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

The Molecular Physiology and Toxicology of Inward Rectifier Potassium Channels in Insects

Inward rectifier K⁺ (Kir) channels have been studied extensively in mammals, where they play critical roles in health and disease. In insects, Kir channels have recently been found to be key regulators of diverse physiological processes in several tissues. The importance of Kir channels in insects has positioned them to serve as emerging targets for the development of insecticides with novel modes of action. In this article, we provide the first comprehensive review of insect Kir channels, highlighting the rapid progress made in understanding their molecular biology, physiological roles, pharmacology, and toxicology. In addition, we highlight key gaps in our knowledge and suggest directions for future research to advance our understanding of Kir channels and their roles in insect physiology. Further knowledge of their functional roles will also facilitate their exploitation as targets for controlling arthropod pests and vectors of economic, medical, and/or veterinary relevance.

Two inwardly rectifying potassium channels, Irk1 and Irk2, play redundant roles in Drosophila renal tubule function

Inwardly rectifying potassium channels play essential roles in renal physiology across phyla. Barium-sensitive K(+) conductances are found on the basolateral membrane of a variety of insect Malpighian (renal) tubules, including Drosophila melanogaster. We found that barium decreases the lumen-positive transepithelial potential difference in isolated perfused Drosophila tubules and decreases fluid secretion and transepithelial K(+) flux. In those insect species in which it has been studied, transcripts from multiple genes encoding inwardly rectifying K(+) channels are expressed in the renal (Malpighian) tubule. In Drosophila melanogaster, this includes transcripts of the Irk1, Irk2, and Irk3 genes. The role of each of these gene products in renal tubule function is unknown. We found that simultaneous knockdown of Irk1 and Irk2 in the principal cell of the fly tubule decreases transepithelial K(+) flux, with no additive effect of Irk3 knockdown, and decreases barium sensitivity of transepithelial K(+) flux by ∼50%. Knockdown of any of the three inwardly rectifying K(+) channels individually has no effect, nor does knocking down Irk3 simultaneously with Irk1 or Irk2. Irk1/Irk2 principal cell double-knockdown tubules remain sensitive to the kaliuretic effect of cAMP. Inhibition of the Na(+)/K(+)-ATPase with ouabain and Irk1/Irk2 double knockdown have additive effects on K(+) flux, and 75% of transepithelial K(+) transport is due to Irk1/Irk2 or ouabain-sensitive pathways. In conclusion, Irk1 and Irk2 play redundant roles in transepithelial ion transport in the Drosophila melanogaster renal tubule and are additive to Na(+)/K(+)-ATPase-dependent pathways.

Cloning and functional characterization of inward-rectifying potassium (Kir) channels from Malpighian tubules of the mosquito Aedes aegypti

Inward-rectifying K(+) (Kir) channels play critical physiological roles in a variety of vertebrate cells/tissues, including the regulation of membrane potential in nerve and muscle, and the transepithelial transport of ions in osmoregulatory epithelia, such as kidneys and gills. It remains to be determined whether Kir channels play similar physiological roles in insects. In the present study, we sought to 1) clone the cDNAs of Kir channel subunits expressed in the renal (Malpighian) tubules of the mosquito Aedes aegypti, and 2) characterize the electrophysiological properties of the cloned Kir subunits when expressed heterologously in oocytes of Xenopus laevis. Here, we reveal that three Kir subunits are expressed abundantly in Aedes Malpighian tubules (AeKir1, AeKir2B, and AeKir3); each of their full-length cDNAs was cloned. Heterologous expression of the AeKir1 or the AeKir2B subunits in Xenopus oocytes elicits inward-rectifying K(+) currents that are blocked by barium. Relative to the AeKir2B-expressing oocytes, the AeKir1-expressing oocytes 1) produce larger macroscopic currents, and 2) exhibit a modulation of their conductive properties by extracellular Na(+). Attempts to functionally characterize the AeKir3 subunit in Xenopus oocytes were unsuccessful. Lastly, we show that in isolated Aedes Malpighian tubules, the cation permeability sequence of the basolateral membrane of principal cells (Tl(+) > K(+) > Rb(+) > NH(4)(+)) is consistent with the presence of functional Kir channels. We conclude that in Aedes Malpighian tubules, Kir channels contribute to the majority of the barium-sensitive transepithelial transport of K(+).

Sulphonylurea sensitivity and enriched expression implicate inward rectifier K+ channels in Drosophila melanogaster renal function

Insect Malpighian (renal) tubules are capable of transporting fluid at remarkable rates. Secondary active transport of potassium at the apical surface of the principal cell must be matched by a high-capacity basolateral potassium entry route. A recent microarray analysis of Drosophila tubule identified three extremely abundant and enriched K(+) channel genes encoding the three inward rectifier channels of Drosophila: ir, irk2 and irk3. Enriched expression of inward rectifier channels in tubule was verified by quantitative RT-PCR, and all three IRKs localised to principal cells of the main segment (and ir and irk3 to the lower tubule) by in situ hybridisation, suggesting roles both in primary secretion and reabsorption. A new splice form of irk2 was also identified. The role of inward rectifiers in fluid secretion was assessed with a panel of selective inhibitors of inward rectifier channels, the antidiabetic sulphonylureas. All completely inhibited fluid secretion, with IC(50)s of 0.78 mmol l(-1) for glibenclamide and approximately 5 mmol l(-1) for tolbutamide, 0.01 mmol l(-1) for minoxidil and 0.1 mmol l(-1) for diazoxide. This pharmacology is consistent with a lower-affinity class of inward rectifier channel that does not form an obligate multimer with the sulphonylurea receptor (SUR), although effects on non-IRK targets cannot be excluded. Glibenclamide inhibited fluid secretion similarly to basolateral K(+)-free saline. Radiolabelled glibenclamide is both potently transported and metabolised by tubule. Furthermore, glibenclamide is capable of blocking transport of the organic dye amaranth (azorubin S), at concentrations of glibenclamide much lower than required to impact on fluid secretion. Glibenclamide thus interacts with tubule in three separate ways; as a potent inhibitor of fluid secretion, as an inhibitor (possibly competitive) of an organic solute transporter and as a substrate for excretion and metabolism.

Basolateral ion transport mechanisms during fluid secretion byDrosophilaMalpighian tubules: Na+ recycling,Na+:K+:2Cl– cotransport and Cl– conductance

Mechanisms of ion transport during primary urine formation by the Malpighian tubule of Drosophila melanogaster were analyzed through measurements of fluid secretion rate, transepithelial ion flux, basolateral membrane potential (Vbl) and intracellular activities of K+ (aKi) and Cl–(aCli). Calculation of the electrochemical potentials for both ions permitted assessment of the possible contributions of K+ channels, Na+:K+:2Cl–cotransport, and K+:Cl– cotransport, to net transepithelial ion secretion across the basolateral membrane. The data show that passive movement of both K+ and Cl– from cell to bath is favoured across the basolateral membrane, indicating that both ions are actively transported into the cell. Contributions of basolateral K+ channels or K+:Cl– cotransporters to net transepithelial ion secretion can be ruled out. After prior exposure of tubules to ouabain, subsequent addition of bumetanide reduced fluid secretion rate, K+ flux and Na+ flux, indicating a role for a Na+:K+:2Cl– cotransporter in fluid secretion. Addition of the K+ channel blocker Ba2+ had no effect on aKi or aCli. Addition of Ba2+ unmasked a basolateral Cl– conductance and the hyperpolarization of Vbl in response to Ba2+ was Cl–-dependent. A new model for fluid secretion proposes that K+ and Cl– cross the basolateral membrane through a Na+-driven Na+:K+:2Cl–cotransporter and that most of the Na+ that enters the cells is returned to the bath through the Na+/K+-ATPase.

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+.

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.