Distribution of sulfate ion across semi-permeable membranes

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

The clinical chemistry of inorganic sulfate

Although inorganic sulfate is an essential and ubiquitous anion in human biology, it is infrequently assayed in clinical chemistry today. Serum sulfate is difficult to measure accurately without resorting to physicochemical methods, such as ion chromatography, although many other techniques have been described. It is strongly influenced by a variety of physiological factors, including age, diet, pregnancy, and drug ingestion. Urinary excretion is the principal mechanism of disposal for the excess sulfate produced by sulfur amino acid oxidation, and the kidney is the primary site of regulation. In renal failure, sulfoesters accumulate and hypersulfatemia contributes directly to the unmeasured anion gap characteristic of the condition. In contrast, sulfate in urine is readily assayed by a number of means, particularly nephelometry after precipitation as a barium salt. Sulfate is most commonly assayed today as part of the clinical workup for nephrolithiasis, because sulfate is a major contributor to the ionic strength of urine and alters the equilibrium constants governing saturation and precipitation of calcium salts. Total sulfate deficiency has hitherto not been described, although genetic defects in sulfate transporters have been associated recently with congenital osteochondrodystrophies that may be lethal. New insights into sulfate transport and its hormonal regulation may lead to new clinical applications of sulfate analysis in the future.

Methionine-induced acidosis in the weanling rat

Whole body balances of non-metabolizable base (NB) were studied in weanling rats fed a cereal-based diet with or without added L-methionine. In response to L-methionine loading (2.5 mmol day-1) the rats exhibited a significant reduction in rates of food intake (109 vs. 160 g per 8 days) and body growth (3 vs. 52 g per 8 days); fractional oxidation of absorbed dietary amino acid sulphur increased from 0.28 to 0.64; and mean urinary sulphate excretion increased from 2.3 to 14.8 mmol per 8 days. Mean rates of renal excretion of NB and filtered titratable organic acid decreased from 20 to -11 mmol per 8 days and from 22 to 8 mmol per 8 days. Balances of calcium and NB remained at reference values despite a decrease in whole blood 'base excess' from -1.0 to -6.4 mmol l-1. The concentration of NB in plasma rose but slightly. It is concluded that L-methionine loading in the weanling rat leads to extracellular non-carbonic acidosis subject to renal compensation. This acidosis is due not to retention of H+ released by ionization of endogenous sulphuric acid but possibly to accumulation of (acid) organic metabolites of methionine which are efficiently conserved by the kidneys. The rise in sulphuric acid production leads to an adaptive decrease in fractional reabsorption of filtered sulphate. Even during inhibited growth, absorbed dietary NB is retained and deposited in the skeleton, probably as calcium carbonate.

Reabsorption of inorganic sulfate in the rat kidney: evidence for an adaptive depression of TmSO4 during SO4 loading

Clearance methods were employed to study reabsorption of inorganic sulfate (SO4) in the rat kidney. During stepwise elevation of the So4 concentration in plasma by sulfate infusions (SO4 titration), the reabsorption of S)4 increased and reached the maximal reabsorptive capacity for inorganic sulfate (TmSO4). During further SO4 loading TMSO4 decreased by about 50%. At the same time GFR was stable, and the decline of the TmSO4 was not due to volume loading during infusion, since in time-controls with Ringer infusion alone the reabsorption of inorganic sulfate was almost unchanged. The decrease of the TmSO4 was also observed in thyroparathyroidectomized animals. It is concluded that the decline of TmSO4 was a result of the SO4 load per se and may be mediated by an unknown adaptive process.

Sulphate excretion by dogs following ingestion of ammonium sulphate or meat

1. When normal conscious dogs were given small doses of (NH4)2SO4 by stomach tube, the rate of excretion of SO4 increased due to increase in plasma SO4 without change in creatinine clearance. After eating meat, increased SO4 excretion was due to increase in glomerular filtration rate without change in plasma SO4. 2. The calculated rate of tubular reabsorption of SO4 did not change significantly either after (NH4)2SO4 or after meat. 3. The rate of production of SO4 after meat was estimated. In 4 hr approximately one fifth of the S in meat was converted to inorganic SO4; in the same time approximately one third of the N in the meat was converted into urea. 4. Half of the SO4 in the doses of (NH4)2SO4 was absorbed from the gut; all of the NH was converted into urea. (NH4)2SO4 thus produced acidosis. 5. After (NH4)2SO4, increased excretion of NH4+ accompanied the increased excretion of SO4-but Na+, K+ in urine also increased.

Potassium depletion in severe heart disease

Exchangeable sodium and potassium, total body water, and sulphate space were measured in 42 patients with severe valvular heart disease who were free of oedema. Compared with normal subjects of the same height, no increase in exchangeable sodium was found but a mean potassium depletion of 27% was shown. This depletion was not related to diuretic therapy, and no relationship between the degree of depletion and postoperative arrhythmias was found. It is concluded that the major cause of the low exchangeable potassium is the reduction in cell mass that occurs in chronic heart disease, and that there is no significant fall in the intracellular potassium concentration.

SERIAL DETERMINATIONS OF EXTRACELLULAR FLUID VOLUME USING THE RADIOSULPHATE SPACE METHOD

The fate of injected and ingested radiosulphate was investigated. The appearance of radiosulphate in bile was shown to be very rapid. Ingested radiosulphate was rapidly and completely absorbed, peak blood levels occurring at 1 to 2 hours.The sulphate space method of measuring extracellular fluid volume was studied and a method adopted requiring two serum samples taken at 1/2 and 6 hours following injection. This procedure provided a convenient means of making independent observations of extracellular fluid volume as often as four times a day in individual subjects. When values for the sulphate space were compared in the same individuals under conditions similar with respect to diet, activity, and time of day, the coefficient of variation was calculated to be ± 5.7% or about ± 1% of body weight. Values among well-nourished subjects with no known abnormalities of water or electrolyte balance gave a mean of 17.5% body weight with a range of 13.5 to 22% body weight. Values among obese subjects averaged 12.8% body weight.Studies of the diurnal variation of the sulphate space made every 6 hours throughout the day suggested that in man, under conditions of uniform intake and normal activity, there is a diurnal variation of extracellular fluid volume, higher values occurring at night.