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

Intracellular Na+, K+ and Cl- activities in Acheta domesticus Malpighian tubules and the response to a diuretic kinin neuropeptide

The mechanism of primary urine production and the activity of a diuretic kinin, Achdo-KII, were investigated in malpighian tubules of Acheta domesticus by measuring intracellular Na(+), K(+) and Cl(-) activities, basolateral membrane voltage (V(b)), fluid secretion and transepithelial ion transport. Calculated electrochemical gradients for K(+) and Cl(-) across the basolateral membrane show they are actively transported into principal cells, and basolateral Ba(2+)-sensitive K(+) channels do not contribute to net transepithelial K(+) transport and fluid secretion. A basolateral Cl(-) conductance was revealed after the blockade of K(+) channels with Ba(2+), and a current carried by the passive outward movement of Cl(-) accounts for the hyperpolarization of V(b) in response to Ba(2+). Ion uptake via Na(+)/K(+)/2Cl(-) cotransport, driven by the inwardly directed Na(+) electrochemical gradient, is thermodynamically feasible, and is consistent with the actions of bumetanide, which reduces fluid secretion and both Na(+) and K(+) transport. The Na(+) gradient is maintained by its extrusion across the apical membrane and by a basolateral ouabain-sensitive Na(+)/K(+)-ATPase. Achdo-KII has no significant effect on the intracellular ion activities or V(b). Electrochemical gradients across the apical membrane were estimated from previously published values for the levels of Na(+), K(+) and Cl(-) in the secreted fluid. The electrochemical gradient for Cl(-) favours passive movement into the lumen, but falls towards zero after stimulation by Achdo-KII. This coincides with a twofold increase in Cl(-) transport, which is attributed to the opening of an apical Cl(-) conductance, which depolarises the apical membrane voltage.

X-ray microanalysis of Rb+ entry into cricket Malpighian tubule cells via putative K+ channels

Elemental imaging by X-ray microanalysis of fully frozen-hydrated samples shows that when Malpighian tubules of the black field cricket(Teleogryllus oceanicus L.) are incubated in saline in which Rb+ has been substituted for K+, Rb+ replaces much of the cellular K+ in the main segment of control Malpighian tubules and this is prevented by incubation in saline containing Ba2+, a non-selective K+ channel blocker. Similarly the amount of cellular K+ is greatly reduced when tubules incubated in normal, i.e. K+ containing, saline are exposed to Ba2+. By considering the amounts of cellular K and Rb remaining in the main segments of tubules incubated in either K+ containing saline or Rb+ containing saline after Ba+ treatment, it is suggested that: (a) a major part (56%) of cellular K+ enters by Ba2+ sensitive K+ channels and that Rb+ can also enter by this route; (b) a smaller fraction (26%) of cellular K+ enters by a previously proposed Na+/K+/2Cl– co-transporter, which can also transport Rb+; (c) a previously proposed Na+/K+-ATPase is responsible for maintaining a K+ pool (18%) in the tubule cells that is not exchangeable by Rb+; and (d) entry by this Na+/K+-ATPase is not available to Rb+.

Insights into the Malpighian tubule from functional genomics

Classical physiological study of the Malpighian tubule has led to a detailed understanding of fluid transport and its control across several species. With the sequencing of the Drosophila genome, and the concurrent development of post-genomic technologies such as microarrays,proteomics, metabolomics and systems biology, completely unexpected roles for the insect Malpighian tubule have emerged. As the insect body plan is simpler than that of mammals, tasks analogous to those performed by multiple mammalian organ systems must be shared out among insect tissues. As well as the classical roles in osmoregulation, the Malpighian tubule is highly specialized for organic solute transport, and for metabolism and detoxification. In Drosophila, the adult Malpighian tubule is the key tissue for defence against insecticides such as DDT; and it can also detect and mount an autonomous defence against bacterial invasion. While it is vital to continue to set insights obtained in Drosophila into the context of work in other species, the combination of post-genomic technologies and physiological validation can provide insights that might not otherwise have been apparent for many years.

The cation-chloride cotransporter, masBSC, is widely expressed in Manduca sexta tissues

Cation-chloride cotransporters, including the Na-K-Cl cotransporter, play an important role in epithelial ion transport in insects. We have determined the tissue distribution of Manduca sexta bumetanide sensitive cotransporter (masBSC), a putative Na-K-Cl cotransporter that was originally cloned from M. sexta Malpighian tubules. We developed a polyclonal antibody (M6) against a C-terminal fragment of masBSC. masBSC protein was detected by M6 at an apparent molecular mass of approximately 220kDa in M. sexta foregut, midgut, hindgut, Malpighian tubule, salivary gland, fat body, trachea, and nerve cord. Higher expression was observed in the foregut than in other tissues. M6 stained the apical membrane of midgut epithelial cells in cross-sections of third instar larvae. The transcript of masBSC was detected by RT-PCR in midgut, Malpighian tubule, hindgut, trachea, nerve cord, and fat bodies. Taken together, these findings demonstrate that masBSC is widely expressed in M. sexta. While the specific function of masBSC remains unknown, the wide distribution indicates a role of masBSC in a broad range of tissues.

Na+ competes with K+ in bumetanide-sensitive transport by Malpighian tubules ofRhodnius prolixus

We examined the effects of bathing saline Na+/K+ratio, bumetanide and hydrochlorothiazide on fluid and ion transport by serotonin-stimulated Malpighian tubules of Rhodnius prolixus. Previous pharmacological and electrophysiological studies indicate that a bumetanide-sensitive Na+/K+/2Cl–cotransporter is the primary route for basolateral ion entry into the cell during fluid secretion. The goal of this study was to resolve the apparent conflict between relatively high secretion rates by tubules bathed in K+-free saline and the evidence that Na+/K+/2Cl– cotransporters described in other systems have an absolute requirement for all three ions for translocation. Our measurements of fluid secretion rate, ion fluxes and electrophysiological responses to serotonin show that fluid secretion in K+-free saline is bumetanide sensitive and hydrochlorothiazide insensitive. Dose–response curves of secretion rate versusbumetanide concentration were identical for tubules bathed in K+-free and control saline with IC50 values of 2.6×10–6 mmol l–1 and 2.9×10–6 mmol l–1, respectively. Double-reciprocal plots of K+ flux versus bathing saline K+ concentration showed that increasing Na+concentration in the bathing fluid increased Kt but had no effect on Jmax, consistent with competitive inhibition of K+ transport by Na+. We propose that the competition between Na+ and K+ for transport by the bumetanide-sensitive transporter is part of an autonomous mechanism by which Malpighian tubules regulate haemolymph K+ concentration.

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.

Mechanisms of K+ transport across basolateral membranes of principal cells in Malpighian tubules of the yellow fever mosquito, Aedes aegypti

The mechanisms of K+ entry from the hemolymph into principal cells of Malpighian tubules were investigated in the yellow fever mosquito, Aedes aegypti. The K+ channel blocker Ba2+ (5 mmol l–1) significantly decreased transepithelial (TEP) fluid secretion (Vs) from 0.84 nl min–1 to 0.37 nl min–1 and decreased the K+ concentration in secreted fluid from 119.0 mmol l–1 to 54.3 mmol l–1 with no change in the Cl– concentration. Even though the Na+ concentration increased significantly from 116.8 mmol l–1 to 144.6 mmol l–1, rates of TEP ion secretion significantly decreased for all three ions. In addition,Ba2+ had the following significant electrophysiological effects: it depolarized the TEP voltage (Vt) from 19.4 mV to 17.2 mV,increased the TEP resistance (Rt) from 6.4 kΩcm to 6.9 kΩcm, hyperpolarized the basolateral membrane voltage of principal cells (Vbl) from –75.2 mV to –88.2 mV and increased the cell input resistance from 363.7 kΩ to 516.3 kΩ. These effects of Ba2+ reflect the block of K+ channels that, apparently, are also permeable to Na+. Bumetanide (100μmol l–1) had no effect on TEP fluid secretion and electrical resistance but significantly decreased TEP K+ secretion,consistent with the inhibition of electroneutral Na+/K+/2Cl– cotransport. TEP Na+ secretion significantly increased because other Na+entry pathways remained active. Bumetanide plus Ba2+ completely inhibited TEP electrolyte and fluid secretion, with fast and slow kinetics reflecting the Ba2+ block of basolateral membrane K+channels and the inhibition of Na+/K+/2Cl– cotransport, respectively. The single and combined effects of Ba2+ and bumetanide suggest that(1) K+ channels and Na+/K+/2Cl– cotransport are the primary mechanisms for bringing K+ into cells, (2) K+ channels mediate a significant Na+ influx, (3) Na+ has as many as four entry pathways and (4) the mechanisms of TEP K+ and Na+ secretion are coupled such that complete block of TEP K+ renders the epithelium unable to secrete Na+.

Environmental suppression of Neurospora crassa cot-1 hyperbranching: a link between COT1 kinase and stress sensing

cot-1 mutants belong to a class of Neurospora crassa colonial temperature-sensitive (cot) mutants that exhibit abnormal polar extension and branching patterns when grown at restrictive temperatures. cot-1 encodes a Ser/Thr protein kinase that is structurally related to the human myotonic dystrophy kinase which, when impaired, confers a disease that involves changes in cytoarchitecture and ion homeostasis. When grown under restrictive conditions, cot-1 cultures exhibited enhanced medium acidification rates, increased relative abundance of sodium, and increased intracellular glycerol content, indicating an ion homeostasis defect in a hyperbranching mutant. The application of ion transport blockers led to only mild suppression of the cot-1 phenotype. The presence of increased medium NaCl or sorbitol, H(2)O(2), or ethanol levels significantly suppressed the cot-1 phenotype, restored ion homeostasis, and was accompanied by reduced levels of cyclic AMP-dependent protein kinase (PKA) activity. The cot-1 phenotype could also be partially suppressed by direct inhibition of PKA with KT-5720. A reduced availability of fermentable carbon sources also had a suppressive effect on the cot-1 phenotype. In contrast to the effect of extragenic ropy suppressors of cot-1, environmental stress-related suppression of cot-1 did not change COT1 polypeptide expression patterns in the mutant. We suggest that COT1 function is linked to environmental stress response signaling and that altering PKA activity bypasses the requirement for fully functional COT1.

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.

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].

Characterization of Rb uptake into Sf9 cells using cation chromatography: evidence for a K-Cl cotransporter

To assess cation-chloride cotransporter activity in Sf9 cells, cation chromatography was used to measure initial uptake rates of Rb. Rb exchanged with cellular K, with 30% of cellular K replaced after a 40 min exposure to Rb. Rb uptake into Sf9 cells was not inhibited by 50 µmol l(-1) ouabain. Rb uptake was approximately 65% inhibited by 250 µmol l(-1) bumetanide added to the assay solution, and was more than 95% inhibited when cells were pre-incubated for 20 min with bumetanide (100 and 1000 µmol l(-1)). Uptake of Rb and Cl followed simple Michaelis-Menten kinetics, with a K(m) for Rb of 17.1+/-2.2 mmol l(-1) and a K(m) for Cl of 93.7+/-5.6 mmol l(-1). Rb uptake was not dependent upon extracellular Na. Two min exposures to solutions with reduced [Na] or [Cl] produced small but significant changes in cellular Na content. We conclude that the primary Rb uptake pathway in Sf9 cells is a K-Cl cotransporter and that cation chromatography can be used to effectively study kinetic parameters of cotransporter function in tissue culture cells. Characterization of baseline cation-chloride cotransporter activity in Sf9 cells strengthens their utility as a tool for expression and characterization of exogenous proteins.

Control of ion and fluid transport by putative second messengers in different segments of the Malpighian tubules of the black field cricket Teleogryllus oceanicus

The differences in second messenger control of secretion were investigated in the distal and main segments of the Malpighian tubules of the black field cricket Teleogryllus oceanicus. Secretion by the main segment was considerably increased by corpora cardiaca extract and db-cAMP. Corpora cardiaca had no effect on secreted fluid composition or intracellular elemental composition but db-cAMP increased Na(+) and Cl(-) transport, as measured by x-ray microanalysis of secreted fluids and cells. Secretion by the main segment was considerably increased by forskolin and by Sp-cAMP. Secretion in the distal segment was abolished by corpora cardiaca extract but was unaffected by db-cAMP and only slightly reduced by 8-bromo-cAMP. However, Sp-cAMP increased secretion but forskolin reduced secretion. The responses of the distal segment suggest the possibility of a multiplicity of controls through different protein kinases and adenylyl cyclases. Secretion rate in the main segment was also increased by cGMP but distal segment secretion was unaffected. Secretion from both segments was increased by 5-HT. In the main segment secretion rate was increased by Ca-ionophore and thapsigargin and decreased by verapamil. This suggests a role for Ca(2+) as a controlling second messenger. In the distal segment only Ca-ionophore had an effect on secretion rate, which was reduced. Secretion rates in both segments were decreased in Ca-free saline. In saline in which Sr(2+) replaced Ca(2+), secretion rate in the main segment was greatly increased whilst that of the distal segment was decreased, suggesting that Sr(2+) could substitute for Ca(2+) as a second messenger.

Effects of inhibitors and specific ion-free salines on segmental fluid secretion by the Malpighian tubules of the black field cricket Teleogryllus oceanicus

The effects of inhibitors and specific ion-free salines on fluid secretion rates in the distal and main segments showed that there were major differences in secretory mechanisms in the two segments. Both main and distal segments of the Malpighian tubules were sensitive to DIDs, SITS and acetazolamide but in different ways. The evidence suggests that the main segment does not contain a Cl(-)/HCO(3)(-) exchanger in the basal membrane, whereas the distal segment may do so. Secretion in both segments was K(+) dependent. Ba(2+) markedly reduced fluid secretion by the main segment and K(+) entry into the cells of the main segment is suggested to be predominantly via K(+) channels. Entry of K(+) may be primarily by other routes, such as Na K ATPase, in the distal segment. In the distal segment secretion was highly Mg(2+) dependent. Both segments were sensitive to amiloride analogs suggesting the presence of apical cation/H(+) exchangers.