Studies on ouabain-binding to (Na+ + K+)-ATPase from Malpighian tubules of the locust, Locusta migratoria L

K(+) transport in Malpighian tubules of Tenebrio molitor L: a study of electrochemical gradients and basal K(+) uptake mechanisms

Malpighian tubules of the mealworm Tenebrio molitor were isolated for intracellular measurement of basolateral (V(bl)) and, indirectly, apical (V(ap)) membrane potentials. In control Ringer (50 mmol l(-1) K(+), 140 mmol l(-1) Na(+)), V(bl) was 24 mV, cell negative, and V(ap) was 48 mV, cell negative with reference to the lumen. Ion substitution experiments involving K(+) and Na(+) indicated that both V(bl) and V(ap) were sensitive to the bathing K(+) concentration, with the change in V(ap) being 60-77% that of V(bl). A 10-fold drop in bath [K(+)] irreversibly decreased fluid secretion rates from 6.38+/-0.95 nl x min(-1) (mean +/- S.E.M.) to 1.48+/-0.52 nl x min(-1) (N=8). In the presence of 6 mmol l(-1) Ba(2+), a blocker of basal K(+) channels, fluid secretion rates reversibly decreased and the hyperpolarization of both V(bl) and V(ap) seen in 50 mmol l(-1) and 140 mmol l(-1) K(+) indicated a favourable electrochemical gradient for basal K(+) entry. In 5 mmol l(-1) K(+), Ba(2+) induced two different responses: V(bl) either hyperpolarized by approximately 10 mV or depolarised by approximately 14 mV, according to the electrochemical gradient for K(+), which was either inward or outward in low bath [K(+)]. Rubidium, a 'permeant' potassium substitute, caused a hyperpolarization of V(bl), indicating the specificity of K(+) channels found in Tenebrio tubule cells. Other possible K(+) uptake mechanisms located in the basolateral membrane were investigated. Blocking of the putative electroneutral Na(+)/K(+)/2Cl(-) cotransporter by 10 micromol l(-1) bumetanide reversibly decreased fluid secretion rates, with no detectable change in membrane potentials. Ouabain (1 mmol l(-1)), an Na(+)/K(+)-ATPase inhibitor, irreversibly decreased fluid secretion rates but had no effect on electrical potential differences either in the absence or presence of Ba(2+). The results implicate K(+) channels, the Na(+)/K(+)/2Cl(-) contransporter and the Na(+)/K(+)-ATPase in basal K(+) and fluid transport of Tenebrio tubule cells.

The dependence of electrical transport pathways in Malpighian tubules on ATP

The relationship between the intracellular ATP concentration [ATP](i) and the electrical properties of principal cells was investigated in Malpighian tubules of the yellow fever mosquito, Aedes aegypti. Under control conditions, [ATP](i) was 0.91 mmol l(-1), the input resistance of the principal cell (R(pc)) was 334.1 k Omega, and the basolateral membrane was marked by a large K(+)-conductance and a membrane voltage (V(bl)) of -75.8 mV. Peritubular cyanide (CN, 0.3 mmol l(-1)) reduced [ATP](i) to 0.08 mmol l(-1) in less than 2 min; however, V(bl) dropped to -8 mV and R(pc) increased to 3150.8 k Omega in 8 min, while the K(+)-conductance of the basolateral membrane disappeared. Upon washout of CN, V(bl) and R(pc) returned to control values within 2 min, and the basolateral membrane recovered its K(+)-conductance. The recovery of normal [ATP](i) took 15 min. Dose-dependence and EC(50) values for the CN-inhibition of V(bl) and the increase in R(pc) were strikingly similar (184.0 micromol l(-1) and 164.4 micromol l(-1)). Similar effects of metabolic inhibition were observed with dinitrophenol (DNP), but the EC(50) values were 50.3 micromol l(-1) and 71.7 micromol l(-1) for the effects on V(bl) and R(pc), respectively. Barium, a blocker of K(+)-channels, significantly hyperpolarized V(bl) to -89.1 mV and increased R(pc) to 769.4 k Omega under control conditions, but had no effects during metabolic inhibition. These results illustrate a temporal relationship between [ATP](i) and electrogenic and conductive transport pathways in principal cells that is consistent with the role of ATP as an integrator of transport steps at apical and basolateral membranes of the cell. When [ATP](i) drops to levels that are 10% of control, the V-type H(+)-ATPase is inhibited, preventing the extrusion of K(+) to the tubule lumen. At the same time, basolateral membrane K(+)-channels close, preventing the entry of K(+) from the hemolymph. Intracellular K(+) homeostasis is thus protected during metabolic inhibition, allowing the cell to re-establish K(+) transport when ATP is synthesized again.

Properties of the V-type ATPase from the excretory system of the usherhopper, Poekilocerus bufonius

The bafilomycin A(1) and N-ethylmaleimide (NEM)-sensitive (V-type) ATPase was partially purified from the apical membrane-rich fractions of excretory system (Malpighian tubules and hind gut) of P. bufonius. Enzymatic activity was inhibited by bafilomycin A(1) (IC(50) = 1.3 nM) and NEM (IC(50) = 10.1 microM). The V-type ATPase activity is confined to the apical membrane fraction, while the activity of Na(+)/K(+) -ATPase forms the major part of the basal membrane fraction. The optimal pH required for maximal activity of V-type ATPase was pH 7.5. The effect of 30 mM of various salts on ATPase activity was investigated. NaCl and KCl caused increases of 175% and 184%, respectively. Other chloride salts also caused an increase in activity in the following ascending order: RbCl, LiCI, choline Cl, NaCI, KCl and tris-HCl. The activity of V-type ATPase was stimulated by a variety of different anions and cations, and HCO(3)(-) was found to be the most potent cationic activator of ATPase activity. The present results show that the properties of V-type ATPase of P. bufonius are similar to those reported for other insect tissues.

Intracellular ion activities in Malpighian tubule cells ofRhodnius prolixus: evaluation of Na+-K+-2Cl-cotransport across the basolateral membrane

Intracellular ion activities (aion) and basolateral membrane potential (Vbl) were measured in Malpighian tubule cells of Rhodnius prolixus using double-barrelled ion-selective microelectrodes. In saline containing 103mmoll-1Na+, 6mmoll-1 K+ and 93mmoll-1Cl-, intracellular ion activities in unstimulated upper Malpighian tubules were 21, 86 and 32mmoll-1, respectively. In serotonin-stimulated tubules, aCl was unchanged, whereas aNa increased to 33mmoll-1 and aK declined to 71mmoll-1. Vbl was -59mV and -63mV for unstimulated and stimulated tubules, respectively. Calculated electrochemical potentials(Δμ/F) favour passive movement of Na+ into the cell and passive movement of Cl- out of the cell in both unstimulated and serotonin-stimulated tubules. Passive movement of K+ out of the cell is favoured in unstimulated tubules. In stimulated tubules, Δμ/F for K+ is close to 0 mV.

The thermodynamic feasibilities of Na+-K+-2Cl-, Na+-Cl-and K+-Cl- cotransporters were evaluated by calculating the net electrochemical potential (Δμnet/F) for each transporter. Our results show that a Na+-K+-2Cl- or a Na+-Cl- cotransporter but not a K+-Cl- cotransporter would permit the movement of ions into the cell in stimulated tubules. The effects of Ba2+ and ouabain on Vbl and rates of fluid and ion secretion show that net entry of K+ through ion channels or the Na+/K+-ATPase can be ruled out in stimulated tubules. Maintenance of intracellular Cl- activity was dependent upon the presence of both Na+ and K+ in the bathing saline. Bumetanide reduced the fluxes of both Na+ and K+. Taken together, the results support the involvement of a basolateral Na+-K+-2Cl- cotransporter in serotonin-stimulated fluid secretion by Rhodnius prolixus Malpighian tubules.

An investigation into the effects of inhibitors of fluid production by Locusta Malpighian tubule Type I cells on their secretion and elemental composition

The intracellular elemental concentrations of K, Na, Cl, P, Mg and Ca within Type I cells of the Malpighian tubules of Locusta migratoria have been measured using electron probe X-ray microanalysis. The distribution of Na, K and Cl was not homogeneous within the cells and concentration gradients exist from basal to apical surfaces. The rate of secretion and the cationic composition of the secreted tubule fluid have also been determined. Furosemide (1 mM) inhibited fluid secretion by about 60%, raised the [Na(+)] but did not significantly alter the [K(+)] of the secreted tubule fluid. When Rb(+) replaced K(+) in the saline fluid secretion was also inhibited by about 60%, but no additional inhibition occurred by the simultaneous inclusion of furosemide. Thus, Rb(+) and furosemide probably act at the same transport site, and Rb(+) cannot substitute for K(+) at the basal membrane cotransporter. Bafilomycin (1 µM) dramatically inhibited fluid production by 85%, the [K(+)] of the secreted fluid was reduced by about 30% but the [Na(+)] was almost doubled. Furosemide, in common with other inhibitors of fluid secretion acting at the basal surface (ouabain and Rb(+)), caused a fall in intracellular [K] and a rise in [Na]. Bafilomycin, in common with N-ethyl maleimide, which acts at the apical surface, increased the intracellular [K] but did not affect the [Na].

X-ray microanalysis of the Malpighian tubules of the black field cricket Teleogryllus oceanicus: the roles of Na K ATPase and the Na K 2Cl cotransporter

Both main and distal segments of the Malpighian tubules were sensitive to ouabain and furosemide but in different ways. Oubain had no effect on secretion rate by the main segment but in the secreted fluid Na(+) concentration increased substantially whereas K(+) decreased. Similarly intracellular elemental Na concentration increased and K decreased. Furosemide decreased the secretion rate of the main segment by 80%. The Na(+) concentration in the secreted fluid increased markedly but K(+) was not affected. Intracellular elemental Na concentration also increased but K was unchanged. In the distal segments both ouabain and furosemide decreased secretion rate by 40% but although ouabain had no effect on the composition of the secreted fluid, furosemide caused a substantial reduction in the concentrations of Mg(2+) and Cl(-) and a substantial increase in Na(+) and K(+) concentrations. The evidence suggests that the main segment contains a Na K ATPase and possibly a Na K 2Cl cotransporter whereas the distal segment may contain a Na K ATPase and a furosemide sensitive Mg(2+) transporter. K(+) entry into the cells of the main segment may be partially effected by a Na K 2Cl cotransporter but may be primarily via Na K ATPase in the distal segment.

Use of Rb(+) and Br(-) as tracers for investigating ion transport by X-ray microanalysis in the Malpighian tubules of the black field cricket Teleogryllus oceanicus

Substitution of Rb(+) for K(+) in the incubation saline for in vitro preparations of Malpighian tubules had little effect on tubule function. Secretion rates increased by 10% for whole tubules, 9% for distal segments and 10% for main segments. In the secreted fluids Rb(+) almost completely replaced K(+). Within the cells of the main segment of the tubules Rb replaced the majority of the intracellular K. Treatment by ouabain in Rb saline resulted in a considerable increase in intracellular Na and Cl concentrations but no change in Rb concentration. This suggests that Rb(+) did not enter the cell via Na K ATPase and that the latter was not directly involved in Rb(+) secretion and by inference K(+) secretion. Substitution of Br(-) for Cl(-) in the incubation saline resulted in a 30% reduction in secretion rate from the distal segments but only a 10% reduction for the main segment. Cl(-) was almost completely replaced by Br(-) in fluid from both main and distal segments. In cells of the main segment the intracellular concentration of Br(-) did not exceed 30mmol kg(-1) dry weight and the Cl(-) concentration was unchanged in the apical region of the cell and increased in the basal region. These data suggest that Br(-) was transported across the tubule epithelium by a paracellular route and that the basal cell membrane is relatively impermeable to Cl(-). By inference Cl(-) may also be transported by a paracellular route.

The action of inhibitors of protein kinases on fluid and ion secretion by Malpighian tubules of Locusta migratoria, L

Fluid production in Locusta Malpighian tubules was stimulated by corpora cardiaca extract (c. 100%) and dibutyryl cAMP (c. 50%). Chelerythrine and staurosporine (Protein kinase C, PKC inhibitors) inhibited it in the range 0.07-60&mgr;M (IC(50)3&mgr;M), whereas Rp-cAMP (Protein kinase A, PKA inhibitor) caused inhibition over the concentration range 10-1000&mgr;M (IC(50)264&mgr;M). The protein phosphatase inhibitor, okadaic acid, was also inhibitory over the concentration range 0.1-1000nM (IC(50) 91nM). CC extract stimulation increased fluid [Na(+)] from 41 to 59mM and decreased [K(+)] from 127 to 107mM; stimulation with cAMP had no such effect. The PKC inhibitors reduced the [K(+)] in the secreted fluid from 126 to 107mM but had no effect on the [Na(+)]. Subsequent addition of CC extract stimulated fluid production and caused an increase in [Na(+)] from 41 to about 50mM. The addition of Rp-cAMP reduced fluid production but caused a decrease in [Na(+)] from 37 to 28mM and an increase in its [K(+)] from 124 to 148mM. Fluid production by Rp-cAMP inhibited tubules was not stimulated by corpora cardiaca extract or cAMP, but [Na(+)] rose to 36mM. Protein phosphorylation plays a role in the regulation of fluid production probably via the apical and basal membrane cation transporters.

Different ouabain sensitivities of Na+/K(+)-ATPase from Poekilocerus bufonius tissues and a possible physiological cost

1. The properties of Na+/K(+)-transporting ATPase in microsomal preparation from mid-gut of the grasshopper, Poekilocerus bufonius, were investigated and compared with the same enzyme from brain and excretory system. 2. Two components of ATPases activity are present in the three tissues studied. 3. The physiochemical properties of Na+/K(+)-transporting ATPase from mid-gut, brain and excretory system (hind-gut plus Malpighian tubules) are essentially the same. 4. The calculated values of PI50 were 2 (I50 = 1 x 10(-2) M), 3.7 (I50 = 2 x 10(-4) M) and 6.4 (I50 = 3.98 x 10(-7)) for Na+/K(+)-ATPase from mid-gut, excretory system and brain, respectively. The mid-gut contains the most ouabain-resistant Na+/K(+)-ATPase. 5. The results suggest that P. bufonius have developed some tolerance to toxic cardiac glycosides (CGS), but there is a possibility of autotoxicity as indicated by the presence of ouabain-sensitive ATPase from brain tissue. 6. It was concluded that the dissimilarities of Na+/K(+)-ATPases from different tissues of P. bufonius are probably due to tissue-dependent differences in ouabain sensitivity (or isoenzymes pattern) available in the same insect. 7. The atrophy of female flight muscle of P. bufonius suggests the possibility of physiological cost inflicted on insects consuming poisonous plants.

Correlation between the lipophilicity of substituted phenols and their inhibition of the Na+/K+-ATPase of Chinese hamster ovary cells

The Na+/K+-ATPase of Chinese Hamster Ovary (CHO) cells, a plasma membrane bound protein was used as a test system to evaluate the toxicity of several phenol derivatives on membranes. Taking only 2 physico-chemical parameters into consideration, viz., the logarithm of the octanol/water partition coefficient as an indicator for the lipophilicity and the sigma-Hammett constant as a measure for the polarity of the phenol substitutes, it was possible to predict the toxicity with high significance. A multivariate regression analysis calculated a correlation coefficient of 0.99. The results confirm studies performed in our laboratory on cytotoxicity and on functional membrane proteins of fungal and mammalian cells [1,2], suggesting a common mechanism of toxicity by the action of hydrophobic xenobiotics on biomembranes. Taking into account the different sensitivities of the test systems, Quantitative Structure-Activity Relationship (QSAR) analyses could help to explain the basic toxicity of several classes of environmental chemicals.

Binding of ouabain to normal and tumoral pancreatic islet cells

The binding of [3H(G)]ouabain was examined in both rat pancreatic islets and tumoral islet cells (RINm5F line). In the islets, the specific binding was considered too low to allow for extensive investigations, taking into account the limited number of islets readily available. In the RINm5F cells, which bound more ouabain than normal islet cells, the time course for binding, its temperature dependency, its competitive inhibition by K+, the number of binding sites, their affinity, and the turnover of ATP per binding site were all similar to those found in other cell types. An incomplete dissociation of [3H(G)]ouabain by unlabelled glycoside and the stimulation of binding by D-glucose were noticed, both phenomena being possibly attributable, in part at least, to the functional response of the RINm5F cells to D-glucose with resulting activation of exocytosis-endocytosis coupling.