Diuretic Hormone—Extraction and Chemical Properties

Abstract

The control of diuresis in insects and in particular the blood sucking bug Rhodnius prolixus has been described by Maddrell (1980) in the previous chapter. However, the characterization of an insect diuretic hormone (DH) has proved to be an elusive goal. The hormonal control of diuresis has been reported for several insects (see Table 4–1 for a summary). In general, it appears that at least two distinct anatomical designs exist for the synthesis, storage, and release of diuretic hormones and these clearly have a direct bearing on the method of extraction employed. The diuretic hormones of several insects listed in Table 4–1 appear to be synthesized by neurosecretory cells in the brain and are transported to, and stored by, the corpora cardiaca, from where they are subsequently released. In other insects, such as R. prolixus and Glossina austeni, which feed on blood and have a requirement for very high rates of urine excretion, the hormone is synthesized in the posterior neurosecretory cells of the mesothoracic ganglionic mass (R. prolixus) or the thoracic ganglion (G. austeni). The hormone appears to be stored mainly in the tissue of synthesis and partly at the site of release on the abdominal nerves. An additional complication is that all parts of the central nervous system of R. prolixus contain extractable diuretic activity (Maddrell 1963). Furthermore, 5-hydroxytryptamine (5-HT), a powerful mimic of the authentic diuretic hormone from R. prolixus, has been identified in the central nervous tissue of insects (Welsh and Moorhead 1960; Hiripi and Salanki-Rozsa 1974). Consequently, some caution is required to en-sure that substances that are biologically active but not normally destined for circulation in the hemolymph are distinguished from the authentic hormone.