinine concentration, and histopathologic changes after oral administration of 1,1-DCE to male and female rats was determined. Fasting males were susceptible to 400 mg/kg with urea nitrogen elevated 24, 48, and 72 hr later whereas fed males were not. Doses of 200, 400, and 600 mg/kg caused urea nitrogen elevations, and 400 mg/kg caused elevated creatinine. By these criteria, males were more susceptible than females. The time-course of the biochemical changes indicative of nephrotoxicity coincided with plasma enzyme changes associated with hepatotoxicity or were slightly preceded by them. In male rats histopathologic changes observed including vacuolization, tubular dilitation, and necrosis, coincided closely with increases in kidney wet weight. The prevalence of histopathologic lesions. however, was at least as great in the females as the males and may have been slightly more prolonged, but the other indicators of nephrotoxicity were not affected by 1,1-DCE.

Insulin extraction from the renal peritubular circulation in the chicken.

The chicken has a renal portal circulation with a variable amount of hind limb venous blood draining into the post-glomerular circulation before entering the systemic circulation. By injecting [125I]iodoinsulin into a saphenous vein we were thus able to deliver labeled hormone directly into the peritubular circulation on the same side. At intervals up to 6 min after the injection of [l25I]-iodoinsulin, total radioactivity per gram wet weight of kidney was 63% greater on the injected side as compared to the opposite side. Similarly immunoprecipitable radioactivity (less than 4% of total radioactivity) was 55% greater on the injected side. The difference between the two sides represents the accumulation of insulin extracted from peritubular blood during its first passage through the kidney. As most of the difference in radioactivity between the 2 kidneys was not precipitable with insulin antibody, it is evident that degradation of extracted insulin must occur. That [l25I]iodoinsulin accumulates in and is de...

Behavioral effects of asymptomatic lead exposure during neonatal development in rats

Long-Evans rats intubated daily from 3 to 21 days of age with 0, 10, 30, or 90 mg/kg of lead acetate in aqueous solution were tested postweaning on eight behavioral measures. The three levels of lead exposure had no effect on growth and all animals were overtly free of signs of poisoning. However, neonatal lead exposure produced subtle behavioral effects including increased motor activity (jiggle cages) and impairment of motor coordination (rotarod test), response inhibition ability (operant delayed response), and reversal learning of a tactually cued conditional discrimination. The lead treatment had no effect on simple learning (turning response in E-maze), complex visually cued learning (conditional discrimination in E-maze), and visual acuity (optokinetic test). Acquisition and extinction of passive shock avoidance were not affected by lead exposure, but both acquisition and extinction of active avoidance were affected. Lead treatment also decreased hematocrit, increased blood lead concentration, and increased adrenal and kidney wet weights.

Influence of cadmium on calcium transfer through the duodenal wall in rats.

Five-week-old female albino rats were given different doses of cadmium chloride by gastric intubation daily for 1 (or 2) weeks. They were killed on 8th (or 15th) day of the experiment. Calcium-45 was used as marker to assess calcium transfer through and its retention in the duodenal wall by the everted gut sac method of Wilson and Wiseman. In all animals liver, kidney and femur wet weight was also determined.

Isolation of the basal and lateral plasma membranes of rat kidney tubule cells

A method was developed to isolate renal basolateral membranes from cortical kidney tubule cells of single rats. The isolated membrane fraction was characterized by the measurement of marker enzyme activities and by electron microscopy. 1. 1. After centrifugation of crude plasma membranes on a discontinuous sucrose density gradient the basolateral membranes accumulated at a sucrose density of p = 1.14–1.15 μ/ ml . The yield was 147 μg membrane protein/g kidney wet weight. Protein recovery was 0.1 %. 2. 2. (Na + + K +)-ATPase was enriched 22-fold from the homogenate. The recovery was 2.6 %. The (Na + + K +)/Mg 2+-ATPase ratio was 4.1. 3. 3. The contamination by brush borders was small. Alkaline phosphatase was 1.6-fold enriched and 0.2 % was recovered. Aminopeptidase was 1-fold enriched with a recovery of 0.1 %. The contamination by mitochondria, lysosomes and endoplasmic reticulum was negligible. 4. 4. In electron micrographs the basolateral membranes showed a typical triple layered profile and were characterized by the presence of junctional complexes, gap junctions or tight junctions.

The Concentrations of Free and Bound Magnesium in Rat Tissues: RELATIVE CONSTANCY OF FREE Mg2+ CONCENTRATIONS

Abstract The levels of total magnesium in rat liver, brain, and kidney have been determined, and the distribution of the magnesium between free and bound forms has been calculated. Total tissue magnesium measured by atomic absorption was found to be 6.92, 8.25, and 10.4 µmoles per g wet weight of brain, kidney cortex, and liver, respectively. The calculation of the intracellular free magnesium concentrations (free [Mg2+]) in these tissues was approached experimentally in two different ways. In the first method the concentration of the magnesium binding sites and the corresponding stability constants were determined experimentally for the various fractions of tissue and were used together with the total [Mg] to calculate the free [Mg2+]. Operationally, the tissue was separated into three fractions: the insoluble binding sites (those sedimented at 40,000 x g for 30 min), the soluble binding sites (large molecules of molecular weight g1000 remaining in the supernatant after centrifugation), and the (small molecules of molecular weight l1000). Using various tissue dilutions and magnesium concentrations, the binding to insoluble and soluble binding sites was studied with centrifugation and equilibrium dialysis techniques. By Scatchard-type plots estimates of the concentrations of these binding sites and the corresponding constants could be made. The concentrations of insoluble binding sites were estimated to be 3.0, 2.0, and 2.8 µmoles per g wet weight of brain, kidney, and liver, respectively, with corresponding binding constants of 4,300, 1,860, and 3,740 kg wet weight of tissue per mole of binding sites. The concentration of soluble binding sites was estimated to be 4.7 and 10.6 µmoles per g wet weight in kidney and liver, respectively, with corresponding binding constants of 727 and 532 kg wet weight of tissue per mole of binding sites. From these values and the concentrations of a number of metabolites measured in freeze-clamped tissue and their binding constants, the free [Mg2+] could be calculated from a complex polynomial of the form: [see PDF for equation] where free [Mg2+] = [MgB1]/([B1] - [MgB1])K1 and MgB1 is the concentration of the magnesium complex and K1 the binding constant of the first binding agent; and Bi and Ki are the concentration and binding constant respectively of the ith binding agent. The second method for the calculation of the free [Mg2+] involves the use of the equilibrium constant of aconitate hydratase (EC 4.2.1.3). The measured Σcitrate/Σisocitrate (concentrations of total citrate/total ls(+)-isocitrate) in freeze-clamped tissue was compared with the equilibrium ratios in vitro for the aconitate hydratase reactions at various free [Mg2+], 38°, ionic strength = 0.25, [K+] = 125 mm, and [Na+] = 25 mm. Free intracellular [Mg2+] was calculated to range between 0.6 and 1.3 µmoles per g wet weight in all tissues studied. The values obtained with the two independent methods showed good agreement. From the similarity of these values with those estimated by other authors in various extracellular fluids, it is implied that there is little or no free [Mg2+] gradient between intracellular and extracellular fluids in spite of very different total magnesium concentrations between tissue and extracellular fluid. It is also implied that the free [Mg2+] like pH is relatively constant in various tissues and extracellular fluids, the free [Mg2+] being buffered by relatively large concentrations of bound magnesium and magnesium-binding sites.