A comparative study on the system of active transport of organic acids in Malpighian tubules of insects

Osmoregulatory capacity at low temperature is critical for insect cold tolerance

At low temperature many insects lose extracellular ion homeostasis and the capacity to mitigate homeostatic imbalance determines their cold tolerance. Extracellular homeostasis is ensured by the osmoregulatory organs and recent research has emphasized key roles for the Malpighian tubules and hindgut in modulating insect cold tolerance. Here, we review the effects of low temperature on transport capacity of osmoregulatory organs and outline physiological processes leading from cold exposure to disruption of ion homeostasis and cold-injury in insects. We show how cold adaptation and cold acclimation are associated with physiological modifications to transport capacity in Malpighian tubules and hindgut. These responses mitigate loss of homeostasis and we highlight how further study of molecular and cellular mechanisms are critical to fully appreciate the adaptations that facilitate insect cold tolerance.

In vivo synergism of ampicillin, gentamicin, ceftaroline and ceftriaxone against Enterococcus faecalis assessed in the Galleria mellonella infection model

BACKGROUND

The unfavourable safety profile of aminoglycosides and the synergistic effects observed in vitro have prompted the development of novel dual β-lactam therapies, e.g. ampicillin/ceftriaxone or ampicillin/ceftaroline, for the treatment of Enterococcus faecalis endocarditis.

OBJECTIVES

For comparison with in vitro chequerboard assay results, a partial chequerboard setup of ampicillin/gentamicin, ampicillin/ceftriaxone and ampicillin/ceftaroline against E. faecalis was established in the Galleria mellonella larval infection model.

METHODS

Discrimination of synergistic and additive interactions was based on the evaluation of larval survival, bacterial quantity in the haemolymph and a pathology score index (internal to the workgroup). Single and multiple dosing schemes based on the half-life of ampicillin were applied. Pharmacokinetic data of the antibiotics in the larvae were determined via agar plate diffusion assays.

RESULTS

Ampicillin and ceftriaxone exhibited strain-specific synergistic interactions in the larvae under both dosing regimens, while the other two combinations showed additive effects. Ampicillin/ceftaroline was inferior to ampicillin/ ceftriaxone. Not all synergistic effects observed in vitro could be replicated in the larvae.

CONCLUSIONS

Our results suggest superior efficacy of ampicillin/ceftriaxone for the treatment of high-inoculum enterococcal infections, for at least some strains, but question the benefit of the current standard of adding the nephrotoxic gentamicin compared with the safer ceftriaxone. This is the first study to develop a scheme for differentiation between additive and synergistic effects in larvae and apply a multiple-antibiotic dosing scheme based on the pharmacokinetics of ampicillin. The model allows the analysis of synergistic effects of antimicrobials in an in vivo setting, but the clinical correlation warrants further study.

Genetic knockdown of a single organic anion transporter alters the expression of functionally related genes in Malpighian tubules of Drosophila melanogaster

Insects excrete a wide variety of toxins via the Malpighian (renal) tubules. Previous studies have implicated three transporters in the secretion of the organic anion (OA) methotrexate (MTX) by the Drosophila Malpighian tubule: Drosophila multidrug resistance-associated protein (dMRP, CG6214), a multidrug efflux transporter (MET, CG30344), and an organic anion transporting polypeptide 58Dc (OATP58Dc, CG3380). RNA interference (RNAi) knockdown and P-element insertion mutation of single OA transporter genes were used to evaluate the importance of these three putative transporters in the secretion of MTX by the Malpighian tubules of Drosophila melanogaster. A major finding is that genetic knockdown of a single OA transporter gene leads to reductions in the expression of at least one other OA transporter gene and in secretion of MTX by Malpighian tubules isolated from flies reared on a standard diet. The pattern of changes indicates that decreases in MTX secretion do not correspond to decreases in dMRP expression in all of the RNAi lines. Genetic knockdown of a single OA transporter gene also alters the extent of upregulation of multiple OA transporter genes in the tubules in response to dietary MTX. Knockdown of dMRP is associated with a decrease in MET expression but an increase in OATP expression when flies are reared on MTX-enriched diet. Our results indicate that dMRP and MET are not the dominant MTX transporters in the tubules when flies are reared on MTX-enriched diets. At least one additional transporter, and possibly OATP, are required for MTX secretion. The implications of our results for studies using genetic knockdown techniques to identify OA transporters in whole tissues such as Malpighian tubules are discussed.

First evidence of epithelial transport in tardigrades: a comparative investigation of organic anion transport

We investigated transport of the organic anion Chlorophenol Red (CPR) in the tardigrade Halobiotus crispae using a new method for quantifying non-fluorescent dyes. We compared the results acquired from the tardigrade with CPR transport data obtained from Malpighian tubules of the desert locust Schistocerca gregaria. CPR accumulated in the midgut lumen of H. crispae, indicating that organic anion transport takes place here. Our results show that CPR transport is inhibited by the mitochondrial un-coupler DNP (1 mmol l–1; 81% reduction), the Na+/K+-ATPase inhibitor ouabain (10 mmol l–1; 21% reduction) and the vacuolar H+-ATPase inhibitor bafilomycin (5 μmol l–1; 21% reduction), and by the organic anions PAH (10 mmol l–1; 44% reduction) and probenecid (10 mmol l–1; 61% reduction, concentration-dependent inhibition). Transport by locust Malpighian tubules exhibits a similar pharmacological profile, albeit with markedly higher concentrations of CPR being reached in S. gregaria. Immunolocalization of the Na+/K+-ATPase α-subunit in S. gregaria revealed that this transporter is abundantly expressed and localized to the basal cell membranes. Immunolocalization data could not be obtained from H. crispae. Our results indicate that organic anion secretion by the tardigrade midgut is transporter mediated with likely candidates for the basolateral entry step being members of the Oat and/or Oatp transporter families. From our results, we cautiously suggest that apical H+ and possibly basal Na+/K+ pumps provide the driving force for the transport; the exact coupling between electrochemical gradients generated by the pumps and transport of ions, as well as the nature of the apical exit step, are unknown. This study is, to our knowledge, the first to show active epithelial transport in tardigrades.

Effects of acute or chronic exposure to dietary organic anions on secretion of methotrexate and salicylate by Malpighian tubules of Drosophila melanogaster larvae

The effects of dietary exposure to organic anions on the physiology of isolated Malpighian tubules and on tubule gene expression were examined using larvae of Drosophila melanogaster. Acute (24 h) or chronic (7 d) exposure to type I organic anions (fluorescein or salicylate) was associated with increased fluid secretion rates and increased fluxes of both salicylate and the type II organic anion methotrexate. By contrast, chronic exposure to dietary methotrexate was associated with increased fluid secretion rate and increased flux of methotrexate, but not salicylate. Exposure to methotrexate in the diet resulted in increases in the expression of a multidrug efflux transporter gene (MET; CG30344) in the Malpighian tubules. There were also increases in expression of genes for either a Drosophila multidrug resistance-associated protein (dMRP; CG6214) or an organic anion transporting polypeptide (OATP; CG3380), depending on the concentration of methotrexate in the diet. Exposure to salicylate in the diet was associated with an increase in expression of dMRP and with decreases of MET and OATP. Exposure to dietary salicylate or methotrexate was also associated with different patterns of expression of heat shock protein genes. The results suggest that exposure to specific type I or type II organic anions has multiple effects and results not only in increased organic anion transport but also in increased rates of inorganic ion transport, which drives osmotically-obliged fluid secretion. Increased fluid secretion may enhance secretion of organic anions by eliminating diffusive backflux from the tubule lumen to the hemolymph.

Physiological and molecular characterization of methotrexate transport by Malpighian tubules of adult Drosophila melanogaster

A radioisotope tracer technique and quantitative PCR were used to study the mechanisms and regulation of transepithelial transport of the type II organic anion methotrexate (MTX) by the Malpighian tubules of Drosophila melanogaster. Transport of MTX was saturable and Na(+)-independent; the kinetic parameters J(max) and K(t) were 437fmolmin(-1) and 23.5microM, respectively. The transport of MTX was competitively inhibited by phenol red and probenecid; non-competitively inhibited by salicylate, verapamil and MK-571; and uncompetitively inhibited by Texas Red. Dietary exposure to 0.1mM MTX led to dramatic increases in gene expression for several members of the ABC family of transporters in both the Malpighian tubules and the gut. Our results suggest that multiple transporters are upregulated in response to dietary exposure to MTX. Increased levels of the protein products which may result from expression of these genes may enhance elimination of toxic compounds such as MTX or its metabolites.

Transepithelial transport of salicylate by the Malpighian tubules of insects from different orders

The organic anion salicylate is a plant secondary metabolite that protects plants against phytophagous insects. In this study, a combination of salicylate-selective microelectrodes and a radioisotope tracer technique was used to study the transepithelial transport of salicylate by the Malpighian tubules of 10 species of insects from five orders. Our results show that salicylate is transported into the lumen of the Malpighian tubules in all the species evaluated, except Rhodnius prolixus. The transepithelial transport of salicylate by the Malpighian tubules of Drosophila simulans, Drosophila erecta, Drosophila sechellia, and Acheta domesticus was saturable, Na(+)-dependent and inhibited by alpha-cyano-4-hydroxycinnamic acid. This transport system resembles that previously found in tubules of Drosophila melanogaster. In contrast, transepithelial transport of salicylate by Malpighian tubules of Tenebrio molitor, Plagiodera versicolora, Aedes aegypti, and Trichoplusia ni was unaffected by Na(+)-free bathing saline. The presence of both salicylate and salicylate metabolites in the secreted fluid samples from the Malpighian tubules of A. domesticus, R. prolixus, T. molitor, and T. ni indicates that insect Malpighian tubules may both transport and metabolize salicylate. The highest capacities to rid the hemolymph of salicylate were found in T. molitor, P. versicolora and Drosphila spp. Our results suggest that transport of salicylate by the Malpighian tubules might contribute to elimination of this organic anion from the hemolymph, particularly in some species that encounter high levels of organic anion in the diet.

Changing salinity induces alterations in hemolymph ion concentrations and Na+ and Cl- transport kinetics of the anal papillae in the larval mosquito, Aedes aegypti

Mosquito larvae are found in diverse aquatic habitats ranging from freshwater to hypersaline water and must often deal with rapid changes in habitat salinity. We transferred larvae of Aedes aegypti from freshwater to 30% seawater, or vice versa, and measured the time course of changes in their hemolymph ion concentrations, using ion-selective microelectrodes. We also reported the Michaelis-Menten kinetics of Na(+) and Cl(-) transport by the anal papillae for the first time using the scanning ion-selective electrode technique (SIET). Hemolymph concentrations of Na(+), Cl(-) and H(+) increased within 6 h, when larvae were transferred from freshwater to seawater and decreased within 6 h, when transferred from seawater to freshwater. Kinetic parameters for Na(+) and Cl(-) transport by the anal papillae were altered after only 5 h following transfer between freshwater (FW) and 30% seawater (30%SW). The J(max) (maximum transport rate) for both ions decreased when larvae were transferred to 30%SW, whereas the K(t) (a measure of transporter affinity) increased for Na(+) transport but was unaltered for Cl(-) transport, suggesting that Na(+) and Cl(-) uptake are independent. Data reveal significant changes in ion transport by the anal papillae of mosquito larvae when they are faced with changes in external salinity such that Na(+) and Cl(-) uptake decrease in higher salinity. The alterations in Na(+) and Cl(-) uptake may be a consequence of changes in hemolymph ion levels when larvae encounter altered salinity. The rapid changes in ion transport described here compliment the previously observed long term alterations in the morphology and ultrastructure of the anal papillae.

Transepithelial transport of fluorescent p-glycoprotein and MRP2 substrates by insect Malpighian tubules: confocal microscopic analysis of secreted fluid droplets

Transport of fluorescent substrates of p-glycoprotein (P-gp) and multidrug resistance-associated protein 2 (MRP2) by insect Malpighian tubules was examined using confocal laser scanning microscopy (CLSM). Isolated tubules of the cricket Teleogryllus commodus accumulated the MRP2 substrate Texas Red in the cells and lumen at concentrations up to 20 and 40 times,respectively, those in the bathing medium. Quantitative CLSM analysis of fluorochrome transport in some cricket tubules and all Drosophilatubules was not practical because of interfering effects of concretions in the cells and lumen. Samples of fluid secreted by tubules set up in Ramsay assays were therefore collected in hollow rectangle glass capillaries. Transepithelial dye flux was calculated as the product of fluid secretion rate(measured in the Ramsay assay) and dye concentration (measured by CLSM of the fluid samples). Dose–response curves for transport and the ratio of dye concentration in the secreted fluid to that in the bathing medium (S/M) were determined for Texas Red as well as for P-gp substrates (rhodamine 123,daunorubicin), the organic anion fluorescein and the organic cation quinacrine. Transepithelial transport of Texas Red was reduced by the MRP2 inhibitors MK571 and probenecid. Transport of daunorubicin was reduced by the P-gp inhibitors verapamil and quinacrine and also by the organic cation tetraethylammonium. The results indicate the presence of P-gp-like and MRP2-like transporters in the Malpighian tubules of both species.

Specificity of the fluorescein transport process in Malpighian tubules of the cricketAcheta domesticus

We demonstrate the presence of an efficient, multispecific transport system for excretion of organic anions in the Malpighian tubules of the cricket Acheta domesticus using fluorescein (FL) as a model substrate. Malpighian tubules rapidly accumulated FL via a high affinity process(Km=7.75 μmol l–1); uptake was completely eliminated by the prototypical organic anion transport inhibitor probenecid (1 mmol l–1), but not by p-aminohippuric acid (3 mmol l–1). FL uptake was inhibited by monocarboxylic acids at a high concentration (3 mmol l–1), and inhibition was more effective with an increase in the carbon chain of the monocarboxylic acid (37% inhibition by 5-carbon valeric acid, and 89% inhibition by 7-carbon caprylic acid). Likewise, tests using a series of aliphatic glutathione conjugates indicated that only the compound with the longest side-chain(decyl-glutathione) significantly inhibited FL uptake (81% inhibition). FL uptake was inhibited by a number of xenobiotics, including a plant alkaloid(quinine), herbicides (2,4-dichlorophenoxyacetic acid and 4-(2,4-dichlorophenoxy)-butyric acid), and the insecticide metabolites malathion monocarboxylic acid (MMA) and 3-phenoxybenzoic acid (PBA),suggesting that this transport system plays an active role in excretion of xenobiotics from Acheta by Malpighian tubules. HPLC quantification of MMA and PBA accumulation into Malpighian tubules verified that MMA accumulation was via a mediated transport process, but suggested that PBA accumulation was by nonspecific binding. The presence of a transport system in Malpighian tubules that handles at least one pesticide metabolite(MMA) suggests that transport processes could be a mechanism conferring resistance to xenobiotic exposure in insects.

Ion-selective microelectrode analysis of salicylate transport by the Malpighian tubules and gut ofDrosophila melanogaster

Transport of the organic anion salicylate by the Malpighian tubules and gut of larval and adult fruit flies was studied using two salicylate-selective microelectrode methods. The first method combined the high selectivity of tridodecylmethylammonium-based electrodes for salicylate with the self-referencing ion-selective microelectrode technique for non-invasive spatial and temporal analysis of salicylate flux. Measurements with this technique revealed secretion of salicylate across the main and distal segments of the Malpighian tubule as well as the midgut, ileum and rectum. The second method used a salicylate-selective microelectrode to measure the concentration of salicylate in fluid droplets secreted by isolated DrosophilaMalpighian tubules set up in a Ramsay secretion assay. Transepithelial salicylate flux was calculated as the product of fluid secretion rate and secreted fluid salicylate concentration. Measurements with this method revealed that salicylate transport was active and saturable; the kinetic parameters Jmax and Kt were 2.72 pmol min-1 tubule-1 and 0.046 mmol l-1,respectively. Measurements of transepithelial salicylate flux determined by both microelectrode methods were in good agreement. Transepithelial flux measurements measured by microelectrodes were also validated by comparing them with measurements of radiolabelled salicylate levels in secreted droplets. Salicylate concentrations in haemolymph samples were measured with salicylate-selective microelectrodes after injection of salicylate into the haemocoel or after insects were fed salicylate-rich diets. The rate of salicylate secretion by Malpighian tubules in vitro was sufficient to account for the measured rate of decline of salicylate concentration in the haemolymph in vivo.

Organic cation transport by Malpighian tubules ofDrosophila melanogaster: application of two novel electrophysiological methods

Transport of the prototypical organic cation tetraethylammonium (TEA) by the Malpighian tubules, ureters and gut of Drosophila melanogasterwas studied using two novel electrophysiological techniques. Both techniques exploited the high selectivity of the cation exchanger potassium tetra-p-chlorophenylborate for tetraalkylammonium compounds relative to inorganic cations such as K+. In the first technique, TEA fluxes were measured using a non-invasive self-referencing TEA-selective microelectrode positioned in the unstirred layer near the surface of each tissue. TEA fluxes from bath to lumen as large as 6 pmol cm–2s–1 were measured across the lower (reabsorptive) segment of the Malpighian tubule and the ureter bathed in saline containing 0.1 mmol l–1 TEA. Corresponding bath-to-lumen fluxes across the secretory main segment of the Malpighian tubule and the posterior midgut were∼1 pmol cm–2 s–1. TEA transport by the lower Malpighian tubule was enhanced by hyperpolarization of the basolateral membrane potential and was inhibited by cimetidine, quinidine, vinblastine and verapamil. In the second technique, TEA concentration was measured using a TEA-selective microelectrode positioned in droplets of fluid secreted by Malpighian tubules set up in saline droplets under oil in a Ramsay assay. Results from the Ramsay assay confirmed the dominant role of the lower Malpighian tubule in net transepithelial secretion of TEA and inhibition of TEA transport by cimetidine. Kinetic parameters (Jmax and Kt) were determined using both approaches.

Inorganic and organic anion transport by insect renal epithelia

Insect renal organs typically exhibit high rates of transport of inorganic and organic anions, and therefore provide useful models for the study of epithelial anion transport and its control. Isolated Malpighian tubules of some species secrete a volume of iso-osmotic fluid equal to their own volume in 10-15 s, which means that cellular Cl(-) content is exchanged every 3-5 s. Anion transport can also be achieved against extreme thermodynamic gradients. The concentration of K(+) and Cl(-) in the lumen of the Malpighian tubules of some desert beetles approaches or exceeds saturation. A basolateral Na(+):K(+):2Cl(-) cotransporter plays an important role in vectorial ion transport in Malpighian tubules of many species, but there is also evidence for coupling of Cl(-) transport to the movement of a single cationic species (Na(+) or K(+)). Although an apical vacuolar H(+)-ATPase plays a primary role in energizing transepithelial secretion of chloride via channels or cotransporters in the secretory segment of the Malpighian tubule, several different ATPases have been implicated in reabsorption of Cl(-) by the lower Malpighian tubule or hindgut. Chloride transport is known to be controlled by several neuropeptides, amines and intracellular second messengers. Insect renal epithelia are also important in excretion of potentially toxic organic anions, and the transporters involved may play a role in resistance to insecticides of natural or anthropogenic origin.

Novel aspects of the transport of organic anions by the malpighian tubules of Drosophila melanogaster

Para-aminohippuric acid (PAH) is a negatively charged organic ion that can pass across the epithelium of Malpighian tubules. Its mode of transport was studied in Malpighian tubules of Drosophila melanogaster. PAH transport was an active process, with a K(m) of 2. 74 mmol l(−)(1) and a V(max) of 88.8 pmol min(−)(1). Tubules had a low passive permeability to PAH, but PAH transport rates (832 nmol min(−)(1)mm(2)) and concentrative ability ([PAH](secreted fluid):[PAH](bath)=81.2) were the highest measured to date for insects. Competition experiments indicated that there were two organic anion transporters, one that transports carboxylate compounds, such as PAH and fluorescein, and another that transports sulphonates, such as amaranth and Indigo Carmine. PAH transport appears to be maximal in vivo because the rate of transport by isolated tubules is not increased when these are challenged with cyclic AMP, cyclic GMP, leucokinin I or staurosporine. Basolateral PAH transport was inhibited by ouabain and dependent on the Na(+) gradient. The Malpighian tubules appeared not to possess an organic acid/ α -keto acid exchanger because PAH accumulation was not affected by low concentrations (100 μmol l(−)(1)) of α -keto acids (α -ketoglutarate, glutarate, citrate and succinate) or the activity of phosphokinase C. PAH transport may be directly coupled to the Na(+) gradient, perhaps via Na(+)/organic acid cotransport. Fluorescence microscopy showed that transport of the carboxylate fluorescein was confined to the principal cells of the main (secretory) segment and all the cells of the lower (reabsorptive) segment. Organic anions were transported across the cytoplasm of the principal cells both by diffusion and in vesicles. The accumulation of punctate fluorescence in the lumen is consistent with exocytosis of the cytoplasmic vesicles. Apical PAH transport was independent of the apical membrane potential and may not occur by an electrodiffusive mechanism.

Fluid secretion by isolated Malpighian tubules of Drosophila melanogaster Meig.: effects of organic anions, quinacrine and a diuretic factor found in the secreted fluid

Para-aminohippuric acid (PAH, 0.2 and 1 mmol l(−)(1)) had no effect on the basal fluid secretion rate (FSR) of isolated Malpighian tubules of Drosophila melanogaster Meig. and did not affect stimulation of the FSR induced by adenosine 3′,5′-monophosphate (cAMP). Phenol Red (phenolsulphonphthalein, PSP; 0.5 and 1 mmol l(−)(1)) slowed the FSR and abolished stimulation of the FSR by cAMP. Diodrast (1 mmol l(−)(1)) slightly, but significantly, reduced the FSR and greatly reduced the stimulation of the FSR normally provoked by cAMP and by the 3′,5′-monophosphates of guanosine (cGMP), inosine (cIMP) and uridine (cUMP). However, stimulation of the FSR by the 3′, 5′-monophosphate of cytidine (cCMP) was little affected by diodrast. Probenecid (0.2 or 1 mmol l(−)(1)) consistently stimulated the FSR, on average by approximately 25 %, but did not markedly inhibit the subsequent stimulation of the FSR by cAMP, cGMP or cIMP. However, the FSR of tubules stimulated by cGMP was temporarily lowered by probenecid. Quinacrine (0.1 mmol l(−)(1)) slowed basal FSR by an average of approximately 30 %, but subsequent stimulation of the FSR by cAMP was not noticeably affected. Both 0.1 mmol l(−)(1) cAMP and 1 mmol l(−)(1) probenecid stimulated adenylate cyclase activity in extracts of Malpighian tubules, but cIMP, cGMP, cUMP and diodrast were without effect in this regard. Uptake of radioactivity from a solution containing 500 nmol l(−)(1) [(3)H]cAMP and 9.5 μmol l(−)(1) cAMP was reduced by more than 90 % by 1 mmol l(−)(1) PSP, by approximately 40 % by 0.2 mmol l(−)(1) probenecid, by 36 % by 1 mmol l(−)(1) diodrast and by 30 % by 1 mmol l(−)(1) PAH. Neither 0.01 mmol l(−)(1) ouabain nor 0.1 mmol l(−)(1) quinacrine affected the uptake of [(3)H]cAMP by the Malpighian tubules. Fluid secreted by isolated Malpighian tubules of Drosophila melanogaster contains a factor that stimulated the FSR on average by approximately 50 %. The presence in the secreted fluid of cGMP at a concentration of 8.3 μmol l(−)(1) did not explain the stimulatory effect on FSR. These results support the existence of a carrier-mediated uptake of cyclic nucleotides into the Malpighian tubules of Drosophila melanogaster, possibly involving a multispecific transporter.

Anti-diuresis in the blood-feeding insect Rhodnius prolixus Stål: antagonistic actions of cAMP and cGMP and the role of organic acid transport

Secretion of primary urine by upper Malpighian tubules of the blood-sucking insect Rhodnius prolixus has recently been shown to be inhibited by cyclic GMP (cGMP). In the present work, we have demonstrated that cGMP has effects antagonistic to those of cAMP in Rhodnius tubules and have further characterized the effects of cGMP on tubular secretion. Cyclic GMP inhibited secretion at all concentrations from 5x10(-6) to 10(-3)M, though this inhibition was partially or wholly reversed by large (2mM) doses of cAMP. While sub-maximal concentrations of cGMP did not significantly alter [K(+)] and [Na(+)] of secreted fluid, high external [cGMP] reduced secretion to minimal levels and caused [K(+)] and [Na(+)] to approach pre-stimulation levels. Cyclic GMP does not appear to affect the permeability of the lower Malpighian tubule to water. Both cAMP and cGMP likely enter tubule cells by way of an organic acid transporter whose activity is induced by feeding. Sensitivity of the tubules to exogenous cGMP and cAMP, which is assumed to be a function of transport activity, reaches a peak approximately 5 days after the blood meal and declines rapidly thereafter. Transport of anions into upper tubules involves at least two different transporters: one for acylamides (e.g., p-aminohippuric acid) and another for sulphonates (e.g., amaranth, phenol red). Amaranth and phenol red blocked the actions of both cGMP and cAMP, whereas p-aminohippuric acid was without effect. This suggests that cyclic nucleotides enter by way of the sulphonate transporter.