Cell sodium activity and sodium pump function in frog skin

Theoretical considerations underlying Na(+) uptake mechanisms in freshwater fishes

Ion and acid-base regulating mechanisms have been studied at the fish gill for almost a century. Original models proposed for Na(+) and Cl(-) uptake, and their linkage with H(+) and HCO(3)(-) secretion have changed substantially with the development of more sophisticated physiological techniques. At the freshwater fish gill, two dominant mechanisms for Na(+) uptake from dilute environments have persisted in the literature. The use of an apical Na(+)/H(+) exchanger driven by a basolateral Na(+)/K(+)-ATPase versus an apical Na(+) channel electrogenically coupled to an apical H(+)-ATPase have been the source of debate for a number of years. Advances in molecular biology have greatly enhanced our understanding of the basic ion transport mechanisms at the fish gill. However, it is imperative to ensure that thermodynamic principles are followed in the development of new models for gill ion transport. This review will focus on the recent molecular advances for Na(+) uptake in freshwater fish. Emphasis will be placed on thermodynamic constraints that prevent electroneutral apical NHE function in most freshwater environments. By combining recent advances in molecular and functional physiology of fish gills with thermodynamic considerations of ion transport, our knowledge in the field should continue to grow in a logical manner.

Effects of intracellular signals on Na+/K(+)-ATPase pump activity in the frog skin epithelium

The effects of intracellular signals (pHi, Na+i, Ca2+i, and the electrical membrane potential), on Na+ transport mediated by the Na+/K+ pump were investigated in the isolated Rana esculenta frog skin. In particular we focussed on pHi sensitivity since protons act as an intrinsic regulator of transepithelial Na+ transport (JNa) by a simultaneous control of the apical membrane Na+ conductance (gNa) and the basolateral membrane K+ conductance (gK). pHi changes which modify JNa, gNa and gK, do not affect the Na+ transport mediated by the pump as shown by kinetic and electrophysiological studies. In addition, no changes were observed in the number of 3H-ouabain binding sites in acid-loaded epithelia. Our attempts to modify cellular Ca2+ (by using Ca(2+)-free/EGTA Ringer solution or A23187 addition) also failed to produce any significant effects in the Na+ pump turnover rate or the number of 3H-ouabain binding sites. The Na+ pump current was found to be sensitive to the basolateral membrane potential, saturating for very positive (cell) potentials and a reversal potential of -160 mV was calculated from I-V relationships of the pump. Changes in Na+i considerably affected the Na+ pump rate. A saturating relationship was found between pump rate and Nai+ with maximal activation at Nai+ greater than 40 mmol/l; a high dependence of the pump rate and of the number of 3H-ouabain binding sites was observed in the physiological range of Nai+. We conclude that protons (in the physiological pH range) which act directly and simultaneously on the passive transport pathways (gNa and gK), have no direct effect on the Na+/K+ pump rate. After an acid load, the inhibition of JNa is primarily due to the reduction of gNa. This results in a reduction of Nai and the pump turnover rate then becomes dependent on other pathways of Na+ entry such as the basolateral membrane Na+/H+ exchanger.

Sodium transport and intracellular sodium activity in cultured human nasal epithelium

Human airway epithelia are predominantly Na(+)-absorbing epithelia. To investigate the mechanisms for Na+ absorption across airway epithelia, the driving forces and paths for Na+ translocation across each membrane were examined with double-barreled Na(+)-selective microelectrodes in cultured human nasal epithelium (HNE). Under control conditions, intracellular Na+ activity (acNa) was 23 +/- 1 mM (n = 44 preparations, 393 impalements). Amiloride (10(-4) M) hyperpolarized the apical membrane and increased the fractional apical membrane resistance but did not affect acNa. Exposure to Na(+)-free luminal solution induced bioelectric responses similar to amiloride but also reduced acNa to 8 +/- 1 mM. Reduction of luminal Na+ concentration ([Na+]) in the presence of amiloride also reduced acNa without further changes in bioelectric parameters. Reduction of serosal [Na+] decreased aNac, a response blocked by bumetanide (10(-4) M). Ouabain (10(-4) M, serosal) led to a reduction in equivalent short-circuit current (Ieq) and increase in acNa. We conclude that 1) acNa is higher in HNE than in most mammalian epithelial cells, 2) the apical membrane expresses a conductive Na+ path, and 3) the basolateral membrane transports Na+ via the Na(+)-K(+)-adenosinetriphosphatase and a Na(+)-K(+)-2Cl- cotransport system.

Ethanol and (Na+,K+)-ATPase: alteration of Na(+)-K+ selectivity

Relative internal concentrations of Na+ and K+ are important in regulating (Na+,K+)-ATPase in situ. Ethanol is known to inhibit (Na+,K+)-ATPase and to reduce K+ affinity, but the concentrations required for these effects in vitro are large compared with those probably attainable in vivo. Yet, there is evidence suggesting that ethanol has physiologically relevant effects on (Na+,K+)-ATPase. We have investigated the effects of ethanol on selectivity for Na+ versus K+. At 150 mM, ethanol had little effect on (Na+,K+)-ATPase activity under the usual assay conditions, slightly (but nonsignificantly) reduced K+ affinity, and had no effect on extrapolated Na+ affinity in the absence of K+. However, ethanol had marked effects on cation selectivity, doubling the Ki for K+ on Na+ affinity and halving the Ki for Na+ on K+ affinity. These data show that ethanol, at concentrations too small for effects on (Na+,K+)-ATPase activity under optimal assay conditions, can alter its responses to changes in Na+ or K+.

Subacute noradrenergic agonist infusions in vivo increase Na+, K+-ATPase and ouabain binding in rat cerebral cortex

In order to investigate the specificity of noradrenergic effects on Na+, K+-ATPase, we infused noradrenergic agonists into the cerebral ventricles of rats, with or without depletion of forebrain norepinephrine. Infusion of norepinephrine, isoproterenol, or phenylephrine increased ouabain binding in intact rats, whereas clonidine infusion decreased binding. Depletion of forebrain norepinephrine by destruction of the dorsal noradrenergic bundle reduced ouabain binding. Norepinephrine infusion reversed the effect of dorsal bundle lesion; isoproterenol and phenylephrine increased ouabain binding in lesioned rats, but did not restore the effect of the lesions. Clonidine had no effect in lesioned rats. Effects on Na+, K+-ATPase activity were similar, but smaller. These results suggest that stimulation of both alpha 1- and beta-noradrenergic receptors may be necessary for optimal Na+, K+-ATPase, and that clonidine reduces Na+, K+-ATPase indirectly through decreased norepinephrine release.

Influence of serosal Cl on transport properties and cation activities in frog skin

The effects of serosal substitution of isosmotic Na2SO4-Ringer solution for NaCl-Ringer solution were studied in the short-circuited frog skin (Rana pipiens, Northern variety). Despite prompt changes of transepithelial measurements, initial cellular effects were slight. After 30 to 45 min, however, the transcellular current had decreased and the cell electrical potential had depolarized, in association with decrease of the apical membrane fractional resistance and basolateral membrane conductance. Apical membrane slope conductance was unaffected. Similar effects were obtained with isolated epithelia. With the use of gluconate or NO3 in place of Cl, the effects on cellular current and conductance were minimal or insignificant, despite changes of the cell potential, fractional resistance, and basolateral conductance similar to those seen with sulfate. Following prolonged exposure to serosal SO4-Ringer, the extent of depolarization induced by raising the serosal K concentration decreased, indicating diminution of basolateral K conductance and the existence of other basolateral conductances. Equilibration in serosal gluconate-Ringer enhanced polarization on serosal restoration of Cl or removal of Na, again indicating a time-dependent change in the basolateral conductance pattern. Depolarization on removal of serosal Cl was not attributable to inhibition of the pump. Nor was it the result of decrease of the K equilibrium potential EK: exposure to serosal SO4-Ringer decreased cell K activity aKc from 104 +/- 6 to 58 +/- 4 mM (n = 5), but EK was reduced only slightly; exposure to serosal gluconate increased aKc and EK. Serosal sulfate lowered the cell Na activity aNac, but the electrochemical potential difference for Na across the apical surface was unaffected. The concurrent decrease of both aKc and aNac following serosal substitution of SO4 for Cl raises questions concerning mechanisms of osmoregulation.

Voltage dependence of cellular current and conductances in frog skin

Knowledge of the voltage dependencies of apical and basolateral conductances is important in determining the factors that regulate transcellular transport. To gain this knowledge it is necessary to distinguish between cellular and paracellular currents and conductances. This is generally done by sequentially measuring transepithelial current/voltage (It/Vt) and conductance/voltage (gt/Vt) relationships before and after the abolition of cellular sodium transport with amiloride. Often, however, there are variable time-dependent and voltage-dependent responses to voltage perturbation both in the absence and presence of amiloride, pointing to effects on the paracellular pathway. We have here investigated these phenomena systematically and found that the difficulties were significantly lessened by the use of an intermittent technique, measuring It and gt before and after brief (less than 10 sec) exposure to amiloride at each setting of Vt. I/V relationships were characterized by these means in frog skins (Rana pipiens, Northern variety, and Rana temporaria). Cellular current, Ic, decreased with hyperpolarization (larger serosa positive clamps) of Vt. Derived Ic/Vt relationships between Vt = 0 and 175 mV (serosa positive) were slightly concave upwards. Because values of cell conductance, gc, remained finite, it was possible to demonstrate reversal of Ic. Values of the reversal potential Vr averaged 156 +/- 14 (SD, n = 18) mV. Simultaneous microelectrode measurements permitted also the calculation of apical and basolateral conductances, ga and gb. The apical conductance decreased monotonically with increasing positivity of Vt (and Va). In contrast, in the range in which the basolateral conductance could be evaluated adequately (Vt less than 125 mV), gb increased with more positive values of Vt (and Vb). That is, there was an inverse relation between gb and cellular current at the quasi-steady state, 10-30 sec after the transepithelial voltage step.

Cell and luminal activities of chloride, potassium, sodium and protons in the late distal tubule of Necturus kidney

1. Double-barrelled (selective vs. conventional) microelectrodes were used to assess the steady-state activities (a) of the ions Cl-, K+, Na+ and H+ in peritubular blood capillaries (abld) and in cell (acell) and lumen (alum) of the late distal tubule (l.d.t.) of Necturus. 2. a(cell)cl, a(lum)cl and a(bld)cl were 5.5 +/- 0.3, 11.8 +/- 1.0 and 70.5 +/- 0.1 mM, respectively. They were used to compute the chemical potentials for Cl- across the three diffusive barriers of the tissue. Basolateral and apical membrane potentials were -74.3 +/- 1.1 and -60.1 +/- 2.0 mV, respectively (cell negative); the lumen was thus negative with respect to blood, by 13.6 +/- 1.5 mV. The electrochemical potential difference (e.p.d.) for Cl- of 42 mV across the apical membrane opposes Cl- absorption, implying active apical Cl- uptake, since Cl- is known to be absorbed in the l.d.t. Basolateral Cl- exit is favoured by an e.p.d. of 10 mV. 3. a(cell)K, a(lum)K and a(bld)K were 65.8 +/- 0.8, 2.5 +/- 0.1 and 2.5 +/- 0.1 mm, respectively. The electrochemical distribution of K+ indicates that K+ absorption, if present, proceeds against an adverse apical e.p.d. of 18 mV. Basolateral K+ distribution is close to its electrochemical equilibrium, suggesting high K+ permeability at this membrane. 4. a(cell)Na was 9.0 +/- 0.4 mM, a(bld)Na 71.0 +/- 0.3 mM, and a(lum)Na was approximated at about 9 mM. Diffusive Na+ entry from lumen to cell is favoured by an e.p.d. close to 65 mV. Basolateral Na+ exit must be active, since it proceeds against an e.p.d. of 130 mV. 5. Cell, luminal and blood pH were 7.14 +/- 0.03, 6.52 +/- 0.08 and 7.37 +/- 0.04, respectively. The luminal electrochemical potential of H+ is higher than that of cell (by 91 mV) and blood (by 34 mV) indicating that proton secretion into the lumen must be active. 6. The e.p.d. of each ion across the epithelium opposes, by its orientation, the established direction of net transepithelial ion transport, suggesting that the shunt pathway may serve only for back-diffusion.