Malpighian tubule

Far less is known about excretion by terrestrial insects than by terrestrial mammals. Metabolism can take place in the midgut and fat body. Excretion can occur via the malpighian tubules. 

In culture medium containing 80 pg Cu/cm2 for 4 weeks Survival reduced 35% copper elevated in cytoplasm, Malpighian tubule epithelial cells, and other tissues 6... 

The removal of OA from the region of the receptor by diffusion, reuptake into neighboring cells, and subsequent metabolism e.g. by N-acetylation or perhaps by the action of monoamine oxidase in the Malpighian tubules. 

Kay The only time we have done that is with Malpighian tubules in culture, where we can almost get single-cell distinction in culture. I wouldn t trust our conclusions on that, which is that we saw tau polymorphism in individual Malpighian tubules in culture. 

Borden J. H. and Slater C. E. (1969) Sex pheromone of Trypodendron lineatum production in the female hindgut-Malpighian tubule region. Ann. Ent. Soc. Am. 62, 454-455. 

Malpighian tubules The excretory organs of insects a set of long tubules that open into the gut. 

Most of these studies have established that the enzyme is located in the mitochondrial fraction of tissue homogenates, although the enzyme in the fat body and Malpighian tubules of Locusta is reportedly associated with the microsomal fraction (30). All studies concur that the enzyme is a cytochrome P-450-mediated mixed-function oxidase and its requirements for NADPH and 02 and sensitivity to inhibitors such as carbon monoxide, metyrapone, etc., support this conclusion. As yet, there are no reports as to whether the enzyme is associated with an iron sulfur protein similar to the adrenodoxin of the mammalian mitochondrial steroid... 

As can be seen from Table I a substantial number of alkaloids display significant insect toxicity, including nicotine, pipeline, lupine alkaloids, caffeine, gramine, strychnine, berberine, ephedrine, and steroidal alkaloids. Only the specialists can tolerate the respective alkaloids. The tobacco homworm (Manduca sexto), for example, can grow on a diet with more than 1% nicotine without any adverse effects. Most of the nicotine is either degraded or directly eliminated via the Malpighian tubules and in feces 182). Because nicotine binds to the acetylcholine (ACH) receptor, it is likely that in Manduca this receptor has been modified in such a way that ACH can still bind, but not nicotine (so-called target site modification). 

Nitrobenzene reductase activity has been detected in the fat body, gut, and Malpighian tubules of the Madagascar cockroach, G. portentosa (Rose and Young, 1973). Anaerobic conditions are essential for activity. The enzymes in the microsomes are strongly NADH dependent, whereas those in the soluble fraction are strongly NADPH dependent. Activity is enhanced by the addition of flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) or riboflavin. It appears that the true substrate for the nitroreductase is FMN and that the reduction of the nitro compounds occurs nonenzymatically (Figure 8.15). Similar results are obtained using azofuchsin as substrate. 

Rafaeli et ai. have recently reported the existence of an adrenocorticotropin (ACTH)-like material in locust CC (5.) detectable by immunoassay. They also reported that ACTH at 10 mM stimulated liquid excretion from, and elevated levels of cAMP in, locust Malpighian tubules. Extracts of CC produced similar effects in the traditional in vitro assay (fi.). They believe that a diuretic peptide in locusts has at least immunological similarity to ACTH. While the partial sequence reported (4.) by Morgan et ai. has no apparent similarity to ACTH, it is conceivable that these groups could be investigating the same factor. 

To check whether the diuretic hormone acts on Malpighian tubules via cAMP, intact tubules were incubated with synthetic F2 for various periods of time, then extracted and assayed for cAMP using an RIA. Addition of the synthetic antiparallel dimer elevated tissue cAMP concentration at all times tested. 

Only one of the four reports on characterization of diuretic peptides relied on a direct assay of the effects of extracts on Malpighian tubules (2 S.). A more common strategy has been the use of a less laborious bioassay. 

The related achetakinins double the rate of fluid secretion by isolated Acheta Malpighian tubules (Coast, G. M., et al. J. Insect Phvsiol.. in press.)- In addition, some leucokinins stimulate fluid secretion and/or depolarize transepithelial membrane potentials in the malpighian tubules of the yellow fever mosquito, Aedes aegvpti. in a fashion similar to several uncharacterized peptides isolated from mosquito head extracts (23). Thus, the neuropeptide family of leucokinins, achetakinins, and homologs may function in the control of water and ion balance, as well as myotropic activity, in a number of insects. To determine the relationships of the C-terminal sequence of the leucokinins to myotropic activity, analogs were synthesized and tested on the isolated cockroach hindgut. 

High performance liquid chromatography with coulometric electrochemical detection was used to separate and quantify octopamine and N-acetyl octopamine in haemolymph and incubated tissues of the American cockroach, Periplaneta americana. Injected octopamine is rapidly removed from the insect haemocoel and this removal of octopamine occurs concomitantly with elevated haemolymph levels of N-acetyl octopamine. Studies with isolated tissues indicate that the gut, especially the Malpighian tubules, contribute substantially to the removal of octopamine by N-acetylation. The inclusion of the formamidine pesticide, N-demethylchlordimeform in incubations containing octopamine and Malpighian tubules inhibits the N-acetylation of octopamine by the incubated tissue. 

Under the experimental conditions, the capacity of the Malpighian tubules to remove OA may be calculated as 261 pmoles/min, which is compatible with a calculated rate of production of N-acetyl OA of 250 pmoles/min. The OA level of haemolymph following flight-induced excitation rises to 65 nM (17) and, again assuming a haemolymph volume of 175 yl this amounts to 11.38 pmoles/insect. These calculations indicate that the capacity of Malpighian tubules... 

Table IV. Rate of Removal of -Octopamine and Production of N-Acetyl -Octopamine by Incubated Malpighian Tubules of Periplaneta americana...

The present study provides a convenient protocol for monitoring the ability of Malpighian tubules and other tissues to remove OA from incubation medium and convert it to N-acetyl OA. The results obtained with DCDM also indicate the potential to interfere with the process. Studies are currently in progress to determine the capacity of other tissues to effect N-acetylation of OA and to screen a spectrum of potential blockers of the effect. 

In insects, ecdysone is secreted from the prothoracic glands (PG) at every moulting and pupal stage. Conversion of ecdysone to 20-hydroxyecdysone occurred in fat body, Malpighian tubules, gut and body wall tissues [189]. Although ecdysone may have direct hormonal effects, it is generally held that ecdysone serves as a prehormone that is converted to 20-hydroxyecdysone which functions as the active hormone. This is because in many bioassay systems, 20-hydroxyecdysone appears to be much more active than ecdysone. 

The (myo) kinin family of peptides is especially a very interesting one, because its members have two very different functional activities in insects. The first members of this family were originally isolated on the basis of stimulation of the cockroach hindgut in vitro from extracts of whole heads of L. maderae [151]. This species contains 8 isoforms of kinins. Shortly after their first isolation, it was reported that these kinins also stimulate diuretic activity of isolated Malpighian tubules of the yellow fever mosquito Aedes aegypti [175], This function of kinins has now been shown regularly in conspecific diuretic assays with a number of other insect species. To date, the structure of kinins (see Table 5) have been fully elucidated in insects from the American cockroach (8 isoforms), the house cricket, Acheta domesticus (5 isoforms), the migratory locust (1 isoform), the mosquitoes, Culex salinarius and A. 

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