Cell sodium activity and sodium pump function in frog skin

Summary Cell Na activity,a _Na ^c , was measured in the short-circuited frog skin by simulaneous cell punctures from the apical surface with open-tip and Na-selective microelectrodes. Skins were bathed on the serosal surface with NaCl Ringer and, to reduce paracellular conductance, with NaNO₃ Ringer on the apical surface. Under control conditionsa _Na ^c averaged 8±2mm (n=9,sd). Apical addition of amiloride (20 m) or Na replacement reduceda _Na ^c to 3mm in 6–15 min. Sequential decreases in apical [Na] induced parallel reductions ina _Na ^c and cell current,I _c . On restoring Na after several minutes of exposure to apical Na-free solutionI _c rose rapidly to a stable value whilea _Na ^c increased exponentially, with a time constant of 1.8±0.7 min (n=8). Analysis of the time course ofa _Na ^c indicates that the pump Na flux is linearly related toa _Na ^c in the range 2–12mm. These results indicate thata _Na ^c plays an important role in relating apical Na entry to basolateral active Na flux.     

Transmembrane effects in the sodium pump system. II. The ratio of the sodium efflux to the sodium concentration in frog muscle in media with various sodium substitutes

The dependence of sodium efflux on the internal sodium concentration on sodium-free magnesium, Tris, coline and lithium media was investigated on frog striated muscle. In all the sodium-substituted media, the efflux concentration curve was found to be dependent on the external rubidium concentration, being S-shaped at the saturating external rubidium (potassium) concentration and becoming close to linear at the low external rubidium concentration (0.5-1.0 microM). The maximal sodium efflux at saturating levels of internal sodium concentrations remains unchanged with various sodium substitutes in the medium, whereas the affinity constant of internal sodium sites is dependent on the external cations.     

Protons as substitutes for sodium and potassium in the sodium pump reaction

The role of protons as substitutes for Na+ and/or K+ in the sodium pump reaction was examined using inside-out membrane vesicles derived from human red cells. Na+-like effects of protons suggested previously (Blostein, R. (1985) J. Biol. Chem. 260, 829-833) were substantiated by the following observations: (i) in the absence of extravesicular (cytoplasmic) Na+, an increase in cytoplasmic [H+] increased both strophanthidin-sensitive ATP hydrolysis (nu) and the steady-state level of phosphoenzyme, EP, and (ii) as [H+] is increased, the Na+/ATP coupling ratio is decreased. K+-like effects of protons were evidenced in the following results: (i) an increase in nu, decrease in EP, and hence increase in EP turnover (nu/EP) occur when intravesicular (extracellular) [H+] is increased; (ii) an increase in the rate of Na+ influx into K+(Rb+)-free inside-out vesicles and (iii) a decrease in Rb+/ATP coupling occur when [H+] is increased. Direct evidence for H+ being translocated in place of cytoplasmic Na+ and extracellular K+ was obtained by monitoring pH changes using fluorescein isothiocyanate-dextran-filled vesicles derived from 4',4-diisothiocyano-2',2-stilbene disulfonate-treated cells. With the initial pHi = pHo = pH 6.2, a strophanthidin-sensitive decrease in pHi was observed following addition of ATP provided the vesicles contained K+. This pH gradient was abolished following addition of Na+. With alkali cation-free inside-out vesicles, a strophanthidin-sensitive increase in pH was observed upon addition of both ATP and Na+. The foregoing changes in pHi were not affected by the addition of tetrabutylammonium to dissipate any membrane potential and were not observed at pH 6.8. These ATP-dependent cardiac glycoside-sensitive proton movements indicate Na,K-ATPase mediated Na+/H+ exchange in the absence of extracellular K+ as well as H+/K+ exchange in the absence of cytoplasmic Na+.     

Viscosity behaviour and persistence length of sodium xanthan in aqueous sodium chloride

Zero-shear-rate intrinsic viscosities [eta] of sodium xanthan in aqueous NACl at 25 degrees C were determined for five samples ranging in weight- average molecular weight from 2 x 10(5) to 4 x 10(6) at salt concentrations Cs between 0.005 and 1 M, at which the polysaccharide maintains its double-helical structure. The measured [eta] for every sample was almost independent of Cs, in contrast to usual observations on flexible polyelectrolytes. The persistence length q of sodium xanthan was determined as a function of Cs by use of the theory of Yamakawa et al. for [eta] of an unperturbed worm-like cylinder, and from its Cs dependence the intrinsic persistence length q(o) ( = q at infinite ionic strength) was estimated to be 106 nm. This q(o) value was roughly twice as large as that of double-stranded DNA, indicating a high intrinsic rigidity of the xanthan double helix. The electrostatic contribution ( = q - q(o)) to q was only about 10% even at the lowest Cs of 0.005 M. Thus, it was concluded that above Cs = 0.005 M, the double- helical structure of sodium xanthan is hardly stiffened by electrostatic interactions between charged groups.     

Inhibition of sodium for sodium exchange by phlorizin in frog sartorius muscle

The effects of phlorizin (2 X 10(-3) mol X l-1) on the Na transport of frog (Rana esculenta) sartorius muscle were investigated in glucose-free medium. Phlorizin decreased the rate coefficient of 24Na efflux by about 40%. The degree of inhibition was comparable to that caused by ouabain (10(-4) mol X l-1). Phlorizin could evoke a further reduction in the 24Na efflux also in the presence of ouabain. The intracellular Na content of the phlorizin-treated muscles remained unchanged, in contrast to a 60% increase induced by ouabain. 42K uptake was not affected by phlorizin. Data indicate that the ouabain-sensitive Na-K pump was not involved in the action of phlorizin. At the same time, phlorizin failed to alter the residual 24Na efflux measured in Li-Ringer solution containing ouabain. When Na: Na exchange was restored by replacing Na into the washout solution in the presence of ouabain, the increase of 24Na efflux was significantly diminished by phlorizin. Phlorizin reduced the 24Na uptake into a compartment with a half time of 6 min by about 40% without affecting the intracellular compartment. The results suggest that phlorizin inhibits the ouabain-insensitive Na: Na exchange in a superficial Na compartment.     

The influence of external sodium ions on the sodium pump in erythrocytes

1. A study has been made of the interaction between Na(+) and K(+) on the adenosine triphosphatase activity of erythrocyte ;ghosts', and on the K(+) influx and Na(+) efflux of intact erythrocytes. The adenosine triphosphatase activity and the ion movements were greater at a low external K(+) concentration in the absence of Na(+) than they were in the presence of 150mm-Na(+). The inhibition by external Na(+) of K(+) influx had an inhibitory constant of 5-10mm. 2. Activation by K(+) of kidney microsomal adenosine triphosphatase was retarded by Na(+), and activation by Na(+) was retarded by K(+). Fragmented erythrocyte membranes behaved similarly. 3. These observations suggest that there is competition between Na(+) and K(+) at the K(+)-sensitive site of the membrane.     

Relation between intracellular sodium and active sodium transport in rabbit colon: Current-voltage relations of the apical sodium entry mechanism in the presence of varying luminal sodium concentrations

The current-voltage relations of the amiloride-sensitive Na entry pathway across the apical membrane of rabbit descending colon, exposed to a high K serosal solution, were determined in the presence of varying mucosal Na activities, (Na)m, ranging from 6.2 to 99.4 mM. These relations could be closely fit to the "constant field" flux equation yielding estimates of the permeability of the apical membrane to Na, PmNa, and the intracellular Na activity, (Na)c. The following empirical relations emerged: (Na)c increased hyperbolically with increasing (Na)m; PmNa decreased hyperbolically with increasing (Na)m and linearly with increasing (Na)c; spontaneous variations in Na entry rate at constant (Na)m could be attributed entirely to parallel, spontaneous variations in PmNa; the rate of Na entry increased hyperbolically with increasing (Na)m obeying simple Michaelis-Menten kinetics; the relation between (Na)c and "pump rate," however, was sharply sigmoidal and could be fit by the Hill equation assuming strong cooperative interactions between Na and multiple sites on the pump; the Hill coefficient was 2-3 and the value of (Na)c at which the pump-rate is half-maximal was 24 mM. The results provide an internally consistent set of relations among Na entry across the apical membrane, the intracellular Na activity and basolateral pump rate that is also consistent with data previously reported for this and other Na-absorbing epithelia.     

钠离子通道疾病及其抑制剂生物学功能研究进展

电压门控型钠离子通道(Voltage-gated sodium channel,VGSC)广泛分布于兴奋性细胞,是电信号扩大和传导的主要介质,在神经细胞以及心肌细胞兴奋传导等方面发挥重要作用。钠离子通道结构和功能的异常会改变细胞的兴奋性,从而导致多种疾病的发生,如神经性疼痛、癫痫,以及心律失常等。目前临床上多采用钠离子通道抑制剂治疗上述疾病。近些年,研究人员陆续从动物的毒液中分离纯化出具有调控钠离子通道功能的神经毒素。这些神经毒素多为化合物或小分子多肽。现已有医药研发公司将这些天然的神经毒素进行定向设计改造成钠离子通道靶向药物用于临床疾病的治疗。此外,来源于七鳃鳗Lampetra japonica口腔腺的富含半胱氨酸分泌蛋白(Cysteine-rich buccal gland protein,CRBGP)也首次被证明能够抑制海马神经元和背根神经元的钠离子电流。以下针对钠离子通道疾病及其抑制剂生物学功能的最新研究进展进行分析归纳。     

Ion uptake in Pinus banksiana treated with sodium chloride and sodium sulphate

Within its wide range across Canada, jack pine is exposed to salinity from both natural and anthropogenic sources. To compare the effects of Cl and SO₄ on salt injury, sand and solution-culture grown jack pine ( Pinus banksiana Lamb.) seedlings were treated with nutrient solutions containing 60 or 120 m M NaCl, 60 m M Na₂SO₄, or a mixture of 60 m M NaCl and 30 m M Na₂SO₄. After 5 weeks of salt treatments, concentrations of Cl, K, Na, and SO₄ were determined in roots, stem and needles of the current and previous years growth, and in necrotic needles. To determine the role of water uptake in the absorption and translocation of salts in plants, total transpiration was measured as the loss of water from a sealed system and related to total plant uptake of Cl, Na, and SO₄. Sodium uptake and root-to-shoot transport rates were greater in treatments containing Cl. A delay in root-to-shoot transport of both Na and Cl indicates retention of these ions in the roots. Electrolyte leakage of needles was more closely related to treatment Cl concentrations than treatment Na concentrations. The transport of Na ions to the shoot was related to the presence of Cl, but was not related to transpiration rate.     

Lowest neonatal serum sodium predicts sodium intake in low birth weight children

Forty-one children aged 10.5 +/- 0.2 years (range, 8.0-15.0 yr), born with low birth weight of 1,218.2 +/- 36.6 g (range, 765-1,580 g) were selected from hospital archives on the basis of whether they had received neonatal diuretic treatment or as healthy matched controls. The children were tested for salt appetite and sweet preference, including rating of preferred concentration of salt in tomato soup (and sugar in tea), ratings of oral spray (NaCl and sucrose solutions), intake of salt or sweet snack items, and a food-seasoning, liking, and dietary questionnaire. Results showed that sodium appetite was not related to neonatal diuretic treatment, birth weight, or gestational age. However, there was a robust inverse correlation (r = -0.445, P < 0.005) between reported dietary sodium intake and the neonatal lowest serum sodium level (NLS) recorded for each child as an index of sodium loss. The relationship of NLS and dietary sodium intake was found in both boys and girls and in both Arab and Jewish children, despite marked ethnic differences in dietary sources of sodium. Hence, low NLS predicts increased intake of dietary sodium in low birth weight children some 8-15 yr later. Taken together with other recent evidence, it is now clear that perinatal sodium loss, from a variety of causes, is a consistent and significant contributor to long-term sodium intake.     

Endocrine disorders of sodium regulation. Role of adrenal steroids in genetic defects causing sodium loss or sodium retention

Salt and water homoeostasis is tightly regulated by a variety of control mechanisms with the adrenal steroid hormone aldosterone playing a central role. Defects or disturbances in these systems lead to either salt loss, which is life threatening in the neonatal period, or sodium retention causing hypertension. Rapid and accurate diagnosis is required to avoid severe complications. During the last few years molecular genetic advances have been identified as the basic genetic defects for a number of clinical syndromes. This knowledge has considerably increased our understanding of the basic pathways involved in sodium and water homoeostasis and of the pathophysiology of these syndromes, particularly the hypertension. In this review we have summarized the biochemical, physiological and genetic basis for clinical syndromes presenting with salt loss and failure to thrive as well as the rare but important genetic syndromes causing sodium retention and hypertension. Early diagnosis and identification will help to prevent severe complications, but it has to be emphasized that the complicated cascade of aldosterone action is still relatively poorly understood. Further syndromes may exist which once identified will help to better understand the basic physiology of aldosterone action.     

Regulation of single sodium channels in renal tissue: a role in sodium homeostasis

The kidney is responsible for the maintenance of an organism's body solute and water balance (i.e., Na+ homeostasis). The distal nephron and the cortical collecting duct (CCD) (an example of a tight epithelium) are important sites of regulatory control over the rate of Na+ reabsorption. The Na+ channel, a specialized protein located in the apical membrane of CCD cells, is the specific site of transepithelial Na+ movement. Na+ entry into the cell across the apical membrane occurs by passive diffusion of Na+ down an electrochemical gradient. We have used the patch-voltage clamp method to examine single-channel conductance events of the amiloride-sensitive apical Na+ channel in A6 cells, a model of CCD. Two types of Na+ channel were identified. One type was characterized by low selectivity (Na+ to K+) and high conductance, the other by high selectivity and low conductance. The type and frequency of channel observed depended on the transporting state of the epithelium. In a tissue with poor transport rates, the low-selectivity type of channel was prevalent (the other type of channel was present, but in a very low density). Therefore, the poorly transporting tissue had an overall low apical Na+ conductance. In a tissue with high transport rates, the highly selective channel appeared to be predominant. In this case the net result was a highly Na+ conductive apical membrane.