Insect diuretic hormones
Encyclopedia
Insect diuretic hormones are hormone
s that regulate water balance
through diuretic
action.
The insect
excretory system
, responsible for regulating water balance in the insect, comprises the Malpighian tubules and the hindgut
(the ileum
and rectum
). Malpighian tubules secrete primary urine
, most of which is passed into the hindgut where water, ion
s and essential metabolites are reabsorbed before the fluid is excreted. Excretion is under the control of diuretic
and anti-diuretic factors, or hormones, comprehensively reviewed by Coast et al. (2002).
These factors are produced in neurosecretory cells in the insect nervous system
, and stored and released from neurohaemal sites, such as the corpora cardiaca in the brain
. Before a factor can be attributed with the role of hormone, it needs to meet certain criteria. While there is evidence that some diuretic and antidiuretic factors are indeed circulating neurohormones, this has not been demonstrated for all factors investigated so far.
It has been known for many years that insects possess diuretic and antidiuretic factors, but it has only been comparatively recently that technological advances have allowed for them to be characterised. Antidiuretic factors are less well studied than diuretic factors. They act either on the Malpighian tubules to inhibit urine production, or on the hindgut to stimulate reabsorption. To date, the only insect for which both diuretic and antidiuretic hormones (acting directly on tubules) have been isolated is a beetle, the mealworm
Tenebrio molitor (Tenebrionidae).
Functions of diuretic and antidiuretic hormones include: postprandial diuresis, post-eclosion diuresis, excretion of excess metabolic water, clearance of toxic wastes and restricting metabolite
loss (Coast et al., 2002).
The effects of diuretic factors are tested in fluid secretion experiments, usually conducted using the Ramsay assay, in which isolated Malpighian tubules are placed in droplets of saline solution under liquid paraffin
. The ends of the tubules are drawn out of the saline and wrapped around minuten pins. The diameter of the secreted droplets is measured, whereby the volume, in nanolitres, and hence rate of secretion, can be calculated. Test substances are added to the saline and changes in secretion rate are recorded. Isolated tubules continue to secrete for many hours, and because the Ramsay assay is fairly easy to perform, many peptides are tested using this method. As a result, the effects of diuretic peptides are often tested only on the tubules and their role in other parts of the insect excretory system is not investigated.
There are three main families of diuretic hormones: the corticotropin-releasing factor (CRF)-related peptides, calcitonin
(CT)-like peptides and the insect kinin
s (Coast et al., 2002). These will be reviewed briefly. Others have been identified, but will not be discussed here.
CRF-related peptides are the best characterised and have been isolated from Blattodea, Isoptera, Orthoptera
, Coleoptera, Diptera
and Lepidoptera
. Insect CRF-related peptides are so-called because of their similarity to the CRF-related peptides of vertebrate
s, which indicates a long evolution
ary history. They range from 30 to 47 amino acid
residues in length. Although only a few orders are represented so far, CRF-related peptides are suspected to be ubiquitous in insects. They act by increasing cyclic AMP production in Malpighian tubules and appear to stimulate cation (K+/Na+) transport.
Originally known as the myokinins because of their myotropic activity, the kinin
s were first isolated from the Madeira cockroach, Leucophaea maderae and the cricket
, Acheta domesticus. Kinins are smaller than the CRF-related peptides (typically 6 - 15 residues long). They appear to have a non-selective effect on sodium chloride
and potassium chloride
secretion in tubules by opening a Ca2+-activated anion conductance, thus allowing more Cl- into the tubule. With the increase in available Cl-, additional Na+ and K+ can be transported into the lumen. Their effects are mimicked by the pharmacological
agent, thapsigargin, which is sometimes used in fluid secretion assays when a kinin is not available. Kinins are known from Blattodea, Orthoptera, Lepidoptera and Diptera.
Comparatively recently, a peptide was isolated from the cockroach Diploptera
punctata that showed no similarity to any known insect peptide but did show some similarity to vertebrate calcitonin (Furuya et al., 2000). The D. punctata peptide, subsequently named Dippu-DH31, turned out to be the first example of a whole new family of insect diuretic peptides – the calcitonin (CT)-like peptides. Dippu-DH31 was isolated using a cyclic AMP assay at the same time as a CRF-related peptide from the same insect. Ct-like peptides and CRF-related peptides both stimulate cAMP production by isolated tubules, but it is suspected that CT-like peptides target a different cAMP-dependent effector system or activate a different second messenger pathway. CT-like peptides have since been identified in other orders – Diptera, Lepidoptera, and one has been partially sequenced from Hymenoptera. Unpublished studies show immunologically-related peptides in Tenebrio molitor (Coast et al., 2002).
It appears that all insects possess diuretic factors from two or more families, indicating that fluid balance is very finely controlled. Synergism between the different factors has been demonstrated in a number of species
. However, although it is common, it should not be assumed to be universal. Also, in many cases, it is only the effects on fluid secretion that are measured, and while two factors may both increase fluid secretion, their effects on ion transport may be very different. Further studies examining the composition of the secreted fluid and electrophysiological experiments that explore ion movement, will shed more light on the actual physiological function of these factors in vivo
.
For example, in vivo experiments have demonstrated that the fluid secreted by the Malpighian tubules of the desert beetle, Onymacris plana is directed to the midgut for recycling to the haemolymph. In this way, metabolic wastes are rapidly cleared from the haemolymph without an associated loss of water, indicating that diuretic hormones may not always effect diuresis per se (Nicolson, 1991).
The rectal or cryptonephric complex is a structure in which the terminal parts of the Malpighian tubules are closely associated with the rectum. It withdraws water from the rectal contents, thereby limiting fecal water loss, and is even capable of withdrawing water from unsaturated air. The complex is only present in lepidopteran larvae and some beetle species. Little is known about the hormonal control of fluid uptake by the cryptonephric complex, which is certain to play an important role in the species in which it occurs.
added to first two letters of the species name. For example, a 37-residue diuretic peptide from Tenebrio molitor will be named Tenmo-DH37.
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s that regulate water balance
Water balance
In hydrology, a water balance equation can be used to describe the flow of water in and out of a system. A system can be one of several hydrological domains, such as a column of soil or a drainage basin....
through diuretic
Diuretic
A diuretic provides a means of forced diuresis which elevates the rate of urination. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way.- Medical uses :...
action.
The insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
excretory system
Excretory system
The excretory system is a passive biological system that removes excess, unnecessary or dangerous materials from an organism, so as to help maintain homeostasis within the organism and prevent damage to the body. It is responsible for the elimination of the waste products of metabolism as well as...
, responsible for regulating water balance in the insect, comprises the Malpighian tubules and the hindgut
Hindgut
The hindgut is the posterior part of the alimentary canal. In mammals, it includes the distal third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and rectum.-Blood flow:...
(the ileum
Ileum
The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...
and rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
). Malpighian tubules secrete primary urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
, most of which is passed into the hindgut where water, ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s and essential metabolites are reabsorbed before the fluid is excreted. Excretion is under the control of diuretic
Diuretic
A diuretic provides a means of forced diuresis which elevates the rate of urination. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way.- Medical uses :...
and anti-diuretic factors, or hormones, comprehensively reviewed by Coast et al. (2002).
These factors are produced in neurosecretory cells in the insect nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
, and stored and released from neurohaemal sites, such as the corpora cardiaca in the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. Before a factor can be attributed with the role of hormone, it needs to meet certain criteria. While there is evidence that some diuretic and antidiuretic factors are indeed circulating neurohormones, this has not been demonstrated for all factors investigated so far.
It has been known for many years that insects possess diuretic and antidiuretic factors, but it has only been comparatively recently that technological advances have allowed for them to be characterised. Antidiuretic factors are less well studied than diuretic factors. They act either on the Malpighian tubules to inhibit urine production, or on the hindgut to stimulate reabsorption. To date, the only insect for which both diuretic and antidiuretic hormones (acting directly on tubules) have been isolated is a beetle, the mealworm
Mealworm
Mealworms are the larval form of the mealworm beetle, Tenebrio molitor, a species of darkling beetle. Like all holometabolic insects, they go through four life stages: egg, larva, pupa, and adult...
Tenebrio molitor (Tenebrionidae).
Functions of diuretic and antidiuretic hormones include: postprandial diuresis, post-eclosion diuresis, excretion of excess metabolic water, clearance of toxic wastes and restricting metabolite
Metabolite
Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...
loss (Coast et al., 2002).
The effects of diuretic factors are tested in fluid secretion experiments, usually conducted using the Ramsay assay, in which isolated Malpighian tubules are placed in droplets of saline solution under liquid paraffin
Paraffin
In chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...
. The ends of the tubules are drawn out of the saline and wrapped around minuten pins. The diameter of the secreted droplets is measured, whereby the volume, in nanolitres, and hence rate of secretion, can be calculated. Test substances are added to the saline and changes in secretion rate are recorded. Isolated tubules continue to secrete for many hours, and because the Ramsay assay is fairly easy to perform, many peptides are tested using this method. As a result, the effects of diuretic peptides are often tested only on the tubules and their role in other parts of the insect excretory system is not investigated.
There are three main families of diuretic hormones: the corticotropin-releasing factor (CRF)-related peptides, calcitonin
Calcitonin
Calcitonin is a 32-amino acid linear polypeptide hormone that is producedin humans primarily by the parafollicular cells of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium , opposing the effects of parathyroid hormone . Calcitonin has been found...
(CT)-like peptides and the insect kinin
Kinin
A kinin is any of various structurally related polypeptides, such as bradykinin and kallikrein. They are members of the autacoid family.They act locally to induce vasodilation and contraction of smooth muscle.It is a component of the kinin-kallikrein system....
s (Coast et al., 2002). These will be reviewed briefly. Others have been identified, but will not be discussed here.
CRF-related peptides are the best characterised and have been isolated from Blattodea, Isoptera, Orthoptera
Orthoptera
Orthoptera is an order of insects with paurometabolous or incomplete metamorphosis, including the grasshoppers, crickets and locusts.Many insects in this order produce sound by rubbing their wings against each other or their legs, the wings or legs containing rows of corrugated bumps...
, Coleoptera, Diptera
Diptera
Diptera , or true flies, is the order of insects possessing only a single pair of wings on the mesothorax; the metathorax bears a pair of drumstick like structures called the halteres, the remnants of the hind wings. It is a large order, containing an estimated 240,000 species, although under half...
and Lepidoptera
Lepidoptera
Lepidoptera is a large order of insects that includes moths and butterflies . It is one of the most widespread and widely recognizable insect orders in the world, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...
. Insect CRF-related peptides are so-called because of their similarity to the CRF-related peptides of vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, which indicates a long evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary history. They range from 30 to 47 amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
residues in length. Although only a few orders are represented so far, CRF-related peptides are suspected to be ubiquitous in insects. They act by increasing cyclic AMP production in Malpighian tubules and appear to stimulate cation (K+/Na+) transport.
Originally known as the myokinins because of their myotropic activity, the kinin
Kinin
A kinin is any of various structurally related polypeptides, such as bradykinin and kallikrein. They are members of the autacoid family.They act locally to induce vasodilation and contraction of smooth muscle.It is a component of the kinin-kallikrein system....
s were first isolated from the Madeira cockroach, Leucophaea maderae and the cricket
Cricket (insect)
Crickets, family Gryllidae , are insects somewhat related to grasshoppers, and more closely related to katydids or bush crickets . They have somewhat flattened bodies and long antennae. There are about 900 species of crickets...
, Acheta domesticus. Kinins are smaller than the CRF-related peptides (typically 6 - 15 residues long). They appear to have a non-selective effect on sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
and potassium chloride
Potassium chloride
The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...
secretion in tubules by opening a Ca2+-activated anion conductance, thus allowing more Cl- into the tubule. With the increase in available Cl-, additional Na+ and K+ can be transported into the lumen. Their effects are mimicked by the pharmacological
Pharmacology
Pharmacology is the branch of medicine and biology concerned with the study of drug action. More specifically, it is the study of the interactions that occur between a living organism and chemicals that affect normal or abnormal biochemical function...
agent, thapsigargin, which is sometimes used in fluid secretion assays when a kinin is not available. Kinins are known from Blattodea, Orthoptera, Lepidoptera and Diptera.
Comparatively recently, a peptide was isolated from the cockroach Diploptera
Diploptera
Diploptera is a genus of cockroach, used particularly in molecular research into allatostatins. The species Diploptera punctata, the Pacific beetle cockroach, is the only known viviparous cockroach ....
punctata that showed no similarity to any known insect peptide but did show some similarity to vertebrate calcitonin (Furuya et al., 2000). The D. punctata peptide, subsequently named Dippu-DH31, turned out to be the first example of a whole new family of insect diuretic peptides – the calcitonin (CT)-like peptides. Dippu-DH31 was isolated using a cyclic AMP assay at the same time as a CRF-related peptide from the same insect. Ct-like peptides and CRF-related peptides both stimulate cAMP production by isolated tubules, but it is suspected that CT-like peptides target a different cAMP-dependent effector system or activate a different second messenger pathway. CT-like peptides have since been identified in other orders – Diptera, Lepidoptera, and one has been partially sequenced from Hymenoptera. Unpublished studies show immunologically-related peptides in Tenebrio molitor (Coast et al., 2002).
It appears that all insects possess diuretic factors from two or more families, indicating that fluid balance is very finely controlled. Synergism between the different factors has been demonstrated in a number of species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. However, although it is common, it should not be assumed to be universal. Also, in many cases, it is only the effects on fluid secretion that are measured, and while two factors may both increase fluid secretion, their effects on ion transport may be very different. Further studies examining the composition of the secreted fluid and electrophysiological experiments that explore ion movement, will shed more light on the actual physiological function of these factors in vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
.
For example, in vivo experiments have demonstrated that the fluid secreted by the Malpighian tubules of the desert beetle, Onymacris plana is directed to the midgut for recycling to the haemolymph. In this way, metabolic wastes are rapidly cleared from the haemolymph without an associated loss of water, indicating that diuretic hormones may not always effect diuresis per se (Nicolson, 1991).
The rectal or cryptonephric complex is a structure in which the terminal parts of the Malpighian tubules are closely associated with the rectum. It withdraws water from the rectal contents, thereby limiting fecal water loss, and is even capable of withdrawing water from unsaturated air. The complex is only present in lepidopteran larvae and some beetle species. Little is known about the hormonal control of fluid uptake by the cryptonephric complex, which is certain to play an important role in the species in which it occurs.
Nomenclature
First three letters of genusGenus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
added to first two letters of the species name. For example, a 37-residue diuretic peptide from Tenebrio molitor will be named Tenmo-DH37.