Sensitivity of renal brush-border Na+-cotransport systems to anions

The role of ions, heavy metals, fluoride, and agrochemicals: critical evaluation of potential aetiological factors of chronic kidney disease of multifactorial origin (CKDmfo/CKDu) and recommendations for its eradication

The pollution of water and food through human waste and anthropogenic activities, including industrial waste and agricultural runoff, is a mounting problem worldwide. Water pollution from microbes causes identifiable diarrhoeal illnesses. The consumption of water contaminated with heavy metals, fluoride, and other toxins causes insidious illnesses that lead to protracted, non-communicable diseases and death. Chronic kidney disease of unusual/uncertain/unknown aetiology is one such example, began to manifest in the mid-1960s in several dry-zonal agricultural societies in developing economies that are located around the equator. In Sri Lanka, such a disease is affecting the North Central Province, the rice bowl of the country that first appeared in the mid-1990s. Several potential causes have been postulated, including heavy metals, fluoride, cyanobacterial and algae toxins, agrochemicals, and high salinity and ionicity in water, but no specific source or causative factor has been identified for CKD of multifactorial origin (CKDmfo). Three large studies conducted in the recent past failed to find any of the postulated components (heavy metals, cyanobacterial toxins, fluoride, salinity, or agrochemicals) at levels higher than those deemed safe by the World Health Organization and the US Environmental Protection Agency. At the reported low levels in water and with the heterogeneous geographical distribution, it is unrealistic to expect any of these components individually could cause this disease. However, the additive or synergistic effects of a combination of factors and components, even at lower exposure levels, together with malnutrition and harmful behaviours, and/or a yet-unidentified (or not investigated) toxin, can cause this epidemic. Because the cause is unknown, scientists need to work on broader hypotheses, so that key causative elements are not missed. Taken together the plausibility of multiple factors in the genesis of this disease, the appropriate terminology is CKDmfo, a name that also indicates the need for multi-disciplinary research programs to facilitate identifying the cause(s) and the need for multiple approaches to eradicate it. While some potential causes remain to be investigated, existing data point to polluted water as the main source of this disease. This article evaluates pros and cons of each hypothesis and highlights the importance of among others, providing clean water to all affected and surrounding communities. Available data do not support any of the postulated agents, chemicals, heavy metals, fluoride, salinity/ionicity, or individual agrochemical components, such as phosphate or glyphosate, as causative factors for CKDmfo in Sri Lanka. However, as the CKDmfo name implies, a combination of these factors (or an unknown toxin) together with harmful behaviour and chronic dehydration may cause this disease. Irrespective of the cause, prevention is the only way forward for eradication.

Chloride-dependent amino acid transport in the small intestine: occurrence and significance

The unidirectional influx of amino acids, D-glucose and ions across the brush-border membrane of the small intestine of different species has been measured in vitro with emphasis on characterization of topographic and species differences and on chloride dependence. The regional differences in transport along the small intestine are outlined and shown to be caused by variation in transport capacity, while the apparent affinity constants are unchanged. Rabbit small intestine is unique by exhibiting maximal rates of transport in the distal ileum and a very steep decline in the oral direction from where tissues are normally harvested for preparation of brush-border membrane vesicles. Transport in the guinea pig and rat is much more constant throughout the small intestine. Since the capacity of nutrient carriers is regulated by their substrates it is possible that bacterial breakdown of peptides and proteins in rabbit distal ileum increases the concentration of amino acids leading to an upregulation of the carriers. Chloride dependence is a characteristics of the carrier rather than the transported amino acid, and is used to improve the classification of amino acid carriers in rabbit small intestine. In this species the imino acid carrier, the beta-amino acid carrier, and the beta-alanine carrier, which should be renamed the B0,+ carrier, are chloride-dependent. The steady-state mucosal uptake of classical substrates for these carriers in biopsies from the human duodenum is also chloride-dependent. The carrier of beta-amino acids emerges as ubiquitous and chloride-dependent, and evidence of cotransport with both sodium and chloride is reviewed. A sodium:chloride:2-methyl-aminoisobutyric acid coupling stoichiometry of approx. 2:1:1 is suggested by ion activation studies. Direct measurements of coupled ion fluxes in rabbit distal ileum confirm that sodium, chloride and 2-methyl-aminoisobutyric acid are cotransported on the imino acid carrier with an identical influx stoichiometry. Control experiments and reference to the literature on the electrophysiology of the small intestine exclude alterations of the membrane potential as a feasible explanation of the chloride dependence. Thus, it is concluded that chloride is cotransported with both sodium and 2-methyl-aminoisobutyric acid across the brush-border membrane of rabbit distal ileum.

Characteristics of ethacrynic acid highly sensitive Mg2+-ATPase in microsomal fractions of the rat brain: functional molecular size, inhibition by SITS and stimulation by Cl-

Studies were performed to characterize ethacrynic acid (EA) highly sensitive Mg2+-ATPase isolated from microsomal fractions of the rat brain. The functional molecular sizes of the EA highly sensitive and EA less sensitive Mg2+-ATPases, estimated by a radiation inactivation method, were 480 and 80 kDa, respectively. An anion transport inhibitor, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) inhibited the EA highly sensitive Mg2+-ATPase activity. The type of inhibition was uncompetitive with respect to ATP, and the inhibition was suppressed by anions such as Cl-, Br- and I-. Chloride ions stimulated enzyme activity with an increase in Vmax, but not in Km, for ATP. Anions tested also increased the enzyme activity in the following order of decreasing potency: Cl- greater than Br- greater than CH3COO- = I- greater than SO4(2-) = HCO3- greater than SO3(2-). These results suggest that EA highly sensitive Mg2+-ATPase is a relatively large molecule with anion-sensitive sites that affect the ATP hydrolyzing activity and the SITS binding capacity through anions, with Cl- being the most potent.

L-cystine transport by papain-treated rat renal brush-border membrane vesicles

In papain-treated rat renal brush-border membrane vesicles, cystine uptake was enhanced under sodium gradient conditions. This effect was not observed when sodium was equilibrated across the vesicle membrane or when sodium was completely absent from the incubation medium. The increased rate of cystine uptake occurred within the first two minutes of incubation and coincided with the period of increased flux of sodium known to occur after papain treatment. Under sodium gradient conditions, the Vmax of cystine uptake by treated vesicles was 65% greater while the Km was 25% lower than the value observed in untreated membranes. The increased cystine uptake after papain treatment occurred when medium cystine was in the electroneutral form. In the absence of a sodium gradient, cystine uptake by control membranes was insensitive to changes in membrane potential and this was unaltered after papain treatment. Exposure of the membranes to papain also resulted in a profound decrease in cystine binding which occurs in native membranes incubated with cystine. The fact that cystine uptake is unchanged under sodium equilibration and even enhanced under sodium gradient conditions suggests that the component of cystine binding is not essential for cystine transport and may represent non-specific binding to membrane proteins.

Intestinal sugar transport during ageing

Ageing effects on sugar intestinal transport were studied by using the everted sac and the brush-border membrane vesicle techniques. Four age groups of rats were used: very young, young, adult and old animals. Net transintestinal transport of D-glucose and intracellular sugar accumulation were greater in young than in very young, adult and old rats. Net Na+ transport was high in very young and young animals and then it declined with age. In brush-border membrane vesicle experiments D-glucose overshoot was smaller in the groups of animals where net sugar transport was less. In old rats, however, the overshoot did not occur. Short-circuiting of vesicles with valinomycin showed that the driving forces for sugar accumulation, i.e. the chemical potential gradient of Na+ and the electrical potential gradient, played different roles during ageing. In very young animals the chemical potential gradient seems to be responsible for D-glucose overshoot; in young rats both gradients are important while in adult animals the electrical potential gradient represents the main driving force.

Electrophysiology of succinate transport across rabbit renal brush border membranes

In rabbit renal brush border membrane vesicles, the membrane potential was monitored using a voltage-sensitive optical probe (diS-C3-(5)). The ionic dependence of the electrogenic Na+/succinate co-transporter was determined in the presence of monovalent anions and mono-, di-, and trivalent cations. Na+ and La3+ were the only cations capable of supporting a succinate-dependent membrane depolarization: Li+, K+, Rb+, Cs+, NH4+, Hg2+, Ca2+, Ba2+, Sr2+, Mg2+, Cu2+, Fe2+, Cd2+, Be2+, Pb2+, Zn2+, Mn2+ and Co2+ did not. Succinate increased the Na+ permeability of the brush border membrane in a saturable manner: saturating succinate (3 mM) concentrations increased the Na+/K+ permeability (PNa/PK) ratio from 0.6 to 2.3. In the presence of Na+, Li+ and Hg2+ inhibit the succinate potential: cis-Li+ inhibition is competitive with an apparent Ki of 2 mM, while trans-Li+ is noncompetitive; cis-Hg2+ decreased the maximal depolarization with an inhibitor constant Ki of 8 microM, and this effect was irreversible. Cations having no effect included K+, Rb+, Cs+, NH4+, Ba2+, Ca2+, Mg2+, Sr2+, Cu2+, Fe2+, Cd2+, Co2+, Be2+, Zn2+, Pb2+, Mn2+, and La3+. It is concluded that succinate/Na+ co-transport produces a specific increase in the Na+ conductance of renal brush borders.