. Physiology of nematocytes

Nematocytes are high specialized cells from Coelenterates Cnidaria. They synthesize the nematocyst, a secretory product of Golgi apparatus, consisting of a capsule wall containing an inverted tubule and a capsule fluid in which various toxins are stored. The most relevant aspects of these cells are: i) the biophysics and control of discharge mechanism; ii) toxicology of venom and impact on humans; iii) cell volume regulation capability. These features underlie both aggression and defence strategies of Cnidaria, and, hence, their survival since Precambrian to date. A general description of these features is provided by this presentation. i) Under an adequate chemico-physical stimulus, the nematocyte ejects the tubule that in turn either adheres to or penetrates into the prey, injecting the venomous substances contained in the capsule fluid. This phenomenon is called discharge and it is the most rapid process of exocytosis known today. The complete mechanism of discharge involves signal recognition, transmission, transduction and response. It is observed both in situ and in isolated nematocytes and is under Ca2+control. ii) Toxicological aspects of Cnidaria have been extensively investigated owing to their notable impact on public health. The accidental contact of bathers with Cnidaria may induce both severe local and systemic pathologies, due to the release of venom. Research suggests that the large variety of envenomation should be ascribed mainly to either an aspecific change in membrane permeability or to a specific action of venoms on voltage-dependent channels. To prove the toxicity of capsule fluid, different types of biological assays have been performed, such as cytolytic assay on cultured cells and hemolytic test, which is the most used. iii) The maintenance of cell volume in an anisosmotic medium is essential for cell survival and has been observed in a number of different cells. After exposure to hyposmotic medium (35% shock), cell volume of nematocyte increases rapidly, owing to water influx, and, thereafter decreases more or less slowly towards the control value, showing regulatory volume decrease (RVD). By replacing the hyposmotic medium with an isosmotic one, the cell volume decreases rapidly, reaching the control value. RVD mechanisms consist mainly in an increased conductance to K+ and consequently Cl-, as verified by specific inhibitors as quinine, NPPB, DIDS, gramicidin, and are triggered by Ca2+ from intracellular stores or from the external medium. Nematocytes also regulate their volume in hypertonic conditions (45% shock). After the exposure to hypertonic medium, cell volume decreases, owing to water efflux, and then increases towards the control value. Such a response is termed regulatory volume increase (RVI). RVI mechanism is mainly due to Na+/K+/2Cl- cotransport, as verified.by bumetanide and sodium free medium. Cell volume regulation, both in hypertonic and hypotonic conditions, show the morphological integrity and physiological viability of cell. Based on these features, nematocytes isolated from jellyfish and sea anemones are a suitable model to perform physiological and toxicological investigations.