Stimulatory effect of 5-hydroxytryptamine (serotonin) on secretion by malpighian tubules of insects

Mechanisms of Systemic Osmoregulation in Insects

Water is essential to life. Terrestrial insects lose water by evaporation from the body surface and respiratory surfaces, as well as in the excretory products, posing a challenge made more acute by their high surface-to-volume ratio. These losses must be kept to a minimum and be offset by water gained from other sources. By contrast, insects such as the blood-sucking bug Rhodnius prolixus consume up to 10 times their body weight in a single blood meal, necessitating rapid expulsion of excess water and ions. How do insects manage their ion and water budgets? A century of study has revealed a great deal about the organ systems that insects use to maintain their ion and water balance and their regulation. Traditionally, a taxonomically wide range of species were studied, whereas more recent research has focused on model organisms to leverage the power of the molecular genetic approach. Key advances in new technologies have become available for a wider range of species in the past decade. We document how these approaches have already begun to inform our understanding of the diversity and conservation of insect systemic osmoregulation. We advocate that these technologies be combined with traditional approaches to study a broader range of nonmodel species to gain a comprehensive overview of the mechanism underpinning systemic osmoregulation in the most species-rich group of animals on earth, the insects.

Managing fuels and fluids: Network integration of osmoregulatory and metabolic hormonal circuits in the polymodal control of homeostasis in insects

Osmoregulation in insects is an essential process whereby changes in hemolymph osmotic pressure induce the release of diuretic or antidiuretic hormones to recruit individual osmoregulatory responses in a manner that optimizes overall homeostasis. However, the mechanisms by which different osmoregulatory circuits interact with other homeostatic networks to implement the correct homeostatic program remain largely unexplored. Surprisingly, recent advances in insect genetics have revealed several important metabolic functions are regulated by classic osmoregulatory pathways, suggesting that internal cues related to osmotic and metabolic perturbations are integrated by the same hormonal networks. Here, we review our current knowledge on the network mechanisms that underpin systemic osmoregulation and discuss the remarkable parallels between the hormonal networks that regulate body fluid balance and those involved in energy homeostasis to provide a framework for understanding the polymodal optimization of homeostasis in insects.

Fluid Secretion by Malpighian Tubules of Rhodnius prolixus: Neuroendocrine Control With New Insights From a Transcriptome Analysis

Rhodnius prolixus (the kissing bug and a major vector of Chagas disease) is an obligate blood feeder that in the case of the fifth instar consumes up to 10 times its unfed body weight in a single 20-minute feed. A post-prandial diuresis is initiated, within minutes of the start of gorging, in order to lower the mass and concentrate the nutrients of the meal. Thus, R. prolixus rapidly excretes a fluid that is high in NaCl content and hypo-osmotic to the hemolymph, thereby eliminating 50% of the volume of the blood meal within 3 hours of gorging. In R. prolixus, as with other insects, the Malpighian tubules play a critical role in diuresis. Malpighian tubules are not innervated, and their fine control comes under the influence of the neuroendocrine system that releases amines and neuropeptides as diuretic or antidiuretic hormones. These hormones act upon the Malpighian tubules via a variety of G protein-coupled receptors linked to second messenger systems that influence ion transporters and aquaporins; thereby regulating fluid secretion. Much has been discovered about the control of diuresis in R. prolixus, and other model insects, using classical endocrinological studies. The post-genomic era, however, has brought new insights, identifying novel diuretic and antidiuretic hormone-signaling pathways whilst also validating many of the classical discoveries. This paper will focus on recent discoveries into the neuroendocrine control of the rapid post-prandial diuresis in R. prolixus, in order to emphasize new insights from a transcriptome analysis of Malpighian tubules taken from unfed and fed bugs.

FLUID AND ION SECRETION BY MALPIGHIAN TUBULES OF LARVAL CHIRONOMIDS, Chironomus riparius: EFFECTS OF REARING SALINITY, TRANSPORT INHIBITORS, AND SEROTONIN

Larvae of Chironomus riparius respond to ion-poor and brackish water (IPW, BW) conditions by activating ion uptake mechanisms in the anal papillae and reducing ion absorption at the rectum, respectively. The role that the Malpighian tubules play in ion and osmoregulation under these conditions is not known in this species. This study examines rates of fluid secretion and major cation composition of secreted fluid from tubules of C. riparius reared in IPW, freshwater (FW) and BW. Fluid secretion of tubules from FW and BW larvae was similar but tubules from IPW larvae secrete fluid at higher rates, are more sensitive to serotonin stimulation, and the secreted fluid contains less Na(+) . Therefore in IPW, tubules work in concert with anal papillae to eliminate excess water while conserving Na(+) in the hemolymph. Tubules do not appear to play a significant role in ion/osmoregulation under BW. Serotonin immunoreactivity in the nervous system and gastrointestinal tract of larval C. riparius was similar to that seen in mosquito larvae with the exception that the hindgut was devoid of staining. Hemolymph serotonin titer was similar in FW and IPW; hence, serotonin is not responsible for the observed high rates of fluid secretion in IPW. Instead, it is suggested that serotonin may work in a synergistic manner with an unidentified hormonal factor in IPW. Ion transport mechanisms in the tubules of C. riparius are pharmacologically similar to those of other insects.

Endocrine regulation of insect diuresis in the early postgenomic era1This review is part of a virtual symposium on recent advances in understanding a variety of complex regulatory processes in insect physiology and endocrinology, including development, metabolism, cold hardiness, food intake and digestion, and diuresis, through the use of omics technologies in the postgenomic era

Diuresis, the removal of excess metabolic waste through production of primary urine while maintaining homeostasis, is an important biological process that is tightly regulated by endocrine factors. Several hormonal components that act as diuretic or antidiuretic factors in insects have been identified in the last few decades. Physiological mechanisms responsible for ion and water transport across biological membranes have been intensively studied. The large amount of data rapidly accumulating in the genomics era has led to an increased dependence on reverse genetic and physiological approaches, first identifying candidate genes and subsequently deriving functions. In many cases, the reverse approaches have been highly successful, especially in studies of the receptors for diuretic factors, which are mainly G-protein-coupled receptors. This review summarizes research on insect diuretic and antidiuretic endocrine factors, and their receptors. Emphases of the review are given to the genomics of ligands and their receptors, as well as to their implications for evolution and function.

Identification of the elusive peptidergic diuretic hormone in the blood-feeding bug Rhodnius prolixus: a CRF-related peptide

Probing of a host and ingestion of a blood-meal in a fifth instar Rhodnius prolixus results in a cascade of tightly integrated events. The huge blood-meal is pumped into the anterior midgut during feeding, then modified by diuresis and stored until it is digested. While serotonin is known to be a diuretic hormone in R. prolixus, a peptidergic factor(s) was also known to play a role in diuresis. In the present study we employed molecular techniques and mass spectrometry to determine the sequence of a native CRF-like peptide from R. prolixus (Rhopr DH). In addition, we confirmed the distribution and localization of Rhopr DH using in situ hybridization and immunohistochemistry, and demonstrated its potent biological activity on both the anterior midgut and Malpighian tubules.

Serotonin has kinin-like activity in stimulating secretion by Malpighian tubules of the house cricket Acheta domesticus

Serotonin stimulates secretion by Malpighian tubules (MT) of a number of insects, and functions as a diuretic hormone in Rhodnius prolixus and in larval Aedes aegypti. Serotonin is here shown to be a potent stimulant of secretion by MT of the house cricket, Acheta domesticus, with an apparent EC(50) of 9.4 nmol L(-1), although its diuretic activity is just 25% of the maximum achievable with either the native CRF-related peptide, Achdo-DH, or a crude extract of the corpora cardiaca. In this respect, the diuretic activity of serotonin is similar to that of the cricket kinin Achdo-KI, and when tested together their actions are not additive, which suggests they target the same transport process. Consistent with this suggestion, the activity of serotonin is chloride-dependent and is associated with a non-selective stimulation of NaCl and KCl transport. In common with Achdo-KI, serotonin has no effect on cAMP production by isolated MT, and both act synergistically with exogenous 8bromo-cAMP in stimulating fluid secretion, most likely by promoting the release of Ca(2+) from intracellular stores. A number of serotonin agonists and antagonists were tested to determine the pharmacological profile of receptors on cricket MT. The results are consistent with the diuretic activity of serotonin being mediated through a 5-HT(2)-like receptor.

Biological activity of diuretic factors on the anterior midgut of the blood-feeding bug, Rhodnius prolixus

Probing of a host and ingestion of a blood-meal in a fifth instar Rhodnius prolixus results in a cascade of tightly integrated events, including salivary gland secretion, plasticization of the abdominal cuticle, increased ion and water movement across the anterior midgut (crop) and Malpighian tubules (which rapidly produce urine) and the regular expulsion of urine from the hindgut. In this study we have focussed on the role of the anterior midgut during the rapid postprandial diuresis. The huge blood-meal is pumped into the anterior midgut, during feeding, then modified by diuresis and stored until it is digested. Changes in the anterior midgut activity are rapid. Within minutes of the commencement of feeding there is an increase in the frequency of anterior midgut contractions and diuresis begins with the movement of salt and water across the epithelium of the anterior midgut into the haemolymph. While serotonin, a diuretic hormone in R. prolixus, is known to play a role in the physiological activity of the anterior midgut, we were interested in exploring further the role of serotonin, and other diuretic peptides. We have tested the activity of several peptides, including R. prolixus calcitonin-like diuretic hormone (Rhopr-DH 31), corticotropin-releasing factor (CRF)-like peptide from Zootermopsis nevadensis DH (Zoone-DH) and a kinin from Leucophaea maderae, Leucokinin 1 (LK1). These peptides families are known to be present in the central nervous system of R. prolixus, are putative neurohormones released into the haemolymph after the start of feeding, and have been shown to have activity on a variety of tissues involved in post-feeding diuresis. We show here that both serotonin and Zoone-DH increase the cAMP content of the anterior midgut and that serotonin, Zoone-DH and cAMP analogues increase absorption of water from the anterior midgut, increase the short circuit current and voltage, while decreasing the resistance across the epithelium. While LK1 and Rhopr-DH 31 do not significantly increase absorption, or short circuit current, LK1 does significantly decrease the resistance and transepithelial voltage of the anterior midgut epithelium. All of the factors studied increase the frequency of contractions of the anterior midgut.

Serotonin: A coordinator of feeding-related physiological events in the blood-gorging bug, Rhodnius prolixus

Rhodnius prolixus is an obligatory blood-feeder that can ingest blood meals of up to 10 times its mass. Rapid production of urine commences within 2-3 min of the start of feeding in order to eliminate the load of water and salts, and so there is an increase of Malpighian tubule secretion greater than 1,000 fold in response to feeding. Feeding and post-prandial diuresis in Rhodnius are highly coordinated events, including for example, host recognition, probing, injection of saliva, cuticle plasticization, passage of blood through the digestive system, diuresis and excretion. This review illustrates that many of the known functions of serotonin in Rhodnius are feeding-related. Serotonin coordinates or 'orchestrates' feeding-related physiological events either as a neurotransmitter/neuromodulator, delivered to target tissues in the nerve supply, or as a neurohormone, delivered by the haemolymph. Thus, serotonin has physiological effects upon the salivary glands, cuticle, digestive tract, cardiac muscle, and Malpighian tubules. By discussing these aspects, the review illustrates that serotonin acts in a coordinated manner to prepare Rhodnius for this energy-demanding process of feeding and diuresis.

Characterization of an ecto-phosphatase activity in malpighian tubules of hematophagous bug Rhodnius prolixus

We have characterized a phosphatase activity present on the external surface of intact Malpighian tubules in Rhodnius prolixus. This phosphatase hydrolyses the substrate p-nitrophenyl phosphate at a rate of 3.38 +/- 0.07 nmol Pi x mg(-1) x min(-1). Phosphatase activity decreased with the increase of the pH from 6.4 to 7.6 pH, a range in which tubules cellular integrity was maintained for at least 1 h. Classical inhibitors of acid phosphatase, such as ammonium molybdate, fluoride, vanadate, mpV-PIC, and bpV-PHEN, caused different patters of inhibition. The ecto-phosphatase present an apparent Km of 1.67 +/- 0.34 mM and Vmax of 5.71 +/- 0.37 nmol Pi x mg(-1) x min(-1) for p-NPP. Zinc chloride inhibited 78.2% of ecto-phosphatase activity, with Ki of 0.35 mM. Such inhibition was reversed by incubation with cysteine and GSH, but not DTT, serine, and GSSG, showing that cysteine residues are important for enzymatic activity. Phosphatase activity increased 141% three days after blood meal, and returned to basal levels 2 days later. These results suggest that ecto-phosphatase activity could be involved in a diuretic mechanism, essential in the initial days after a blood meal for the control of Rhodnius homeostasis.

Regulation of intermediary metabolism and water balance of insects by neuropeptides

Neuropeptides regulate all important physiological, developmental, and behavioral processes in insects. Here, I review two major physiological events that are hormonally controlled, namely intermediary metabolism and ion and water transport. Peptides belonging to the family of adipokinetic hormones (AKHs) increase hemolymph carbohydrates, lipids, and proline by activating the enzyme glycogen phosphorylase or lipase in the fat body. Moreover, these pleiotropic and multifunctional peptides inhibit protein-, lipid-, and RNA synthesis, and stimulate the frequency of contraction of certain muscles. Diuretic hormones that are related to the vertebrate corticotropin-releasing factor (CRF-related DHs) or belong to the family of kinins (which also have a myotropic action) or the cardioacceleratory peptides (CAPs), which increase the frequency of the heartbeat, all stimulate the secretion of fluid in Malpighian tubules (MTs) in vitro. Only a few true antidiuretic hormones are known: those from mealworms that inhibit the fluid transport in MTs in vitro, probably neuroparsins that stimulate water absorption by everted rectal sacs in vitro, and the desert locust's ion-transport peptide (ITP). Biosynthesis, release, receptors, mode of action, inactivation, structure-activity studies, and biological functions are discussed for the various peptides.

Regulation of chloride permeability by endogenously produced tyramine in the Drosophila Malpighian tubule

The Malpighian (renal) tubule of Drosophila melanogaster is a useful model for studying epithelial transport. The purpose of this study was to identify factors responsible for modulating transepithelial chloride conductance in isolated tubules. I have found that tyrosine and several of its metabolites cause an increase in chloride conductance. The most potent of these agonists is tyramine, which is active at low nanomolar concentrations; the pharmacology of this response matches that of the previously published cloned insect tyramine receptor. In addition, the tubule appears capable of synthesizing tyramine from applied tyrosine, as shown by direct measurement of tyrosine decarboxylase activity. Immunohistochemical staining of tubules with an antibody against tyramine indicates that the principal cells are the sites of tyramine production, whereas previous characterization of the regulation of chloride conductance suggests that tyramine acts on the stellate cells. This is the first demonstration of a physiological role for an insect tyramine receptor.

Evidence for CRF-like and kinin-like peptides as neurohormones in the blood-feeding bug, Rhodnius prolixus

In Rhodnius prolixus, the rapid post-feeding diuresis is under neurohormonal control. While serotonin has been demonstrated to be a diuretic neurohormone [J Exp Biol 156 (1991) 557], a peptide is also known to be involved. Previously, we have demonstrated the presence of corticotropin releasing factor (CRF)-like and kinin-like peptides in the central nervous system (CNS) of 5th instar Rhodnius [J Exp Biol 202 (1999) 2017; Peptides 22 (2001) 161]. These peptides are present in neurohemal sites of the corpus cardiacum and are co-localized in neurohemal sites on abdominal nerves. While various CRF-like peptides have been demonstrated to increase Rhodnius Malpighian tubule secretion the kinin-like peptides do not [Peptides 23 (2002) 671]. The kinin-like peptides do however, increase hindgut contraction which may contribute to the rapid post feeding diuresis by the mixing of hemolymph and/or hindgut contents and the removal of wastes. The presence of these peptides in neurohemal sites suggests that they could be released into the hemolymph and act as neurohormones. We have used immunohistochemical techniques and radioimmunoassay (RIA) to demonstrate qualitative and quantitative changes of CRF-like and kinin-like peptides in the CNS associated with feeding. As well we have examined Malpighian tubule secretion in response to assays of hemolymph from unfed and fed insects. Hemolymph was also partially purified by Sep-Pak and HPLC and the fractions assayed for kinin-like immunoreactivity and the ability to stimulate Malpighian tubule secretion. The results suggest that both kinin-like and CRF-like peptides are neurohormones in Rhodnius, released in response to feeding.

Localization of endogenous and recombinant Na(+)-driven anion exchanger protein NDAE1 from Drosophila melanogaster

Na(+)-dependent Cl(-)/HCO exchange activity helps maintain intracellular pH (pH(i)) homeostasis in many invertebrate and vertebrate cell types. Our laboratory cloned and characterized a Na(+)-dependent Cl(-)/HCO exchanger (NDAE1) from Drosophila melanogaster (Romero MF, Henry D, Nelson S, Harte PJ, and Sciortino CM. J Biol Chem 275: 24552--24559, 2000). In the present study we used immunohistochemical and Western blot techniques to characterize the developmental expression, subcellular localization, and tissue distribution of NDAE1 protein in D. melanogaster. We have shown that a polyclonal antibody raised against the NH(2) terminus of NDAE1 (alpha CWR57) recognizes NDAE1 electrophysiologically characterized in Xenopus oocytes. Moreover, our results begin to delineate the NDAE1 topology, i.e., both the NH(2) and COOH termini are intracellular. NDAE1 is expressed throughout Drosophila development in the central and peripheral nervous systems, sensilla, and the alimentary tract (Malpighian tubules, gut, and salivary glands). Coimmunolabeling of larval tissues with NDAE1 antibody and a monoclonal antibody to the Na(+)-K(+)-ATPase alpha-subunit revealed that the majority of NDAE1 is located at the basolateral membranes of Malpighian tubule cells. These results suggest that NDAE1 may be a key pH(i) regulatory protein and may contribute to basolateral ion transport in epithelia and nervous system of Drosophila.

Modes of control of insect Malpighian tubules: synergism, antagonism, cooperation and autonomous regulation

Rates of fluid and ion secretion by insect Malpighian tubules are controlled by peptides, including CRF-related peptides and kinins, and in some species by serotonin. It now appears to be a general rule that tubule secretion rate is controlled through the interaction of two or more haemolymph-borne factors. In this review we suggest that these interactions may be classified as synergistic, cooperative, or antagonistic. When presented together, two diuretic factors may act in synergism, so that fluid secretion is stimulated to a greater extent than the sum of their individual effects. Synergism may involve one or more second messenger systems. Alternatively, diuretic factors may act in cooperation, so that although their overall effects are additive, cation and anion transport pathways are controlled separately by distinct second messenger systems. There is also one example of antagonism between factors controlling tubule secretion and between their respective second messengers; one factor is stimulatory, the other is inhibitory. In addition to the complex control of fluid and ion transport by haemolymph-borne factors, sophisticated autonomous regulatory mechanisms have been identified in Malpighian tubules. When triggered by appropriate stimuli, these mechanisms play homeostatic roles, preserving haemolymph osmolality or ionic composition.

The distribution and function of serotonin in the large milkweed bug, Oncopeltus fasciatus. a comparative study with the blood-feeding bug, Rhodnius prolixus

The blood-feeding hemipteran, Rhodnius prolixus, ingests a large blood meal at the end of each larval stage. To accommodate and process this meal, its cuticle undergoes plasticisation, and its gut and Malpighian tubules respectively absorb and secrete a large volume of water and salts for rapid diuresis. Serotonin has been found to be integral to the feeding process in this animal, along with a diuretic peptide(s). The large milkweed bug, Oncopeltus fasciatus, tends to feed in a more continuous and abstemious manner, and therefore may have different physiological requirements than the blood feeder. Unlike R. prolixus, O. fasciatus is lacking serotonin-like immunoreactive dorsal unpaired median neurons in the mesothoracic ganglionic mass, and lacks serotonin-like immunoreactive neurohaemal areas and processes on the abdominal nerves, integument, salivary glands, and anterior junction of the foregut and crop. The salivary glands and crop do, however, respond to serotonin with increased levels of cAMP, while the integument and Malpighian tubules do not. In addition, O. fasciatus Malpighian tubules respond to both O. fasciatus and R. prolixus partially purified CNS extracts, which are likely to contain any native diuretic peptides. Thus, while serotonin and diuretic peptides may be involved in tubule control in R. prolixus, the latter may be of greater importance in O. fasciatus.

The distribution of a CRF-like diuretic peptide in the blood-feeding bug Rhodnius prolixus

The blood-feeding bug Rhodnius prolixus ingests a large blood meal, and this is followed by a rapid diuresis to eliminate excess water and salt. Previous studies have demonstrated that serotonin and an unidentified peptide act as diuretic factors. In other insects, members of the corticotropin-releasing factor (CRF)-related peptide family have been shown to play a role in post-feeding diuresis. Using fluorescence immunohistochemistry and immunogold labelling with antibodies to the Locusta CRF-like diuretic hormone (Locusta-DH) and serotonin, we have mapped the distribution of neurones displaying these phenotypes in R. prolixus. Strong Locusta-DH-like immunoreactivity was found in numerous neurones of the central nervous system (CNS) and, in particular, in medial neurosecretory cells of the brain and in posterior lateral neurosecretory cells of the mesothoracic ganglionic mass (MTGM). Positively stained neurohaemal areas were found associated with the corpus cardiacum (CC) and on abdominal nerves 1 and 2. In addition, Locusta-DH-like immunoreactive nerve processes were found over the posterior midgut and hindgut. Double-labelling studies for Locusta-DH-like and serotonin-like immunoreactivity demonstrated some co-localisation in the CNS; however, no co-localisation was found in the medial neurosecretory cells of the brain, the posterior lateral neurosecretory cells of the MTGM or neurohaemal areas. To confirm the presence of a diuretic factor in the CC and abdominal nerves, extracts were tested in Malpighian tubule secretion assays and cyclic AMP assays. Extracts of the CC and abdominal nerves caused an increase in the rate of secretion and an increase in the level of cyclic AMP in the Malpighian tubules of fifth-instar R. prolixus. The presence of the peptide in neurohaemal terminals of the CC and abdominal nerves that are distinct from serotonin-containing terminals indicates that the peptide is capable of being released into the haemolymph and that this release can be independent of the release of serotonin.

The effect of putative diuretic factors on in vivo urine production in the mosquito, Aedes aegypti

Changes in total urine production were measured in the mosquito, Aedes aegypti, following injection of 5-hydroxytryptamine (5-HT), Culex salinarius diuresin, culekinin depolarizing peptides (CDP-I, II and III) or the A. aegypti leucokinin peptides (ALP-I, II and III). All stimulated total urine production in a dose dependent manner. 5-HT was the least potent in urine production experiments with ED(50) values nearly 100-fold higher than other diuretic agonists. Doses greater than 2x10(-4) &mgr;moles inhibited urine production, suggesting either the occurrence of receptor down regulation, more than one type of 5-HT receptor, or increases in hindgut resorption of urine. The ALPs had relatively low ED(50) values compared to the CDPs suggesting that the endogenous peptides may have higher receptor binding affinities. Injection of mosquitoes with polyclonal antisera raised against either ALP-I or C. salinarius diuresin significantly reduced the response to injections of the respective peptides. The evidence presented above suggests that mosquito leucokinins and the C. salinarius diuresin function in the neuroendocrine regulation of urine production in the mosquito.

Immunogold labelling of serotonin-like and FMRFamide-like immunoreactive material in neurohaemal areas on abdominal nerves of Rhodnius prolixus

The ultrastructure of neurohaemal areas on abdominal nerves of the blood-sucking bug Rhodnius prolixus was investigated. Four types of axon terminals were found, distinguished by the morphology of their neurosecretory granules. By use of post-embedding immunogold labelling, granules in Type I axon terminals were shown to contain serotonin-like immunoreactive material, and granules in Type II axon terminals were shown to contain FMRFamide-like immunoreactive material. There was no colocalization of these materials. It is suggested that Type III terminals contain peptidergic diuretic hormone, which has previously been reported to be present in electron-dense neurosecretory granules in this neurohaemal area. The identity of material in Type IV terminals is unknown.

Cell membrane receptors and regulation of cell function in ticks and blood-sucking insects

Immunoglobulins cross the midgut epithelium and enter the haemolymph of many blood-feeding arthropods without losing their immunological properties. Antigens essential to the survival of the blood-sucking arthropods which may be affected by the small amounts of specific antibody that cross the gut epithelium include membrane receptors or other factors which regulate cell function. Membrane receptors implicated in transmembrane signalling in response to specific neural and endocrine factors fall into three major classes: (1) gated ion channels, (2) agonist-stimulated tyrosine kinases and (3) receptors that interact with GTP-binding (G) proteins. Examples of all three types have been found in insects and ticks. A dopamine receptor interacts with a G-protein essential for controlling fluid secretion by the salivary glands of ixodid ticks. Another receptor in the ixodid tick salivary gland binds a neuropeptide from the tick synganglion and stimulates turnover of plasma membrane phosphoinositides, but its mechanisms of transmembrane signalling and function remain elusive. Another large class of membrane receptors are those concerned with endocytosis. Examples of receptor-mediated endocytosis include incorporation of vitellogenin by developing oocytes in mosquitoes and ticks and uptake of lysed blood-meal components by digest cells of the tick gut. Many cell membrane receptors and possibly hormones could serve as targets for vaccines in blood-feeding insects and ticks. The major challenge is to identify and characterize essential internal receptors and cellular components that are accessible to and affected by specific antibodies that are introduced into the body of blood-feeding arthropods.

Cyclic AMP in the Malpighian tubule fluid and in the urine of Rhodnius prolixus

We demonstrate the presence of cyclic adenosine monophosphate (cAMP) in the fluid secreted by isolated Malpighian tubules of Rhodnius prolixus. In addition, we show that fifth-instar R. prolixus excrete cAMP in the urine after a meal of human blood. Nonstimulated isolated Malpighian tubules secrete small amounts of cAMP that increase about 10-fold after the addition of 5-hydroxytryptamine (5-HT). 5-HT is known to mimic R. prolixus diuretic hormone. The present findings demonstrate that 5-HT also acts via cAMP. The presence of cAMP in the rectal sac of the insect could be of importance in the differentiation of Trypanosoma cruzi and in the cycle of Chagas disease.

Locust diuretic hormone-stimulated synthesis and excretion of cyclic-AMP: a novel Malpighian tubule bioassay

Endogenous cyclic-AMP concentrations in Malpighian tubules of locusts increase as a result of diuretic hormone stimulation, and the level of increase is shown to be dependent on basal intracellular levels of cyclic-AMP. Furthermore, the increase in endogenous cyclic-AMP is in such excess as to cause excretion of high levels of cyclic-AMP in the urine.

5-Hydroxytryptamine stimulates fluid secretion in locust malpighian tubules independently of cAMP

5-Hydroxytryptamine (5-HT) stimulates fluid secretion by semi-isolated Malpighian tubules of Locusta in a dose-dependent manner. The threshold of stimulation is between 10(-8) and 10(-7) M 5-HT; maximal activation occurs at doses greater than 10(-6) M. Relative to the activation induced by diuretic hormone (storage lobe extracts), 5-HT increases the rate of fluid secretion by only 65%. Phentolamine, the alpha-adrenergic blocker, failed to inhibit either DH or 5-HT stimulated secretion. Diuretic hormone raises the levels of intracellular of cAMP, and activates adenylate cyclase in plasma membrane preparations of Locusta Malpighian tubules. 5-HT (10(-4) M) has no effect in either assay system. Thus 5-HT can stimulate fluid secretion independently of cAMP. A hypothetical model for hormone stimulated fluid secretion by Locusta Malpighian tubules, involving dual-receptor activation, is proposed. Other biogenic amines, including octopamine, adrenalin, dopamine, synephrine and the formamidine chlordimeform were tested for their ability to stimulate fluid secretion. Only dopamine showed a weakly stimulatory effect.

Whole-body distribution an Malpighian tubule transport of 2',5-dichloro-4'-nitrosalicylanilide (Bayer 73) and 3-trifluoromethyl-4-nitrophenol in larvae of the aquatic midge Chironomus tentans

1. Residues of the lampricides 14C-Bayer 73 (2',5-dichloro-4'-nitrosalicylanilide) and 14C-TFM (3-trifluoromethyl-4-nitrophenol) were absorbed from aqueous sublethal concn. by aquatic larvae of the midge Chironomus tentans. 2. Toxicant residues were widely distributed during early hours of exposure, but later and during a 24-h post-exposure period, they became concentrated in the trunk and intestinal tract; the posterior movement of Bayer 73 was more rapid than that of TFM. 3. During 4-h tests, Malpighian tubules of the chironomid actively transported Bayer 73 and TFM in vitro, and on a per tubule basis, more TFM was transported than Bayer 73. 4. Potassium cyanide inhibited tubule transport; serotonin had no effect.

From frog skin to sheep rumen: a survey of transport of salts and water across multicellular structures

All higher animals, whether they live in water or on dry land, are faced with the necessity of regulating rather closely their intake and excretion of salts and water in order to maintain the constancy of their internal ionic environment. The kidney is in general the most important organ of the body as far as the excretion of sodium, potassium, chloride and water is concerned, but there are other tissues which also play a part in controlling the ionic balance between the internal and external environments, such as the intestinal mucosa, the skin and urinary bladder in amphibia, the gill epithelium in fishes, the salt gland in marine birds, and the epithelium of the rumen in ruminants. In addition to excretory and absorptive organs of this type, there are others which are secretory and whose function involves the production of fluids differing in ionic composition from the blood plasma. Examples include the glands which secrete saliva and sweat, the oxyntic acid-producing cells of the gastric mucosa, and the epithelium of the stria vascularis which generates the potassium-rich endolymph of the mammalian cochlea. The purpose of this article is to consider briefly what is known about the active transport of salts and water across some typical multicellular secretory tissues, and to attempt in the process to discern what properties they have in common and in what respects they are specialized.