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.

Stimulation of fluid secretion of malpighian tubules of drosophila melanogaster meig. by cyclic nucleotides of inosine, cytidine, thymidine and uridine

External application of the 3',5'-cyclic monophosphates of inosine,cytidine, uridine and thymidine stimulated the fluid secretion rate (FSR)of Malpighian tubules isolated from Drosophila melanogaster. The evidence suggested that the cyclic nucleotides acted intracellularly in some capacity. Receptors of the 'purinergic' type appeared not to be major contributors to fluid secretion; of three purinergic agonists tried,adenosine, adenosine 5'-monophosphate (AMP) and adenosine 5'-triphosphate(ATP), only adenosine had an effect, but this was not observed consistently. None of the purinergic agonists interfered with the stimulation of the FSR by adenosine 3',5'-cyclic monophosphate (cAMP). The maximum stimulation of the fluid-secretion rate by any cyclic nucleotide was approximately double the unstimulated (control) rate. Tubules stimulated to less than maximal FSR by one cyclic nucleotide could be stimulated maximally by an appropriate concentration of another cyclic nucleotide. Malpighian tubules bathed in solutions that contained either[3H]cAMP or [3H]cGMP accumulated radioactivity to a level many times that in the medium. Accumulation of radioactivity by tubules bathed in 430 nmol l-1 [3H]cAMP was suppressed by 1 mmol l-1 non-radioactive cyclic nucleotides in the order cAMP>>cGMP>cIMP>cCMP; neither cTMP nor cUMP suppressed the accumulation of [3H]cAMP. Approximately 35 % of the[3H]cAMP and 80 % of the [3H]cGMP that entered the Malpighian tubule cells was metabolised to compounds that were not identified. It was concluded that cyclic nucleotides enter the Malpighian tubule cells by at least one transport mechanism which is particularly sensitive to purine-based nucleotides.

Why are there no insects in the open sea?

The air-filled tracheal respiratory system of insects prevents them from diving deeply in water. It is argued that this is the major factor in preventing insects from colonizing the open sea: they cannot descend sufficiently deeply in the daytime to escape being eaten by fish.

Hormonally controlled chloride movement across Drosophila tubules is via ion channels in stellate cells

Anion conductance across the Drosophila melanogaster Malpighian (renal) tubule was investigated by a combination of physiological and transgenic techniques. Patch-clamp recordings identified clusters of 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive "maxi-chloride" channels in a small domain of the apical membrane. Fluid secretion assays demonstrated sensitivity to the chloride channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid, diphenylamine-2-carboxylate, anthracene-9-carboxylic acid, and niflumic acid. Electrophysiological analysis showed that the calcium-mediated increase in anion conductance was blocked by the same agents. Vibrating probe analysis revealed a small number of current density hot spots, coincident with "stellate" cells, that were abolished by low-chloride saline or the same chloride channel blockers. GAL-4-targeted expression of an aequorin transgene revealed that the neurohormone leucokinin elicits a rapid increase in intracellular calcium levels in stellate cells that precedes the fastest demonstrable physiological effect. Taken together, these data show that leucokinins act on stellate cells through intracellular calcium to increase transcellular chloride conductance through channels. As electrogenic cation conductance is confined to principal cells, the two pathways are spatially segregated in this tissue.

Neuropeptide stimulation of the nitric oxide signaling pathway in Drosophila melanogaster Malpighian tubules

Activation of the nitric oxide (NO) and guanosine 3', 5'-cyclic monophosphate (cGMP) signaling pathway stimulates fluid secretion by the Drosophila melanogaster Malpighian tubule. The neuropeptide cardioacceleratory peptide 2b (CAP2b) has been previously shown to stimulate fluid secretion in this epithelium by elevating intracellular cGMP levels. Therefore, it was of interest to investigate if CAP2b acts through NO in isolated tubules and thus presumably through stimulation of a tubule NO synthase (NOS). We show here by reverse-transcription polymerase chain reaction that Drosophila NOS (dNOS) is expressed in Malpighian tubules. Biochemical assays of NOS activity in whole tubules show that CAP2b significantly stimulates NOS activity. Additionally, fluid secretion and cyclic nucleotide assays show that CAP2b-induced elevation of intracellular cGMP levels and fluid secretion rates are dependent on the activation of a soluble guanylate cyclase. Treatment of tubules with a specific NOS inhibitor abolishes the CAP2b-induced rise in intracellular cGMP levels. These data indicate that CAP2b stimulates NOS and therefore, endogenous NO production, which, in turn, stimulates a soluble guanylate cyclase. This is the first demonstration of stimulation of an endogenous NOS by a defined peptide in Drosophila.

Separate control of anion and cation transport in malpighian tubules of Drosophila Melanogaster

Microelectrode measurements of basal, apical and transepithelial potentials in the Malpighian tubules of Drosophila melanogaster were obtained under a range of conditions in order to investigate whether each of the three main second messenger systems known to act in the tubules (cyclic AMP, cyclic GMP and Ca2+) acted specifically on either cation or anion transport, or whether they activated both systems. Ion-selective microelectrode determinations of K+ concentration and pH of secreted fluid allowed the role of each signalling system to be analysed further. Stimulation with cyclic nucleotides markedly alters the potential profile across principal cells through the selective activation of an apical electrogenic V-ATPase. By contrast, manipulation of extracellular chloride levels, combined with stimulation with leucokinin, does not affect the potential profile across the principal cells, showing that chloride must pass through another route. The cell-permeant Ca2+ chelator BAPTA-AM was shown to suppress the action of leucokinins (insect peptides that induce rapid fluid secretion), but not those of cyclic AMP, the neuronally derived insect peptide cardioacceleratory peptide 2b (CAP2b) or its intracellular messenger cyclic GMP. This shows that leucokinins act through Ca2+ and not through cyclic nucleotides and that the cyclic nucleotide pathways do not co-activate the intracellular Ca2+ pathway to exert their effects. Taken together, these results show that leucokinin acts through intracellular Ca2+, independently of cyclic AMP or cyclic GMP, to raise the chloride permeability of the epithelium. By contrast, either cyclic AMP or cyclic GMP (upon CAP2b stimulation) acts on the electrogenic cation-transporting apical V-ATPase, with only a negligible effect on anion conductance and without perturbing intracellular [Ca2+]. There is thus a clear functional separation between the control pathways acting on cation and anion transport in the tubules. Given the evidence from D. melanogaster and other species that chloride does not pass through the principal cells, we speculate that these two pathways may also be physically separated within cell subtypes of the tubules.

CAP2b, a cardioacceleratory peptide, is present in Drosophila and stimulates tubule fluid secretion via cGMP

A cardioacceleratory peptide, CAP2b, identified originally in the lepidopteran Manduca sexta, stimulates fluid secretion by Malpighian tubules of the dipteran Drosophila melanogaster. High-performance liquid chromatography analyses of adult D. melanogaster reveal the presence of a CAP2b-like peptide, that coelutes with M. sexta CAP2b and synthetic CAP2b and that has CAP2b-like effects on the M. sexta heart. CAP2b accelerates fluid secretion in tubules stimulated by adenosine 3',5'-cyclic monophosphate (cAMP) but has no effect on tubules stimulated by guanosine 3',5'-cyclic monophosphate (cGMP), implying that it acts through the latter pathway. By contrast, the action of leucokinin is additive to both cAMP and cGMP but not to thapsigargin, suggesting that leucokinin acts by the elevation of intracellular calcium. CAP2b stimulation elevates tubule cGMP levels but not those of cAMP. By contrast, leucokinin has no effect on levels of either cyclic nucleotide. Both CAP2b and cGMP increase transepithelial potential difference, suggesting that stimulation of vacuolar-adenosinetriphosphatase action underlies the corresponding increases in fluid secretion. Overall, the results show that a Drosophila CAP2b-related peptide acts to stimulate fluid secretion by Malpighian tubules through the cGMP-signaling pathway.

Fluid reabsorption and ion transport by the lower Malpighian tubules of adult female Drosophila

The properties of the Malpighian tubules of Drosophila melanogaster change along their length. The upstream main segments secrete K(+)-rich fluid at a high rate. From this, the lower tubules reabsorb significant amounts of water and K+. Under stimulation, K+ reabsorption is accelerated. In addition, the lower tubules acidify the fluid passed to them by the main segments and secrete Ca2+ into it, adding to that transported there by the upstream epithelium. In contrast to the lumen-positive transepithelial potential difference (TEP) of the main segments, the TEP in the lower tubules is much lower and becomes lumen-negative close to their downstream junction with the common ureter. We suggest that the role of the lower tubule is to reduce the flow of K(+)-rich fluid that passes to the hindgut; this allows the hindgut to process the flow of excretory fluid more thoroughly.

A novel role for the nitric oxide-cGMP signaling pathway: the control of epithelial function in Drosophila

The nitric oxide (NO) signaling pathway plays major roles in the vertebrate vascular, nervous, and immune systems. Here we present evidence that all the elements in the NO pathway are present in, and act to control epithelial fluid secretion by, the Malpighian tubules of an insect, Drosophila melanogaster. This finding will allow both a physiological and a molecular genetic dissection of the NO pathway in the same tissue.

The regulation of haemolymph potassium activity during initiation and maintenance of diuresis in fed Rhodnius prolixus

The blood-sucking insect Rhodnius prolixus rapidly eliminates a Na(+)-rich K(+)-poor urine after its large meals. K(+)-rich fluid is first secreted by the upper Malpighian tubules and passes to the lower tubules where most of the potassium is reabsorbed. During the initial stimulation of the tubules, the lower tubules must be activated first to avoid loss of potassium. The major element in this is that they respond more rapidly than do the upper tubules to particular hormonal concentrations rather than that they react to lower hormonal concentrations than do the upper tubules. During subsequent diuresis, regulation of the haemolymph potassium concentration depends on three cooperative homoeostatic mechanisms in the tubules. A fall in potassium concentration of the medium bathing the tubules causes (i) a decrease in the rate of fluid secretion by the upper tubules, (ii) a decrease in potassium concentration in the fluid secreted by the upper tubules and (iii) an increase in the rate of potassium absorption by the lower tubules. The tubules respond in the opposite direction to an increase in potassium concentration of the medium. As a result, the potassium concentration of the urine can be adjusted to match the potassium concentration of the fluids absorbed from the gut, so that the potassium concentration of the insect's haemolymph remains unaltered.

INSECT MALPIGHIAN TUBULES: V-ATPase ACTION IN ION AND FLUID TRANSPORT

Insect Malpighian tubules secrete fluid into the lumen as part of their function as excretory organs. The underlying ion transport is, when stimulated, faster than in any other known tissue. It is driven by the activity of an H+-transporting V-ATPase situated on the luminal cell membranes. This ATPase, together with cation/H+ antiporter(s), constitutes a common cation pump which can transport sodium ions, potassium ions or both. Treatments that selectively slow cation transport across the epithelium cause the secreted fluid to become alkaline, whereas those that selectively reduce the rate of anion passage lead to secretion of acid fluid.

The fate of calcium in the diet of Rhodnius prolixus: storage in concretion bodies in the Malpighian tubules

We have investigated the fate of the large amounts of calcium ingested by Rhodnius prolixus in its meals of blood. 45Ca2+ injected into the haemolymph or fed to fifth-stage Rhodnius reared on rabbits is accumulated at high concentrations in the cells of the upper Malpighian tubules; very little is excreted from the body This 45Ca2+ accumulation goes on continuously for at least 12 days and the rate of uptake is increased several-fold within 3–4 days of a meal. The extent of calcium accumulation in tubule cells is correlated with the presence of intracellular membrane-bound concretion bodies, which are therefore likely sites of calcium deposition. X-ray diffraction showed that the calcium deposits are non-crystalline. Tubules from rabbit-fed fifth-stage Rhodnius contain 410 mmol l-1 calcium; in those from chicken-fed insects the calcium concentration is over 1 mol l-1; and in those fed in vitro on heparinised low-K+ sheep blood the calcium concentration is only 21 mmol l-1. The concentration of calcium in the haemolymph in all these insects was 8 mmol l-1 and its activity determined by an ion-selective electrode was 2.5 mmol l-1. 45Ca2+ deposited in the tubules is readily exchangeable, but the efflux preferentially passes to the haemolymph side of the tubule epithelium. The ability to sequester calcium in the Malpighian tubules may prevent calcium from interfering with reabsorptive processes in the rectum.

5-Hydroxytryptamine: a second diuretic hormone in Rhodnius prolixus

Bioassays of 5-hydroxytryptamine (5-HT) in fifth-instar Rhodnius prolixus haemolymph using Calliphora salivary glands indicate that: (1) biologically active 5-HT is present, (2) in unfed animals there is not enough 5-HT to stimulate Malpighian tubule fluid secretion, and (3) there is enough 5-HT soon after the initiation of feeding to stimulate rapid tubule secretion. The 5-HT receptor antagonists ketanserin and spiperone reversibly and selectively inhibit 5-HT-induced fluid secretion, indicating the presence of specific 5-HT receptors on Rhodnius Malpighian tubules. The data provide evidence that 5-HT is a naturally occurring hormone acting with a previously described peptide hormone to regulate diuresis in this species.

Action of activated 27,000 Mr toxin from Bacillus thuringiensis var. israelensis on Malpighian tubules of the insect, Rhodnius prolixus

The action of activated 27,000 Mr toxin from Bacillus thuringiensis var. israelensis (Bti toxin) on Malpighian tubules of Rhodnius prolixus has been investigated. Its binding to the tubules is slowed by low temperature but is not prevented even at 0 degree C. The binding is less effective at pH 10 than at pH7. Pretreatment of the tubules with 0.1 mmol l-1 ouabain or bumetanide or 1 mumol l-1 5-hydroxytryptamine did not affect the toxicity of the toxin. The toxin causes very large changes in the trans-epithelial potential difference; it changes from 40 mV, lumen negative, often to more than 100 mV, lumen positive. This reflects an initial collapse of the potential of the basal cell membrane, followed by a large positive-going potential change at the luminal cell membrane. Just prior to the effects of the toxin on rapid fluid secretion, the basal cell membrane becomes permeable to sucrose molecules. Raffinose at 170 mmol l-1 in the bathing solution does not protect the tubules from Bti toxin action but dextran, Mr5000, at 60 mmol l-1 significantly delayed failure of fluid secretion and, even more, the onset of staining of the tubule cells with Trypan Blue. Exposing tubules to saline that is calcium-free and/or magnesium-free, or has a composition adjusted to be similar to that of the intracellular milieu, does not affect the time course of failure of fluid secretion induced by the toxin. There is no evidence that effective aggregates of Bti toxin molecules are formed in concentrated solutions.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of sodium transport and fluid secretion by ouabain in an insect malpighian tubule

Ouabain, at all concentrations higher than 2 × 10(−7) mol l-1, stimulates the rate at which the Malpighian tubules of the insect, Rhodnius, transport sodium ions and fluid into the lumen. An effect on paracellular movement of sodium ions is unlikely because ouabain makes the electrical potential of the lumen more positive, which would slow diffusion of sodium into the lumen. Radioactive ouabain binds to the haemolymph-facing sides of the tubule cells but not to the luminal face. This binding is reduced in the presence of elevated levels of potassium or of non-radioactive ouabain. Bound ouabain is only slowly released on washing in ouabain-free saline. The evidence suggests that there is a Na+/K+-ATPase on the outer (serosal) membranes of the tubules. Such a pump would transport sodium in a direction opposed to the flow of ions and water involved in fluid transport; poisoning it with ouabain would remove this brake, and fluid flow and sodium transport would increase, as observed.

The initial stages in the action of an insecticidal delta-endotoxin of Bacillus thuringiensis var. israelensis on the epithelial cells of the malpighian tubules of the insect, Rhodnius prolixus

The effects of the 27 X 10(3) Mr insecticidal delta-endotoxin from Bacillus thuringiensis var. israelensis have been studied using, as a model system, isolated insect Malpighian tubules. At all concentrations of the toxin higher than 1 microgram ml-1 (4 X 10(−8) moll-1) applied to the outer surface of the tubules, fluid secretion failed within about 30 min. Except at very high concentrations, where failure always takes at least 30 s, there was an inverse relationship between the concentration of toxin and the time of failure of toxin-treated tubules. During exposure to toxin, the tubules were initially unaffected for a relatively long period and then rapid failure occurred. If the tubules were removed into toxin-free saline just before failure would have occurred, fluid secretion remained normal for at least 2 h, but on return to the origin toxin-containing saline failure was almost immediate. The toxin was found not to bind to the basement membrane. Ultrastructural changes became evident as tubule failure occurred. These initially involved modifications to the basal side of the cells, but later also to the luminal microvilli. Intercellular junctions became disassociated and cytoplasmic vacuolization occurred. The population of intramembranous particles in the basal membranes became reduced with time. Our findings suggest the following hypothesis for the initial stages in the interaction of the toxin with the tubules. Toxin molecules attach to the accessible cell membranes progressively and irreversibly. They do not readily associate by diffusing laterally in the membrane, so that toxic effects develop only when sufficiently large numbers of them attach close together. The molecules may then associate in some way as a complex, perhaps forming a pore in the membrane. Relatively few such pores lead rapidly to cell failure and death.

The influence of Blastocrithidia triatomae (Trypanosomatidae) on the reduviid bug Triatoma infestans: in vivo and in vitro diuresis and production of diuretic hormone

The homoxenous flagellate Blastocrithidia triatomae disturbs excretion in Triatoma infestans. During the first 4 and 24 h after feeding, infected 5th instars excreted approximately a 2.5-fold smaller volume of urine. Consequently, about double the normal volume of haemolymph could be obtained from these infected bugs 24 h after feeding. Surprisingly, the in vitro secretion rates of Malpighian tubules from uninfected and infected bugs were nearly identical. In addition, the storage and release of diuretic hormone in infected bugs was sufficient to induce normal secretion rates by isolated Malpighian tubules. Therefore, the previously observed ultrastructural alterations in the upper Malpighian tubules of infected bugs do not result in a disturbed in vitro secretion, although in vivo excretion is reduced.

The permeability properties of septate junctions in Malpighian tubules of Rhodnius

This paper describes the structural characteristics and permeability properties of the smooth septate junctions between the upper Malpighian tubule cells of a blood-sucking bug, Rhodnius prolixus. The permeability of the paracellular route was tested only for solutes that could be demonstrated not to cross the epithelium via the cellular route. The intercellular clefts were readily permeated by sucrose, inulin and polyethylene glycol (PEG), showing a higher permeability to molecules of smaller radius (PEG versus sucrose). Negatively charged molecules permeated the clefts more readily than positively charged ones. The effects of pH, urea and luminal flow rate on permeability were studied. The results are discussed in relation to the physiological tightness of the Malpighian tubules to certain solutes and to its function as an excretory epithelium.

Maintenance of function in single epithelial cells spatially isolated from similar cells

We have found in an insect tissue, the Malpighian tubules of Rhodnius, instances of single epithelial cells which, as the result of a possible error in development, lie within the epithelium some distance from the main population of similar cells. This spatial separation makes it possible to measure the transport abilities of these cells. Their transport abilities were found to be the same as the cells in the main population. This finding shows that the maintenance of function in individual cells of epithelial tissues may not depend on direct contact with other similar cells.

Secretion by the Malpighian tubules of Rhodnius prolixus stal: electrical events

Transepithelial and intracellular potentials have been simultaneously recorded from Rhodnius upper Malpighian tubules before and after stimulation of fluid secretion. The transepithelial electrical response to the diuretic hormone mimic 5-hydroxytryptamine (5-HT) was triphasic; recordings of intracellular potential changes indicated that the three phases represented successive events at the apical membrane. Depolarizations produced by increasing the bathing medium potassium concentration indicated that the basal membrane was much more permeable to potassium than to sodium. Electrical responses to chloride-free saline were inconsistent with a significant basal membrane chloride permeability. Chloride movements across the basal membrane were opposed by an electrical gradient of about 65 mV. The results of experiments in which tubules were exposed to chloride-free saline or sodium-free saline suggested that chloride entry into the cells was linked to the entry of Na+ and K+. The effects of furosemide and bumetanide upon secretion and potential changes suggested that chloride crossed the basal membrane through co-transport with Na+ and K+. Chloride probably crosses the apical membrane into the lumen passively in response to a favourable electrical gradient of about 35 mV. Cations must be actively pumped into the lumen against an electrical gradient of 35 mV. Our results support previous evidence for an apical cation pump which actively transports Na and K into the lumen. A tentative model of ionic movements during fluid secretion is presented. It is suggested that the apical cation pump maintains sodium at low intracellular concentrations, thereby maintaining a favourable gradient for entry of Na+ through the proposed basal co-transport step. The suggested stoichiometry is Na+:K+:2 Cl-.

Passage of solutes through walls of Malpighian tubules of Rhodnius by paracellular and transcellular routes

The walls of isolated upper Malpighian tubules of the insect, Rhodnius prolixus, are much more permeable to small relatively unchanged solutes (ethanol, xylose, and mannitol) than to larger or more charged solutes (acetate, glycine, tyrosine, and inulin). The more permeable solutes rapidly reach concentrations in the tubule cells equivalent to their concentrations in the bathing medium; the less permeable solutes do not penetrate into the cells. The time course of accumulation of permeable solute in the cells matches the time course of the appearance of solute in the lumen. Substances injected into the hemolymph of fed R. prolixus appear in the urine at concentrations predictable from the permeability of in vitro tubules, supporting the idea that the in vitro permeability of the tubules is representative of their properties in the intact insect. It is suggested that the rapid transcellular penetration of small solutes through the Malpighian tubules reflects the large areas of cell membrane. The area of cell membrane exceeds that of the paracellular clefts by a factor of 10(5).

Paracellular and transcellular routes for water and solute movements across insect epithelia

Because the frontal area of the intercellular clefts in Malpighian tubules is small, and the osmotic permeability of the cell membranes is large, the route for transepithelial water movement during fluid secretion is transcellular. Water movements appear to be a passive response to osmotic gradients of a few mosmol 1(−1) produced in the cells and in he lumen by active ion transport. The excretory functions of Malpighian tubules are discussed in relation to recent analyses of the routes of passive permeation for non-electrolytes. Uncharged molecules smaller than a disaccharide appear to move at significant rates through the cells whereas molecules as large as inulin traverse the epithelium by a paracellular path. In addition there are specific active transport mechanisms for a variety of organic molecules. The routes and mechanisms proposed for water and solute movements are discussed in relation to comparable studies in other epithelia.

Changes in hormone activity of single neurosecretory cell bodies during a physiological secretion cycle

Single neurosecretory cell bodies were dissected from the ventral ganglionic mass of Rhodnius prolixus, lysed in distilled water, and bioassayed for diuretic hormone (DH) activity on isolated malpighian tubules. DH was found in a least 10 somata within the ganglion; electron micrographs of isolated cells show a large population of elementary neurosecretory granules. Quantitative measures of hormone activity were made by bioassaying somata from unfed 5th instar larvae, and at the following times after feeding; 1 h, 4 h (near the end of DH-mediated diuresis), 1 day, 5 days, 10 days, 17 days and 21 days (just after the moult to adult). DH activity in cell bodies drops significantly within 1 h after feeding, and remains low long after hormone secretion ceases. Restocking of the soma with active hormone occurs during the period 10-21 days after feeding.

Excretion of alkaloids by malpighian tubules of insects

Nicotine is transported at high rates by Malpighian tubules of larvae of Manduca sexta, Pieris brassicae and Rhodnius prolixus and the transport persists in the absence of alkaloid from the diet. In the fluid-secreting portion of Rhodnius tubules this transport is not coupled to ion transport, nor is it dependent on the physiological state of the animal. The transport, which can occur against a steep electrochemical gradient, shows saturation kinetics with a maximal rate of 700 pmol. min-1 per tubule and is half saturated at 2–3 mM. Nicotine transport independent of ion movements also occurs in the lower resorptive parts of Rhodnius tubules. Both portions of Rhodnius tubules can transport morphine and atropine. These alkaloids and nicotine compete with one naother and are presumed to be carried by the smae transport system. Nicotine transport in Rhodnius was unaffected by organic anions, such as amaranth and benzyl penicillin, or by the organic anion transport inhibitor, probenecid. Fluid secretion in 5-HT-stimulated tubules was reduced by atropine and nicotine, probably by blocking the 5-HT receptors. The Malpighian tubules of adult Calliphora erythrocephala and Musca domestica remove nicotine from bathing solutions, an unknown metabolic accumulating in the tubules. Adult P. brassicae and M. sexta do not exhibit transport of nicotine by their Malpighian tubules.

Induction of transport of organic anions in Malpighian tubules of Rhodnius

1.The ability of Rhodnius Malpighian tubules to transport organic anions such as p-aminohippuric acid (PAH) and amaranth depends on the physiological state of the insect. Unfed insects progressively lose this ability, but the transport mechanisms rapidly become activated after a blood meal. 2. This induction occurs in adults as well as larval instars and is not prevented by decapitation. 3. No increase in rates of excretion of organic anions follows a meal of Ringer's solution or a single injection of 0–1 mumoles of PAH into the haemolymph. 4. Accelerated PAH excretion is induced in insects fed (a) blood plasma alone or a suspension of red blood cells in saline, (b) solutions of serum albumin or casein, and (c) whole or diluted milk. 5. These results show that the ingestion of a protein-rich meal is sufficient to induce an accelerated transport of organic anions such as PAH and they suggest that this induction is not controlled by a hormone released in response to abdominal distension but depends instead on the continued presence in the haemolymph of some product of digestion of the meal.

Active Transport of Magnesium by the Malpighian Tubules of the Larvae of the Mosquito, Aedes Campestris

1. Larvae of Aedes campestris can survive in water containing up to 100 mM Mg even though they ingest and absorb into the haemolymph considerable amounts of magnesium-rich fluid.

2. Isolated Malpighian tubules, unlike those of Rhodnius and Carausius secreted fluid containing elevated concentrations of magnesium. This transport displayed saturation kinetics, the half-maximal rate being at approximately 2·5 mM Mg.

3. Active transport of magnesium was demonstrated by the secretion of this cation against a tenfold concentration gradient and an electrical potential difference of 15 mV.

4. Magnesium ions are not required for fluid transport, which proceeds independently of magnesium transport. As a result fluid which is secreted slowly contains higher concentrations of magnesium than that which is secreted more rapidly.

5. Magnesium is transported by isolated Malpighian tubules fast enough to account for the observed excretion of magnesium in living mosquito larvae.

Active Transport by Insect Malpighian Tubules of Acidic Dyes and of Acylamides

Insect Malpighian tubules carry out active transport of two types of organic anion: acylamides (such as p-aminohippuric acid) and sulphonates (such as indigo carmine and amaranth). There are separate mechanisms for the transport of these two classes of compounds.

The degree to which these compounds are concentrated depends critically on the passive permeability of the tubule wall. In the permeable Malpighian tubules of Calliphora, small transported molecules readily escape from the tubule lumen. At low rates of fluid secretion the net rate of dye transport is thereby very much reduced. As a result the rate of dye transport in this insect depends on the rate of fluid secretion, although the processes are not rigidly linked. In the less permeable tubules of Rhodnius and Carausius, dye secretion is not affected by the rate of fluid secretion.

The active transport of these two types of compounds is a means of clearing from the haemolymph the conjugated compounds which are the products of detoxication of potentially toxic products of metabolism.

Potassium-Induced Release of the Diuretic Hormones of Rhodnius Prolixus and Glossina Austeni: Ca Dependence, Time Course and Localization of Neurohaemal Areas

1. Exposure of neurohaemal areas to solutions of elevated K concentration (above 40 mM) causes a maximal release of diuretic hormone in Rhodnius prolixus and Glossina austeni.

2. An involvement of Ca in hormone release is indicated by the reduction caused by low concentrations of this cation (below 2 mM) or by the presence of Mn.

3. During prolonged treatment with K-rich solutions the rate of hormone release is initially high, but then declines. This response parallels that for Ca entry into squid giant axons during maintained potassium depolarization and suggests that the rate of Ca entry controls the rate of hormone release.

4. Tetrodotoxin did not reduce the potassium-induced release of the hormone, suggesting that K acts directly on the neurosecretory axon endings in the neurohaemal areas.

The Passive Permeability of Insect Malpighian Tubules to Organic Solutes

1. The permeability of Malpighian tubules of five insect species to a range of organic solutes has been measured by both in vitro and in vivo techniques.

2. Nearly all the substances tested were found, in in vitro experiments, to penetrate the walls of Malpighian tubules in a manner which, on several criteria, was judged to be passive.

3. The walls of Malpighian tubules are more permeable to small molecules than to large ones; but even inulin (MW 5200) penetrates fast enough to reach concentrations which, in tubules secreting fluid slowly, can be as high in the secreted fluid as 50% of its concentration in the bathing fluid.

4. Inulin injected into Schistocerca and L-fucose injected into Rhodnius appeared in the excreta at rates which could be accurately predicted from the in vitro behaviour of Malpighian tubules of these insects.

5. Taken with the fact that the cuticular lining of the rectum of insects is thought to be not very permeable, the high permeability of insect Malpighian tubules means that the reabsorption of useful compounds of the order of size of disaccharides must to some extent occur before excretory material reaches the rectum.

6. It is suggested that the circulation of lipids and hormones in the form of complexes with proteins is a device to prevent their rapid loss through the excretory system.

Pharmacology of the Malpighian tubules of Rhodnius and Carausius: the structure-activity relationship of tryptamine analogues and the role of cyclic AMP

1. 5-Hydroxytryptamine (5-HT) and a few of its derivatives will cause an acceleration of secretion by the isolated Malpighian tubules of Rhodnius prolixus and Carausius morosus. Substitution in the terminal amino group of 5-HT causes little loss of effect, but changes in the indole ring and the hydroxy group in general abolish activity.

2. Most derivatives of 5-HT which do not stimulate secretion, and many other related amines, act as inhibitors of secretion in Rhodnius. This inhibition is probably of the competitive type.

3. Substances which stimulate secretion in Rhodnius are easily washed off the Malpighian tubules, although they will stimulate secretion at very low concentrations By contrast, competitive inhibitors wash away relatively slowly and inhibit only at much higher concentrations. It is suggested therefore that stimulation involves only momentary interactions between stimulant and receptor sites, but that competitive inhibitors spend considerably longer attached to the sites.

4. Cyclic 3',5'-adenosine monophosphate (cyclic AMP) will stimulate secretion by tubules of both Rhodnius and Carausius. Tubules of both species are very sensitive to cyclic AMP, measurable effects being produced at concentrations of 4 x 10-5 M/l in Rhodnius and 10-4 M/l in Carausius.

5. Aminophylline (theophylline ethylene diamine) at a concentration of 10-4 M/l stimulates secretion by Carausius tubules. This compound and other inhibitors of phosphodiesterase have no stimulatory action on Rhodnius tubules, but any such effect may be masked by the adverse reaction of the tubules to the high concentrations necessary.

6. Inhibitors of 5-HT action such as tryptamine and tyramine also inhibit secretion by Rhodnius Malpighian tubules induced by cyclic AMP. This may be due to a dependence of the action of cyclic AMP on the state of the diuretic hormone receptor site.

7. The evidence is consistent with the idea that 5-HT, other stimulant molecules related to 5-HT and the diuretic hormones interact with Malpighian tubule cells of Rhodnius and Carausius at specific sites, probably on the cell membrane facing the haemolymph. As a result of this, secretion is induced, possibly through the action of intracellular cyclic AMP produced as a response.

Axonal Function and Ionic Regulation in the Central Nervous System of a Phytophagous Insect (Carausius Morosus)

1. Experiments vising intracellular and extracellular recording techniques indicate that, despite the specialized ionic composition of the haemolymph, the axons in the nerve cord of Carausius are conventional in that the action current is largely carried by sodium ions.

2. This effect is achieved by an appreciable regulation of the concentrations of inorganic ions in the extracellular fluid bathing the axon surfaces.

3. The extra-axonal regulation does not appear to result from any significant restriction in the accessibility of cations to the general extracellular system, but from a local regulation which appears to maintain a relatively high concentration of sodium ions at the axon surfaces.

4. It is suggested that such a regulation may be achieved by an extrusion of sodium ions from the glial cells into the restricted extra-axonal spaces demonstrated in the electron micrographs of this preparation.

Membrane Potentials in the Central Nervous System of a Phytophagous Insect (Carausius Morosus)

1. The nerve cord of the stick insect is surrounded by a fat-body sheath. This sheath encloses an extraneural space and thus interposes an additional fluid compartment between the neural lamella and the haemolymph. The axons in the thoracic connectives were found to be relatively small, the largest ones averaging 7--II µ in diameter.

2. The apparent resting potentials of axons, impaled with glass capillary microelectrodes, were found to be relatively small, averaging only 25.1 mV., with an overshoot of 59.3 mV. in action potentials in intact preparations. In the absence of the neural fat-body sheath the resting potentials were increased to a mean value of 40.3 mV., there being no significant alteration in the total amplitude of the action potentials. This effect appears to result from the interpolation of a positive potential of some 15-20 mV. between the indifferent and recording electrodes.

3. The positive potential was abolished, in intact preparations, when the nerve cords were bathed with solutions of elevated chloride concentration. Positive potentials were also obtained when gradients of chloride ions were maintained across the isolated fatbody sheath. It is suggested that the positive potentials may result from a chloride diffusion potential across the neural fat-body sheath.

4. The results are discussed in relation to the ability of the axons of this species to function in ganglia and connectives bathed with solutions of low sodium concentration.

The Ultrastructure of the Perineurium in two Insect Species, Carausius Morosus and Periplaneta Americana

The organization of the perineurium in two insect species (Carausius morosus and Periplaneta americana) has been examined with the electron microscope. In both species this cellular layer has been found to possess an extensive system of tortuous channels between the lateral cell walls. These channels are open at the outer margin adjacent to the fibrous connective-tissue sheath, but appear to be closed at the inner margin by regions of septate desmosomes and/or ‘tight’ junctions. There is an increased surface area at the inner margin of the perineurial cells produced by the presence of long inwardly directed flanges. An electron-dense coat has also been identified on the cytoplasmic side of the type II perineurial cell membranes at points of contact with the underlying extracellular system and at the outer surface adjacent to the connective-tissue sheath. This organization of the perineurium is strikingly similar to that observed in a variety of fluid-secreting epithelia and its possible function in fluid transport is discussed in relation to the available evidence on the physiology of the insect central nervous system. It is suggested, contrary to some earlier suppositions, that the perineurium may not be primarily involved in the control of the extracellular sodium level and that this regulation may be effected at a deeper level in the central nervous tissues.

Nervous Control of the Mechanical Properties of the Abdominal Wall at Feeding in Rhodnius

1. When larvae of Rhodnius feed, the abdominal wall rapidly becomes extensible. This change is brought about peripherally by the abdominal nervous supply; as a consequence, abdominal segments which have been denervated fail to stretch as much at feeding as do neighbouring innervated segments.

2. The stimulus for this change originates in the head. It appears that the sensations accompanying feeding are responsible and that they have a cumulative effect, so that the degree of plasticization of the abdominal wall depends on the length of time during which the insect has been sucking.

3. Neurosecretory axons are the only known efferent supply to the abdominal wall and so it is most probable that they cause the localized plasticization.

4. The evidence suggests that the plasticization of the abdominal cuticle is a result of an increase in its water content.

5. The increase in plasticity of the abdominal wall is temporary, the effect decreasing progressively after feeding.

Neurosecretory Supply to the Epidermis of an Insect

Electron miticroscopy of the abdominal nerves of Rhodnius reveals neurosecretory axons which supply the abdomizinal epidermis. This suggests that thle epidermnal cells of insects are under more localized endocrine control than has previously been supposed.