Transepithelial and intracellular potentials in isolated Malpighian tubules of tenebrionid beetle

Targeting renal epithelial channels for the control of insect vectors

Three small molecules were identified in high throughput screens that 1) block renal inward rectifier potassium (Kir) channels of Aedes aegypti expressed in HEK cells and Xenopus oocytes, 2) inhibit the secretion of KCl but not NaCl in isolated Malpighian tubules, and after injection into the hemolymph, 3) inhibit KCl excretion in vivo, and 4) render mosquitoes flightless or dead within 24h. Some mosquitoes had swollen abdomens at death consistent with renal failure. VU625, the most potent and promising small molecule for development as mosquitocide, inhibits AeKir1-mediated currents with an IC50 less than 100 nM. It is highly selective for AeKir1 over mammalian Kir channels, and it affects only 3 of 68 mammalian membrane proteins. These results document 1) renal failure as a new mode-of-action for mosquitocide development, 2) renal Kir channels as molecular target for inducing renal failure, and 3) the promise of the discovery and development of new species-specific insecticides.

The mechanism of action of the antidiuretic peptide Tenmo ADFa in Malpighian tubules of Aedes aegypti

The mechanism of action of Tenebrio molitor antidiuretic factor 'a' (Tenmo ADFa) was explored in isolated Malpighian tubules of Aedes aegypti. In the Ramsay assay of fluid secretion, Tenmo ADFa (10(-9) mol l(-1)) significantly inhibited the rate of fluid secretion from 0.94 nl min(-1) to 0.44 nl min(-1) without significant effects on the concentrations of Na+, K+ and Cl- in secreted fluid. In isolated perfused tubules, Tenmo ADFa had no effect on the transepithelial voltage (Vt) and resistance (Rt). In principal cells of the tubule, Tenmo ADFa had no effect on the basolateral membrane voltage (Vbl) and the input resistance of principal cells (Rpc). Tenmo ADFa significantly increased the intracellular concentration of cyclic guanosine monophosphate (cGMP) from 2.9 micromol l(-1) (control) to 7.4 micromol l(-1). A peritubular [cGMP] of 20 micromol l(-1) duplicated the antidiuretic effects of Tenmo ADFa without inducing electrophysiological effects. In contrast, 500 micromol l(-1) cGMP significantly depolarized V(bl), hyperpolarized Vt, and reduced Rt and Rpc, without increasing antidiuretic potency beyond that of 20 micromol l(-1) cGMP. A plot of peritubular cGMP concentration vs Vbl revealed a steep dose-response between 300 micromol l(-1) and 700 micromol l(-1) with an EC50 of 468 micromol l(-1). These observations suggest a receptor- and cGMP-mediated mechanism of action of Tenmo ADFa. Tenmo ADFa and physiological concentrations of cGMP (< 20 micromol l(-1)) reduce the rate of isosmotic fluid secretion by quenching electroneutral transport systems. The inhibition reveals that as much as 50% of the normal secretory solute and water flux can stem from electrically silent mechanisms in this highly electrogenic epithelium.

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.

OSCILLATIONS OF THE TRANSEPITHELIAL POTENTIAL OF MOTH OLFACTORY SENSILLA ARE INFLUENCED BY OCTOPAMINE AND SEROTONIN

The biogenic amine octopamine is known to enhance the sensitivity of male moths to their species-specific pheromones in flight-tunnel experiments. This sensitization of pheromone-guided upwind flight is at least partly due to octopamine-dependent increases in the peak nerve impulse frequency of the pheromone response of olfactory receptor neurons. It is not known, however,whether octopamine exerts its effects directly on the electrical properties of the olfactory receptor neurons or indirectly, via modulation of the accessory cells of the sensillum.

In extracellular tip recordings of pheromone-dependent trichoid sensilla on the antennae of male Manduca sexta moths, we investigated the effects of octopamine and serotonin on the transepithelial potential, which is generated by the activity of V-ATPases in sensillar accessory cells. In addition, the action potential activity of unstimulated olfactory receptor neurons was examined in the presence of biogenic amines. Under constant environmental conditions, the transepithelial potential oscillated regularly with periods of 2-8 min and with a 1-25 mV peak-to-peak amplitude over periods of several hours. These oscillatory intervals were interrupted by periods of relatively stable transepithelial potential, correlated with flight activity by the moth. Octopamine reduced the amplitude of the transepithelial potential oscillation and decreased the resistance of the sensillum preparation in a dose-dependent manner. Serotonin altered the waveform of the transepithelial potential, but did not change the resistance of the preparation. Thus, both amines affect the accessory cells, but have different targets in the regulation of the transepithelial potential. Neither amine significantly influenced the spontaneous action potential activity of the olfactory receptor neurons.

Voltage clamping single cells in intact malpighian tubules of mosquitoes

Principal cells of the Malpighian tubule of the yellow fever mosquito were studied with the methods of two-electrode voltage clamp (TEVC). Intracellular voltage (V(pc)) was -86.7 mV, and input resistance (R(pc)) was 388.5 kOmega (n = 49 cells). In six cells, Ba(2+) (15 mM) had negligible effects on V(pc), but it increased R(pc) from 325.3 to 684.5 kOmega (P < 0.001). In the presence of Ba(2+), leucokinin-VIII (1 microM) increased V(pc) to -101.8 mV (P < 0.001) and reduced R(pc) to 340.2 kOmega (P < 0.002). Circuit analysis yields the following: basolateral membrane resistance, 652. 0 kOmega; apical membrane resistance, 340.2 kOmega; shunt resistance (R(sh)), 344.3 kOmega; transcellular resistance, 992.2 kOmega. The fractional resistance of the apical membrane (0.35) and the ratio of transcellular resistance and R(sh) (3.53) agree closely with values obtained by cable analysis in isolated perfused tubules and confirm the usefulness of TEVC methods in single principal cells of the intact Malpighian tubule. Dinitrophenol (0.1 mM) reversibly depolarized V(pc) from -94.3 to -10.7 mV (P < 0.001) and reversibly increased R(pc) from 412 to 2,879 kOmega (P < 0.001), effects that were duplicated by cyanide (0.3 mM). Significant effects of metabolic inhibition on voltage and resistance suggest a role of ATP in electrogenesis and the maintenance of conductive transport pathways.