Electrophysiological evidence for the presence of an apical H(+)-ATPase in Malpighian tubules of Formica polyctena: intracellular and luminal pH measurements

Electrochemical gradients for Na+, K+,Cl– and H+ across the apical membrane in Malpighian (renal) tubule cells ofRhodnius prolixus

Measurements of intracellular and luminal ion activities as well as membrane potential were used to calculate electrochemical gradients for Cl–, Na+, K+ and H+ across the apical membrane during fluid secretion by Malpighian tubules of Rhodnius prolixus. The results show that the contribution of Na+/H+ and/or K+/H+ exchangers to fluid secretion is feasible both in unstimulated and serotonin-stimulated tubules. Similarly, the electrochemical potential for Cl– is consistent with the passive movement of Cl– from cell to lumen through Cl– channels. The contribution of apical K+:Cl– cotransport and/or paracellular Cl– movement to net transepithelial ion transport is thermodynamically unfeasible. pH in the lumen (pH 6.08±0.1, N=6) was more acid than in the bath (pH 7.25±0.01, N=26) and serotonin stimulation produced a significant increase in lumen pH to 6.32±0.04 (N=5). Intracellular pH was 6.97±0.01 and 6.82±0.04 in unstimulated and serotonin-stimulated tubules, respectively. Lumen pH was altered whereas intracellular pH was tightly regulated during serotonin and bumetanide treatment. Furthermore, DIDS or amiloride treatment did not affect intracellular pH. However, intracellular pH shifted 0.25 pH units more acid in Na+-free saline, suggesting that a Na+-dependent pH regulatory mechanism is at play in steady state pH regulation during fluid secretion by Malpighian tubules of Rhodnius prolixus. The data are consistent with a role for a basolateral Na+/H+ exchanger in intracellular pH regulation during fluid secretion.

Transport mechanisms of diuresis in Malpighian tubules of insects

We have studied Malpighian tubules of Aedes aegypti using a variety of methods: Ramsay fluid secretion assay, electron probe analysis of secreted fluid, in vitro microperfusion and two-electrode voltage clamp. Collectively, these methods have allowed us to elucidate transepithelial transport mechanisms under control conditions and in the presence of diuretic peptides. Mosquito natriuretic peptide (MNP), a corticotropin-releasing factor (CRF)-like diuretic peptide, selectively increases transepithelial secretion of NaCl and water, meeting the NaCl loads of the blood meal. The intracellular messenger of MNP is cAMP, which increases the Na+ conductance and activates the Na+/K+/2Cl- -cotransporter in the basolateral membrane of principal cells. Leucokinin non-selectively increases transepithelial NaCl and KCl secretion, which may deal with hemolymph volume expansions or reduce the flight pay load upon eclosion from the aquatic habitat. The non-selective NaCl and KCl diuresis stems from the increase in septate junctional Cl- conductance activated by leucokinin using Ca2+ as second messenger. Fundamental to diuretic mechanisms are powerful epithelial transport mechanisms in the distal segment of the Malpighian tubules, where transepithelial secretion rates can exceed the capacity of mammalian glomerular kidneys in the renal turnover of the extracellular fluid compartment. In conjunction with powerful epithelial transport mechanisms driven by the V-type H+-ATPase, diuretic hormones enable hematophagous and probably also phytophagous insects to deal with enormous dietary loads, thereby contributing to the evolutionary success of insects.

The V-type H(+)-ATPase in Malpighian tubules of Aedes aegypti: localization and activity

The V-type H(+)-ATPase is thought to provide the driving force for transepithelial electrolyte and fluid secretion in Malpighian tubules. To confirm the presence of this proton pump in Malpighian tubules of the yellow fever mosquito Aedes aegypti, we used several antibodies raised against the V-type H(+)-ATPase of Manduca sexta. Western blot analysis confirmed the presence of the V-type H(+)-ATPase in Malpighian tubules of Aedes aegypti. In situ immunostaining identified the V-type H(+)-ATPase at the apical membrane of the mitochondrion-rich brush border of principal cells. The V-type H(+)-ATPase was not found in stellate cells. Measurements of ATPase activity revealed that bafilomycin-sensitive and NO(3)(-)-sensitive ATPase activity accounted for 50-60% of total ATPase activity in crude extracts of Malpighian tubules. No significant ouabain- or vanadate-sensitive Na(+)/K(+)-ATPase activity was detected. These results support the conclusion reached previously in electrophysiological studies that the mechanisms for transepithelial electrolyte secretion in the Aedes Malpighian tubules rely on the V-type H(+)-ATPase as the principal energizer of epithelial transport. Measures of transepithelial Na(+) and K(+) secretion and estimates of the H(+) flux mediated by the V-type H(+)-ATPase suggest a 1:1 stoichiometry for Na(+)/H(+) and K(+)/H(+) exchange transport across the apical membrane.

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.

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.

Rubidium reduces potassium permeability and fluid secretion in Malpighian tubules of Locusta migratoria, L

The basal membrane potential (V(b)) of Locusta Malpighian tubule cells in control saline results from its relatively high permeability to potassium. In the presence of 1 mM barium added to the control saline V(b) hyperpolarized from a mean resting potential of -72.1 mV to -90.1 mV. On substituting rubidium for potassium in the control saline, V(b) also hyperpolarized to a value of -91.4 mV. Rubidium was also similarly effective in hyperpolarizing the basal membrane even in the presence of control concentrations of potassium in the bathing medium. Substitution of rubidium for potassium also effected a approximately 50% reduction in the rate of fluid secretion. The action of inhibitors on V(b) in the presence of rubidium showed that V(b) under these conditions probably originated from the bafilomycin-sensitive electrogenic potential generated across the apical membrane by a V-type ATPase. The responses of V(b) to potassium, barium and rubidium and their inhibition of fluid secretion suggest the presence of a substantial rubidium-blockable potassium conductance located on the basal membrane of Locusta Malpighian tubule cells.

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].

Cloning of the 16-kDa V-ATPase proteolipid subunit from the red imported fire ant Solenopsis invicta buren (Hymenoptera: Formicidae)

V-ATPases are ubiquitous proton pumps found in eukaryotes, and are important in regulating the pH of cell compartments and in creating membrane potentials. The V-ATPase creates a proton gradient that is used as an energy source for the transport of other ions. The 16-kDa proteolipid is the proton-translocating subunit c of V-ATPases. Using PCR methods, we have cloned the fire ant 16-kDa subunit c, providing the first molecular characterization of this protein in a social insect. Northern blot analysis revealed three possible different transcripts. The presence of V-ATPases in ant Malpighian tubules had been previously demonstrated, where they provide the proton gradient necessary for the excretion of other ions and the formation of primary urine. The 16-kDa proteolipid is highly conserved among insects, and in ants may be important to the critical processes of diuresis and olfaction as a key component of the V-ATPase. Arch.

Target organ specificity of major neuropeptide stimulants in locust excretory systems

The major stimulant of ileal fluid reabsorption in Locusta migratoria and Schistocerca gregaria corpora cardiaca, ion-transport peptide (ITP), had no stimulatory action on fluid secretion by isolated Malpighian tubules of S. gregaria, nor did it have a synergistic or antagonistic effect in combination with locustakinin (Lom-K) or Locusta-diuretic hormone (Locusta-DH). Stimulants of locust Malpighian tubules (Lom-K and Locusta-DH) had no action on either active transport of Cl(−) (measured as short-circuit current, I(sc)) or the rate of fluid reabsorption across S. gregaria ilea and recta in vitro. Thus, hormonal control of these major organs of the excretory system appears to be clearly separated. Lom-K and Locusta-DH acted synergistically to stimulate secretion by S. gregaria Malpighian tubules, and the diuretic response was more rapid than the response of the ileum and rectum to hindgut stimulants. Taken together, these data suggest that, in the initial phase of post-prandial diuresis, urine flow will exceed fluid uptake in the hindgut, thereby allowing excess water to be eliminated.

Intracellular and luminal pH measurements of Malpighian tubules of the mosquito Aedes aegypti: the effects of cAMP

In order to understand the critical role that hydrogen ions play in fluid secretion in Malpighian tubules, intracellular and luminal pH and K+ measurements were performed in isolated Malpighian tubules of the yellow fever mosquito (Aedes aegypti). The intracellular pH was 7.03+/-0.05 (n=15 Malpighian tubules (MT)) and the luminal pH was 7.19+/-0.09 (n=99 MT) when bathed in saline at a pH of 7.0. The lumen potential is positive, thus net proton secretion into the lumen is active. The intracellular and the luminal K+ concentrations were 75+/- 9 mM (n=15) and 102+/-13 mM (n=9 MT) respectively. Cyclic AMP analogues accelerated fluid secretion and at the same time acidified the cell without affecting the luminal pH. Both effects were abolished by an isomer of adenosine-3',5' cyclic monophosphothioate (cAMPS), the Rp-cAMPS, known to inhibit protein kinase A. The results suggest that in the presence of cAMP the properties of the cation/H+ exchanger are affected and that this may be a result of phosphorylation of a Na+/2H+ antiporter located on the apical membrane.

Partial identification of a peptide that stimulates the primary urine production of single isolated Malpighian tubules of the forest ant, Formica polyctena

A peptide was purified from a 10% trifluoroacetic acid (TFA) head/thorax extract of 300,000 ants with high performance liquid chromatography (HPLC). Fluid secretion assay of single isolated Malpighian tubules was used as a bioassay. The purity of F. polyctena diuretic peptide (FopDP) after a two step HPLC protocol was confirmed by means of mass spectrometry and revealed a molecular mass of 7514 daltons. Due to lack of material, no enzymatic digestion could be performed and the sequence of only the first 25 amino acids could be determined: VPKYENCVSEVLPAGDRQRCVKVTC. A computer search of sequence data banks did not reveal any significant similarity between FopDP and other known insect diuretic peptides.FopDP had no effect on the basolateral membrane potential and depolarised the apical membrane potential of the Malpighian tubule cells. This effect as well as the stimulatory effect on the primary urine formation in the Malpighian tubule of the ant, could be mimicked with A23187, a calcium ionophore, and by thapsigargin, an inhibitor of the endoplasmic reticulum calcium ATPase. FopDP did not stimulate the cAMP content. The results suggest that FopDP uses an increase of intracellular calcium as cellular transduction mechanism.

Characterization of ATPases of apical membrane fractions from Locusta migratoria Malpighian tubules

Apical and basal membrane fractions from Locusta Malpighian tubules were prepared and were characterized by marker enzyme analysis. The apical membranes contained an azide- and orthovanadate-insensitive ATPase activity that was inhibited by bafilomycin A1 (IC50 = 0.44 nM) and NEM (IC50 = 2.15 microM), and thus was characterized as putative V-type ATPase. The enzyme was stimulated by a variety of monovalent cations (Tris > K = Na > choline > Li = Rb) maximal stimulation occurring at 30-40 mM. It was also stimulated by a variety of monovalent anions (maximal activation 30-40 mM), but was strongly inhibited by nitrate and thiocyanate. SDS-PAGE separation of proteins present in the various membrane fractions was carried out. The apical membrane fraction alone contained a 28 kDa protein band that bound a monoclonal antibody specific for a 28 kDa peptide which was a component of the V-type ATPase from midgut of Manduca sexta and, in native gels, possessed ATPase activity which was also sensitive to both bafilomycin and NEM but not to azide or orthovanadate. Binding of the fluorescent monoclonal antibody was located at the apical boundary of the tubule cells. It was concluded that a V-type ATPase is present at the apical surface of Locusta Malpighian tubule cells and that it is involved in their secretory functioning.

Carbonic anhydrase supports electrolyte transport in Drosophila Malpighian tubules. Evidence by X-ray microanalysis of cryosections

Electron probe X-ray microanalytical studies on the role of carbonic anhydrase in electrolyte transport in the cells of Drosophila Malpighian tubules indicate that carbonic anhydrase delivers protons and bicarbonate ions to ion transport systems in the cell membrane. After injection and after feeding acetazolamide or hydrochlorothiazide, known inhibitors of carbonic anhydrase, the contents of potassium, magnesium and chloride in the apical cytoplasm and in the cytoplasm close to the basal plasma membrane decreased. We explain our measurements by the hypothesis of a basal Mg-H-antiport system in parallel with Cl-HCO(3)-antiport, inhibitable by DIDS. Zinc is supposed to enters cells and intracellular Zn storage vacuoles by a negatively charged Zn-anion-complex in exchange for HCO(3)(-) ions. This antiport is inhibitable by SITS. The content of the Zn storage vacuoles is acid, as shown by red fluorescence after incubation of Malpighian tubules with acridine orange. Red fluorescence is absent after preincubation in a medium containing an inhibitor of carbonic anhydrase. Carbonic anhydrase was demonstrated cytochemically in the Golgi-ER complex, Golgi vesicles and intercellular space. We suppose that carbonic anhydrase is synthesized and stored in the Golgi-ER-complex from where it is released into the tubule lumen.

Effects of dinitrophenol on active-transport processes and cell membranes in the Malpighian tubule of Formica

In Formica Malpighian tubules KCl secretion is driven by a V-type H+ ATPase in the luminal membrane in parallel with a H+/K+ antiporter. The effect of the protonophore dinitrophenol (DNP) was investigated on the isolated, symmetrically perfused tubule. DNP was applied in two different concentrations: 0.2 mmol/l and 1 mmol/l. The effects were fast and rapidly reversible. The equivalent short-circuit current (Isc) was reduced significantly to respectively 25 +/- 3% Cn = 4) and -3 +/- 7% (n = 11) of the control value when 0.2 mmol/l or 1 mmol/l was added to the bath. When 1 mmol/l DNP was applied the transepithelial resistance (Rte) decreased significantly to 74 +/- 11% of the control value (n = 11), and the luminal over basolateral voltage divider ratio (VDR), providing an estimate of luminal over basolateral membrane resistance, decreased to 37 +/- 12% of the control (n = 6). A concentration of 1 mmol/l DNP was also applied from the lumen. The decrease in Isc was significant, but much less pronounced (74 +/- 5% of control; n = 6) and no significant changes in Rte and VDR were observed. It is argued that, when the concentration in the bath is high enough, DNP may cross the cell and have a protonophoric effect not only on the mitochondria but also across the luminal cell membrane explaining the drop in transepithelial and in relative luminal membrane resistance.(ABSTRACT TRUNCATED AT 250 WORDS)