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

Airway submucosal glands from cystic fibrosis swine suffer from abnormal ion transport across the serous acini, collecting duct, and ciliated duct

The human airway is protected by an efficient innate defense mechanism that requires healthy secretion of airway surface liquid (ASL) to clear pathogens from the lungs. Most of the ASL in the upper airway is secreted by submucosal glands. In cystic fibrosis (CF), the function of airway submucosal glands is abnormal, and these abnormalities are attributed to anomalies in ion transport across the epithelia lining the different sections of the glands that function coordinately to produce the ASL. However, the ion transport properties of most of the anatomical regions of the gland have never been measured, and there is controversy regarding which segments express CFTR. This makes it difficult to determine the glandular abnormalities that may contribute to CF lung disease. Using a noninvasive, extracellular self-referencing ion-selective electrode technique, we characterized ion transport properties in all four segments of submucosal glands from wild-type and CFTR^-/- swine. In wild-type airways, the serous acini, mucus tubules, and collecting ducts secrete Cl^- and Na⁺ into the lumen in response to carbachol and forskolin stimulation. The ciliated duct also transports Cl^- and Na⁺ but in the opposite direction, i.e., reabsorption from the ASL, which may contribute to lowering Na⁺ and Cl^- activities in the secreted fluid. In CFTR^-/- airways, the serous acini, collecting ducts, and ciliated ducts fail to transport ions after forskolin stimulation, resulting in the production of smaller volumes of ASL with normal Cl^-, Na⁺, and K⁺ concentration.

Mechanisms and regulation of chloride transport in the Malpighian tubules of the larval cabbage looper Trichoplusia ni

Malpighian tubules (MTs) and the hindgut together constitute the excretory system of insects. Larvae of lepidopterans (butterflies and moths) demonstrate the so-called cryptonephric arrangement, where the distal blind end of each MT is embedded into the rectal complex. The rest of the free tubule is modified into several distinct regions that differ greatly in the transport of cations and water. However, relatively little is known about the transport of counter-anions (e.g., Cl^- and HCO₃^-) by the MTs of lepidopteran larvae. In the current study we used ion-selective microelectrodes to characterize Cl^- transport in the distinct regions of the free MT of the larval Trichoplusia ni. Firstly, we note that Cl^- transport in the MTs is sensitive to the Cl^- concentration of the bathing saline, and several regions of the MTs are capable of either secreting or reabsorbing Cl^-. In the distal ileac plexus (DIP), a region previously characterized by cellular heterogeneity and its ability to switch between cation secretion and reabsorption, principal cells (PCs) toggled between Cl^- reabsorption (in high-Cl^- saline) and Cl^- secretion (in low-Cl^- saline). In contrast, secondary cells (SCs) in the DIP secreted Cl^- regardless of saline Cl^- concentration. Mechanistically, we have detected a number of 'leak' and ligand-gated Cl^- channels (ClC) and demonstrated that Cl^- channels are involved in Cl^- secretion. Additionally, we demonstrated that the lumen-positive transepithelial potential increased in response to glycine. Using the scanning ion-selective electrode technique we demonstrated that glycine stimulated Cl^- secretion by SCs, but not by PCs. In contrast, when MTs were deprived of glycine, a decrease in Cl^- secretion, coupled with a decrease in the TEP, was observed. In contrast to the effects of glycine, an active dose of helicokinin reduced Cl^- secretion by PCs, but not by SCs. Lastly, we detected expression of chloride-bicarbonate exchangers (CBE) in all regions of the free tubule. Scans of H⁺ transport across the tubule indicated that base equivalents are likely reabsorbed across the ileac plexus. Blocking ClC or CBE led to secretion of a more basic fluid, indicating lack of base reabsorption. We suggest that the transport of Cl^- in the MTs of larval lepidopterans (i) may be correlated with the reabsorption of base, (ii) may be sensitive to Cl^- concentration in the haemolymph, and (iii) could be regulated by helicokinin and glycine.

Helicokinin alters ion transport in the secondary cell-containing region of the Malpighian tubule of the larval cabbage looper Trichoplusia ni

Excretion in insects is accomplished by the combined actions of the Malpighian tubules (MTs) and hindgut, which together form the functional kidney. MTs of many insect groups consist of principal cells (PC) and secondary cells (SC). In most insect groups SCs are reported to secrete ions from haemolymph into the tubule lumen. Paradoxically, SCs in the MTs of the lepidopteran cabbage looper T. ni are used to reabsorb Na⁺ and K⁺ back into haemolymph. The current study was designed to investigate the effects and mode of action of the lepidopteran kinin, Helicokinin (HK), on ion transport in the SC-containing region of MT of T. ni. We identified a HK receptor (HK-R) homologue in T. ni and detected its expression in the SC-containing region of the MTs. The mRNA abundance of hk-r altered in response to changes in dietary K⁺ and Na⁺ content. HK-R immunolocalized to both PCs and SCs. Ramsay assays of preparations of the isolated distal ileac plexus (DIP) indicated that [HK] = 10^-8 M: (i) decreased fluid secretion rate in unstimulated and serotonin-stimulated preparations, and (ii) increased [Na⁺]/[K⁺] ratio in the secreted fluid. Scanning ion-selective electrode technique measurements revealed that HK reduced: (i) K⁺ secretion by the PCs, and (ii) Na⁺ reabsorption by the SCs in intact tubules. In vitro incubation of the DIP with HK resulted in reduced mRNA abundance of hk-r as well as Na⁺/K⁺-ATPase subunit α (NKAα), Na⁺/K⁺/Cl^- co-transporter (nkcc), Na⁺/H⁺ exchangers (nhe) 7 and 8, and aquaporin (aqp) 1. Taken together, results of the current study suggest that HK is capable of altering fluid secretion rate and [Na⁺]/[K⁺] ratio of the fluid, and that HK targets both PCs and SCs in the DIP of T. ni.

Molecular mechanisms of bi-directional ion transport in the Malpighian tubules of a lepidopteran crop pest, Trichoplusia ni

Classical studies have described in detail the complex and regionalized morphology of the Malpighian tubule (MT) in larval Lepidoptera. Recent studies revealed unusual aspects of ion transport in the Malpighian tubules of the larva of the cabbage looper, Trichoplusia ni. These included: cation reabsorption via secondary cells (SC); coupling of SCs to neighbouring PCs via gap junctions to enable reabsorption; and a reversal from cation secretion to reabsorption by the principal cells in the distal ileac plexus region of the in situ tubule in response to dietary ion loading. The current paper aimed to identify molecular components of ion transport in the MTs of T. ni and to describe their role in the recently reported reversal of ion transport in ion-loaded animals. Using a combination of molecular, immunohistochemical and electrophysiological techniques, we assigned roles to Na⁺/K⁺-ATPase (NKA), V-type H⁺-ATPase (VA), Na⁺/K⁺/Cl^- co-transporter (NKCC), K⁺/Cl^- co-transporter (KCC), inward-rectifying K⁺ channel (Kir), and Na⁺/H⁺ exchangers (NHE)-7 and -8 in the transport of Na⁺ and K⁺ by the distal ileac plexus of T. ni. The yellow region of the tubule lacked all of the above except VA, and the white region lacked all of the above transporters but expressed an amiloride-sensitive Na⁺ channel (NaC). Overall, the ion transport machinery in the distal ileac plexus of the T. ni tubule shows remarkable similarity to that in tubules of other groups of insects, yet this region transports ions very differently. Shutdown of secretory ATPases and utilisation of the same molecular machinery in the face of changing ion gradients may enable ion transport reversal in lepidopteran MTs. We propose that gap junction-based coupling of the two cell types likely aids in toggling between ion secretion and ion reabsorption in this segment.

The gastric caecum of larval Aedes aegypti: stimulation of epithelial ion transport by 5-hydroxytryptamine and cAMP

We report measurements of ion transport across the gastric caecum of larvae of Aedes aegypti, a vector of yellow fever that inhabits a variety of aquatic habitats ranging from freshwater to brackish water. We provide the first measurements of the effect of 5-hydroxytryptamine (5-HT) on transepithelial potential (TEP), luminal ion concentrations and electrochemical potentials, as well as basolateral membrane potential and H⁺, Na⁺ and K⁺ fluxes. TEP, basolateral membrane potential, and H⁺, K⁺ and Na⁺ fluxes across the gastric caeca declined within 3-6 min after isolation of the entire midgut from the larva. 5-HT restored both the TEP and active accumulation of H⁺, K⁺ and Na⁺ in the lumen. Additionally, 5-HT restored H⁺, K⁺ and Na⁺ fluxes across the distal caecum of freshwater larvae, and restored H⁺ fluxes across the distal caecum of brackish water larvae. There was no effect of 5-HT on ion fluxes across the proximal caecum. We have also shown that 5-HT restores the basolateral membrane potential in cells of the distal, but not proximal, caecum. Effects of 5-HT on TEP and basolateral membrane potential were mimicked by application of cAMP but not by a phorbol ester. We provide a working model which proposes that 5-HT and cAMP stimulate the vacuolar H⁺-ATPase of the distal caecum. Our results provide evidence that the gastric caecum is functionally distinct from the adjacent anterior midgut and we discuss possible roles of the gastric caecum in osmoregulation. We also describe similarities in the arrangement of ion transporters in the caecum with those of the Malpighian tubules.

Effects of cold-acclimation on gene expression in Fall field cricket (Gryllus pennsylvanicus) ionoregulatory tissues

Background

Cold tolerance is a key determinant of temperate insect distribution and performance. Chill-susceptible insects lose ion and water homeostasis during cold exposure, but prior cold acclimation improves both cold tolerance and defense of homeostasis. The mechanisms underlying these processes are mostly unknown; cold acclimation is thought to enhance ion transport in the cold and/or prevent leak of water and ions. To identify candidate mechanisms of cold tolerance plasticity we generated transcriptomes of ionoregulatory tissues (hindgut and Malpighian tubules) from Gryllus pennsylvanicus crickets and compared gene expression in warm- and cold-acclimated individuals.

Results

We assembled a G. pennsylvanicus transcriptome de novo from 286 million 50-bp reads, yielding 70,037 contigs (~44% of which had putative BLAST identities). We compared the transcriptomes of warm- and cold-acclimated hindguts and Malpighian tubules. Cold acclimation led to a ≥ 2-fold change in the expression of 1493 hindgut genes (733 downregulated, 760 upregulated) and 2008 Malpighian tubule genes (1009 downregulated, 999 upregulated). Cold-acclimated crickets had altered expression of genes putatively associated with ion and water balance, including: a downregulation of V-ATPase and carbonic anhydrase in the Malpighian tubules and an upregulation of Na⁺-K⁺ ATPase in the hindgut. We also observed acclimation-related shifts in the expression of cytoskeletal genes in the hindgut, including actin and actin-anchoring/stabilizing proteins, tubulin, α-actinin, and genes involved in adherens junctions organization. In both tissues, cold acclimation led to differential expression of genes encoding cytochrome P450s, glutathione-S-transferases, apoptosis factors, DNA repair, and heat shock proteins.

Conclusions

This is the first G. pennsylvanicus transcriptome, and our tissue-specific approach yielded new candidate mechanisms of cold tolerance plasticity. Cold acclimation may reduce loss of hemolymph volume in the cold by 1) decreasing primary urine production via reduced expression of carbonic anhydrase and V-ATPase in the Malpighian tubules and 2) by increasing Na⁺ (and therefore water) reabsorption across the hindgut via increase in Na⁺-K⁺ ATPase expression. Cold acclimation may reduce chilling injury by remodeling and stabilizing the hindgut epithelial cytoskeleton and cell-to-cell junctions, and by increasing the expression of genes involved in DNA repair, detoxification, and protein chaperones.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3711-9) contains supplementary material, which is available to authorized users.

Effects of rearing salinity on expression and function of ion motive ATPases and ion transport across the gastric caecum of Aedes aegypti larvae

Larvae of Aedes aegypti, the yellow fever vector, inhabit a variety of aquatic habitats ranging from fresh water to brackish water. This study focuses on the gastric caecum of the larvae, an organ that has not been widely studied. We provide the first measurements of H+, K+, and Na+ fluxes at the distal and proximal gastric caecum, and have shown that they differ in the two regions, consistent with previously reported regionalization of ion transporters. Moreover we have shown that the regionalization of vacuolar H+-ATPase and Na+/K+ -ATPase is altered when larvae are reared in brackish water (30% seawater) relative to fresh water. Measurements of luminal Na+ and K+ concentrations also show a 5-fold increase in Na+/K+ ratio in the caecal lumen in larvae reared in brackish water relative to fresh water, whereas transepithelial potential and luminal pH were unchanged. Calculated electrochemical potentials reveal changes in the active accumulation of Na+ and K+ in the lumen of the gastric caecum of fresh water versus brackish water larvae. Together with the results of previous studies of the larval midgut, our results show that the caecum is functionally distinct from the adjacent anterior midgut, and may play an important role in osmoregulation as well as uptake of nutrients.

Serotonin triggers cAMP and PKA-mediated intracellular calcium waves in Malpighian tubules of Rhodnius prolixus

Rhodnius prolixus is a hematophagous insect vector of Chagas disease capable of ingesting up to 10 times its unfed body weight in blood in a single meal. The excess water and ions ingested with the meal are expelled through a rapid postprandial diuresis driven by the Malpighian tubules. Diuresis is triggered by at least two diuretic hormones, a CRF-related peptide and serotonin, which were traditionally believed to trigger cAMP as an intracellular second messenger. Recently, calcium has been suggested to act as a second messenger in serotonin-stimulated Malpighian tubules. Thus, we tested the role of calcium in serotonin-stimulated Malpighian tubules from R. prolixus. Our results show that serotonin triggers cAMP-mediated intracellular Ca(2+) waves that were blocked by incubation in Ca(2+)-free saline containing the cell membrane-permeant Ca(2+) chelator BAPTA-AM, or the PKA blocker H-89. Treatment with 8-Br-cAMP triggered Ca(2+) waves that were blocked by H-89 and BAPTA-AM. Analysis of the secreted fluid in BAPTA-AM-treated tubules showed a 75% reduction in fluid secretion rate with increased K(+) concentration, reduced Na(+) concentration. Taken together, the results indicate that serotonin triggers cAMP and PKA-mediated Ca(2+) waves that are required for maximal ion transport rate.

Investigations of the signaling cascade involved in diuretic hormone stimulation of Malpighian tubule fluid secretion in Rhodnius prolixus

In insects, the excretory system is comprised of the Malpighian tubules (MTs) and the hindgut, which collectively function to maintain ionic and osmotic balance of the haemolymph and rid the organism of toxic compounds or elements in excess. Secretion by the Malpighian tubules of insects is regulated by a variety of hormones including peptidergic factors as well as biogenic amines. In Rhodnius prolixus, two endogenous diuretic hormones have been identified; the biogenic amine serotonin (5-hydroxytryptamine, 5-HT) and the corticotropin releasing factor-related peptide, RhoprCRF. Both factors significantly increase secretion by MTs and are known to elevate intracellular levels of cAMP. Interestingly, applying sub-maximal doses of these two diuretic factors in combination on isolated MTs in vitro reveals synergistic effects as rates of fluid secretion are significantly higher than would be expected if rates of secretion from MTs treated with each factor alone were summed. This observed synergism suggests that different downstream targets may be activated by the two diuretic factors, but that some cellular elicitors may be shared since cAMP is elevated in response to either diuretic hormone. This study investigated the signaling cascade involved in the diuretic hormone regulation of Malpighian tubule fluid secretion. Bioassays were performed in physiological as well as modified salines (e.g. calcium-free) alone or in the presence of a variety of pharmacological compounds that interfere with prospective intracellular targets, such as the apical cation/H(+) exchanger. Intriguingly, only amiloride yielded differential effects on the two diuretics with 5HT-stimulated secretion being blocked, whereas in contrast, RhoprCRF-stimulated secretion was unaffected. In addition, experiments examining the role of extracellular and intracellular calcium on fluid secretion rate showed that both diuretics are dependent on intracellular calcium availability. Finally, fluid secretion stimulated by either diuretic hormone was also sensitive to inhibition of cAMP-dependent protein kinase A. Taken together, these results suggest that each diuretic hormone activates pathways dependent upon intracellular calcium and cAMP.

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.

The Drosophila NKCC Ncc69 is required for normal renal tubule function

Epithelial ion transport is essential to renal homeostatic function, and it is dysregulated in several diseases, such as hypertension. An understanding of the insect renal (Malpighian) tubule yields insights into conserved epithelial ion transport processes in higher organisms and also has implications for the control of insect infectious disease vectors. Here, we examine the role of the Na(+)-K(+)-2Cl(-) (NKCC) cotransporter Ncc69 in Drosophila tubule function. Ncc69 mutant tubules have decreased rates of fluid secretion and K(+) flux, and these phenotypes were rescued by expression of wild-type Ncc69 in the principal cells of the tubule. Na(+) flux was unaltered in Ncc69 mutants, suggesting Na(+) recycling across the basolateral membrane. In unstimulated tubules, the principal role of the Na(+)-K(+)-ATPase is to generate a favorable electrochemical gradient for Ncc69 activity: while the Na(+)-K(+)-ATPase inhibitor ouabain decreased K(+) flux in wild-type tubules, it had no effect in Ncc69 mutant tubules. However, in the presence of cAMP, which stimulates diuresis, additional Na(+)-K(+)-ATPase-dependent K(+) transport pathways are recruited. In studying the effects of capa-1 on wild-type and Ncc69 mutant tubules, we found a novel antidiuretic role for this hormone that is dependent on intact Ncc69, as it was abolished in Ncc69 mutant tubules. Thus, Ncc69 plays an important role in transepithelial ion and fluid transport in the fly renal tubule and is a target for regulation in antidiuretic states.

Recent experiments towards a model for fluid secretion in Rhodnius Upper Malpighian Tubules (UMT)

Three different methods have been used to improve a model for fluid secretion in Upper Malpighian Tubules (UMT) of the blood sucking insect Rhodnius prolixus. (I) In the first, UMT double perfusions in 5th instar Rhodnius were used to measure their fluid secretion rate. They were stimulated to secrete with 5-HT. Double perfusions allowed access separately to the basolateral and the apical cell membranes with pharmacological agents known to block different ion transport functions, namely ATPases, cotransporters and/or countertransporters and ion and water channels: ouabain, bafilomycin A1, furosemide, bumetanide, SITS, acetazolamide, amiloride, DPC, BaCl(2), pCMBS and DTT. The basic assumption is that changes in water movement reflect changes in ion transport mechanisms. (II) Intracellular Na(+) concentrations were measured with a fluorometric method in dissected R. prolixus UMT, under several experimental conditions. (III) ATPase activities were measured in R. prolixus UMT. A tentative model for the function of the UMT cell is presented. We find that (a) at the basolateral cell membrane, fundamental is a Na(+)-K(+)-2Cl(-) cotransporter; of intermediate importance are the Na(+)-K(+)-ATPase and a ouabain-insensitive Na(+)-ATPase, ion channels and Rp-MIP water channels. (b) At the apical cell membrane, most important are a V-H(+)-ATPase; and a K(+) and/or Na(+)-H(+) exchanger.

Natriuresis and diuretic hormone synergism in R. prolixus upper Malpighian tubules is inhibited by the anti-diuretic hormone, RhoprCAPA-α2

Insects contain an array of hormones that coordinate the actions of the excretory system to achieve osmotic and ionic balance. In the hematophagous insect, Rhodnius prolixus, two diuretic hormones have been identified, serotonin (5HT) and a corticotropin releasing factor-related peptide (RhoprDH), and both lead to an increase in fluid secretion by Malpighian tubules (MTs). However, only 5HT activates reabsorption by the lower MTs to recover K(+) and Cl(-). An anti-diuretic hormone (RhoprCAPA-α2) is believed to coordinate the cessation of the rapid diuresis following blood meal engorgement. However, the role of RhoprCAPA-α2 on fluid secretion by MTs stimulated by RhoprDH was previously unknown. Here we demonstrate that, unlike the inhibitory effect on 5HT-stimulated secretion by MTs, RhoprCAPA-α2 does not inhibit secretion stimulated by RhoprDH although it does abolish the synergism that occurs between the two diuretic hormones. In addition, we show that the natriuresis elicited by either diuretic hormone is reduced by RhoprCAPA-α2. Using electrophysiological tools, we investigate the possible mechanism by which this complex regulatory pathway is achieved. Analysis of the pH of secreted fluid as well as the triphasic response in transepithelial potential in MTs treated with diuretic hormones, suggests that RhoprCAPA-α2 does not inhibit the V-type H(+) ATPase. Taken together, these results indicate that RhoprCAPA-α2 functions to reduce the rapid diuresis following blood feeding, and in addition, it inhibits the natriuresis associated with diuretic hormone stimulated MTs. This may reflect an important regulatory mechanism related to the slow diuresis that occurs as the K(+)-rich blood cells are digested.

Role of an apical K,Cl cotransporter in urine formation by renal tubules of the yellow fever mosquito (Aedes aegypti)

The K,Cl cotransporters (KCCs) of the SLC12 superfamily play critical roles in the regulation of cell volume, concentrations of intracellular Cl(-), and epithelial transport in vertebrate tissues. To date, the role(s) of KCCs in the renal functions of mosquitoes and other insects is less clear. In the present study, we sought molecular and functional evidence for the presence of a KCC in renal (Malpighian) tubules of the mosquito Aedes aegypti. Using RT-PCR on Aedes Malpighian tubules, we identified five alternatively spliced partial cDNAs that encode putative SLC12-like KCCs. The majority transcript is AeKCC1-A(1); its full-length cDNA was cloned. After expression of the AeKCC1-A protein in Xenopus oocytes, the Cl(-)-dependent uptake of (86)Rb(+) is 1) activated by 1 mM N-ethylmaleimide and cell swelling, 2) blocked by 100 μM dihydroindenyloxyalkanoic acid (DIOA), and 3) dependent upon N-glycosylation of AeKCC1-A. In Aedes Malpighian tubules, AeKCC1 immunoreactivity localizes to the apical brush border of principal cells, which are the predominant cell type in the epithelium. In vitro physiological assays of Malpighian tubules show that peritubular DIOA (10 μM): 1) significantly reduces both the control and diuretic rates of transepithelial fluid secretion and 2) has negligible effects on the membrane voltage and input resistance of principal cells. Taken together, the above observations indicate the presence of a KCC in the apical membrane of principal cells where it participates in a major electroneutral transport pathway for the transepithelial secretion of fluid in this highly electrogenic epithelium.

A SLC4-like anion exchanger from renal tubules of the mosquito (Aedes aegypti): evidence for a novel role of stellate cells in diuretic fluid secretion

Transepithelial fluid secretion across the renal (Malpighian) tubule epithelium of the mosquito (Aedes aegypti) is energized by the vacuolar-type (V-type) H(+)-ATPase and not the Na(+)-K(+)-ATPase. Located at the apical membrane of principal cells, the V-type H(+)-ATPase translocates protons from the cytoplasm to the tubule lumen. Secreted protons are likely to derive from metabolic H(2)CO(3), which raises questions about the handling of HCO(3)(-) by principal cells. Accordingly, we tested the hypothesis that a Cl/HCO(3) anion exchanger (AE) related to the solute-linked carrier 4 (SLC4) superfamily mediates the extrusion of HCO(3)(-) across the basal membrane of principal cells. We began by cloning from Aedes Malpighian tubules a full-length cDNA encoding an SLC4-like AE, termed AeAE. When expressed heterologously in Xenopus oocytes, AeAE is both N- and O-glycosylated and mediates Na(+)-independent intracellular pH changes that are sensitive to extracellular Cl(-) concentration and to DIDS. In Aedes Malpighian tubules, AeAE is expressed as two distinct forms: one is O-glycosylated, and the other is N-glycosylated. Significantly, AeAE immunoreactivity localizes to the basal regions of stellate cells but not principal cells. Concentrations of DIDS that inhibit AeAE activity in Xenopus oocytes have no effects on the unstimulated rates of fluid secretion mediated by Malpighian tubules as measured by the Ramsay assay. However, in Malpighian tubules stimulated with kinin or calcitonin-like diuretic peptides, DIDS reduces the diuretic rates of fluid secretion to basal levels. In conclusion, Aedes Malpighian tubules express AeAE in the basal region of stellate cells, where this transporter may participate in producing diuretic rates of transepithelial fluid secretion.

The antidiuretic neurohormone RhoprCAPA-2 downregulates fluid transport across the anterior midgut in the blood-feeding insect Rhodnius prolixus

Osmotic balance in insects is regulated by the excretory system, consisting of Malpighian tubules and the gut under the control of diuretic and antidiuretic factors. Terrestrial insects must conserve water, and antidiuresis is the norm, only interrupted by brief diuretic periods. Surprisingly, little is known about antidiuresis in insects. Two antidiuretic strategies have been described. The first antidiuretic mechanism involves the reabsorption of fluid from the primary urine in the hindgut. More recently, a second antidiuretic strategy was reported, consisting of inhibition of primary urine formation by the Malpighian tubules. Recently, we isolated, characterized, and cloned the gene encoding for the antidiuretic neurohormone (the neuropeptide RhoprCAPA-2) acting on the Malpighian tubules of Rhodnius prolixus. Here we describe a third, novel mechanism central to the antidiuretic strategy of R. prolixus, the inhibition of ion and fluid transport across the anterior midgut by RhoprCAPA-2. Our results show that RhoprCAPA-2 (1 micromol/l) reduces serotonin-stimulated fluid transport from 83 +/- 11 to 12 +/- 12 nl/min and equivalent short-circuit current from 20 +/- 4 to 5 +/- 0.7 microA/cm(2) in diuretic hormone-stimulated anterior midgut. RhoprCAPA-2 appears to function independently of intracellular cGMP or Ca(2+) in the midgut. Thus, the antidiuretic neurohormone RhoprCAPA-2 has multiple target tissues, and we hypothesize that RhoprCAPA-2 functions to coordinate the transport activity of the anterior midgut and Malpighian tubules so that the rate of fluid transport into the haemolymph by the anterior midgut matches the transport rate of Malpighian tubules to maintain the volume and ion composition of haemolymph.

Too much of a good thing: how insects cope with excess ions or toxins in the diet

Much of our understanding of the ionoregulatory and excretory physiology of blood-feeding insects can be traced to a series of papers by Simon Maddrell and colleagues in the 1970s and 1980s. These studies of the Malpighian (renal)tubules of Rhodnius prolixus revealed a number of physiological adaptations to the short-term and long-term stresses associated with blood feeding. More recent electrophysiological studies using voltage- and ion-selective microelectrodes have extended our understanding of the mechanisms and control of ion transport by the secretory and reabsorptive segments of the Rhodnius Malpighian tubule. The discovery that the rates of transport of organic anions, urates and Ca2+ are synchronized to coincide with the appearance of the products of blood meal digestion in the haemolymph of Rhodnius has stimulated parallel studies in Drosophila. This recent research has examined how excretory mechanisms for organic cations and organic anions are altered by exposure to such compounds in the diet. These studies also show that the Drosophila Malpighian tubule provides a useful model for analysis of the roles of transporters such as P-glycoproteins and multidrug resistance-associated proteins in the excretion of toxins.

A K(+)/H (+) P-ATPase transport in the accessory cell membrane of the blowfly taste chemosensilla sustains the transepithelial potential

An electrogenic K(+) transport in the tormogen cell of insect chemosensilla is involved in the generation and maintenance of the transepithelial potential (TEP). To gain more information about the K(+) transport system underlying the TEP generation and the location of its components in the plasma membrane of the tormogen cell, we studied the effects of inhibitors of K(+)/H(+) P-ATPase (bafilomycin A1, omeprazole and Na-orthovanadate), of K(+)/Cl(-) co-transport (bumetanide), of Cl(-) channels (NPPB) and of a K(+) channel blocker (BaCl(2)). The relationship between TEP amplitude and spike firing activity was also studied. Experiments were performed on the labellar chemosensilla of the blowfly Protophormia terraenovae using a modified tip-recording technique. Results show that: (a) K(+)/H(+) P-ATPase inhibitors significantly decrease the TEP, when properly applied to the labellum for 20 min, so as to reach the basolateral side of the plasma membrane, while no effect was detected when applied to the apical side, (b) bumetanide, NPPB and BaCl(2) decrease the TEP value only when administered to the apical side, (c) spike activity is positively correlated with the TEP. A model is proposed of the active and passive K(+) transports sustaining the TEP associated with the blowfly chemosensilla.

Intracellular pH homeostasis and serotonin-induced pH changes inCalliphorasalivary glands: the contribution of V-ATPase and carbonic anhydrase

Blowfly salivary gland cells have a vacuolar-type H+-ATPase(V-ATPase) in their apical membrane that energizes secretion of a KCl-rich saliva upon stimulation with serotonin (5-hydroxytryptamine, 5-HT). We have used BCECF to study microfluometrically whether V-ATPase and carbonic anhydrase (CA) are involved in intracellular pH (pHi) regulation,and we have localized CA activity by histochemistry. We show: (1) mean pHi in salivary gland cells is 7.5±0.3 pH units(N=96), higher than that expected from passive H+distribution; (2) low 5-HT concentrations (0.3–3 nmol l–1) induce a dose-dependent acidification of up to 0.2 pH units, with 5-HT concentrations >10 nmol l–1, causing monophasic or multiphasic pH changes; (3) the acidifying effect of 5-HT is mimicked by bath application of cAMP, forskolin or IBMX; (4) salivary gland cells exhibit CA activity; (5) CA inhibition with acetazolamide and V-ATPase inhibition with concanamycin A lead to a slow acidification of steady-state pHi; (6) 5-HT stimuli in the presence of acetazolamide induce an alkalinization that can be decreased by simultaneous application of the V-ATPase inhibitor concanamycin A; (7) concanamycin A removes alkali-going components from multiphasic 5-HT-induced pH changes; (8) NHE activity and a Cl–-dependent process are involved in generating 5-HT-induced pH changes; (9) the salivary glands probably contain a Na+-driven amino acid transporter. We conclude that V-ATPase and CA contribute to steady-state pHi regulation and 5-HT-induced outward H+pumping does not cause an alkalinization of pHi because of cytosolic H+ accumulation attributable to stimulated cellular respiration and AE activity, masking the alkalizing effect of V-ATPase-mediated acid extrusion.

The endocrine control of salt balance in insects

An overview is given of the role of Malpighian (renal) tubules and the hindgut (ileum and rectum) in the excretory process of insects. The review focuses on the mechanism of primary urine production by Malpighian tubules and its control by neurohormones, which includes serotonin and neuropeptides resembling mammalian corticotropin-releasing factor (CRF) and calcitonin. Particular emphasis is given to in vitro studies of the effect of neurohormones on Malpighian tubule ion transport and a consideration of their likely role in the regulation of salt balance in vivo.

A vacuolar-type H+-ATPase and a Na+/H+exchanger contribute to intracellular pH regulation in cockroach salivary ducts

Cells of the dopaminergically innervated salivary ducts in the cockroach Periplaneta americana have a vacuolar-type H+-ATPase(V-ATPase) of unknown function in their apical membrane. We have studied whether dopamine affects intracellular pH (pHi) in duct cells and whether and to what extent the apical V-ATPase contributes to pHiregulation. pHi measurements with double-barrelled pH-sensitive microelectrodes and the fluorescent dye BCECF have revealed: (1) the steady-state pHi is 7.3±0.1; (2) dopamine induces a dose-dependent acidification up to pH 6.9±0.1 at 1 μmol l–1 dopamine, EC50 at 30 nmol l–1dopamine; (3) V-ATPase inhibition with concanamycin A or Na+-free physiological saline (PS) does not affect the steady-state pHi; (4)concanamycin A, Na+ -free PS and Na+/H+exchange inhibition with 5-(N-ethyl-N-isopropyl)-amiloride(EIPA) each reduce the rate of pHi recovery from a dopamine-induced acidification or an acidification induced by an NH4Cl pulse; (5)pHi recovery after NH4Cl-induced acidification is almost completely blocked by concanamycin A in Na+-free PS or by concanamycin A applied together with EIPA; (6) pHi recovery after dopamine-induced acidification is also completely blocked by concanamycin A in Na+-free PS but only partially blocked by concanamycin A applied together with EIPA. We therefore conclude that the apical V-ATPase and a basolateral Na+/H+ exchange play a minor role in steady-state pHi regulation but contribute both to H+extrusion after an acute dopamine- or NH4Cl-induced acid load.

NHE8 mediates amiloride-sensitive Na+/H+ exchange across mosquito Malpighian tubules and catalyzes Na+ and K+ transport in reconstituted proteoliposomes

Following a blood meal, the mosquito Aedes aegypti will have acquired an enormous sodium load that must be rapidly excreted to restore ion homeostasis. It is a process that demands robust sodium and fluid transport capabilities. Even though the identities of the components involved in this ion transport across the mosquito Malpighian tubule epithelia have not been completely determined, electrophysiological studies suggest the contribution of a Na(+)/H(+) exchanger extruding cations into the lumen driven secondarily by the proton gradient created by the V-type H(+)-ATPase in the tubules' apical membrane. We have identified the putative exchanger and designated it AeNHE8. Immunolocalization studies demonstrated that AeNHE8 is expressed in the apical membranes of Malpighian tubules, gastric caecae, and rectum. When heterologously expressed in salt-sensitive yeast cells lacking Na(+) extrusion and Na(+)/H(+) exchange proteins, AeNHE8 rescues the salt-sensitive phenotype and restores the cells' ability to grow in high NaCl media. Furthermore, heterologous expression of AeNHE8 in NHE-deficient fibroblast cells results in an amiloride-sensitive (22)Na(+) uptake. To determine the exchanger's kinetic properties, we reconstituted membranes from yeast cells expressing the protein into lipid proteoliposomes and assayed for cation-dependent H(+) exchange by fluorimetric methods. Our results indicate that AeNHE8 mediates saturable exchange of Na(+) and K(+) for H(+). We propose that AeNHE8 may be coupled to the inward H(+) gradient across the Malpighian tubules and plays a role in the extrusion of excess sodium and potassium while maintaining steady intracellular pH in the principal cells.