Инсектные диуретические гормоны и регуляция водного баланса

Insect diuretic hormones are hormones 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, ions and essential metabolites are reabsorbed before the fluid is excreted. Excretion is under the control of diuretic and anti diuretic factors, or hormones. 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. 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 Unpublished studies show immunologically related peptides in Tenebrio molitor.

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.

First three letters of genus added to first two letters of the species name. For example, a 37 residue diuretic peptide from Tenebrio molitor will be named Tenmo DH37.