The role of calcium(2+) and cAMP in the nematocyst discharge of the sea anemone tentacle

The phylum Cnidarians are aquatic animals, including jellyfish, hydra, sea anemones, and corals. They are the simplest metazoans having a nervous system and are diploblastic. Cnidarians are obligate predators that capture prey using specialized stinging cells called cnidocytes. The cnidocyte contains a nematocyst, which is a capsule containing an inverted, hollow tubule. Prey contacting the tentacle triggers the nematocyst tubule to rapidly evert; a process called discharge. The everting tubule penetrates and envenomates the prey, which is carried to the mouth by tentacle movements.; Generally both chemical and mechanical stimuli are needed to trigger discharge. The cnidocytes, in sea anemones, are surrounded by two or more supporting cells that have chemoreceptors and possibly contact-sensitive mechanoreceptors (CSMs). Therefore, the cnidocyte/supporting cell complex (CSCC) controls nematocyst discharge.; This project explores the chemoreceptor signaling pathway controlling nematocyst discharge from tentacles of the sea anemone, Aiptasia pallida . Chemoreceptors respond to N-acetylated sugars, such as N-acetylneuraminic acid (NANA), which occur on prey surfaces. When stimulated, this chemosensory pathway sensitizes CSMs to trigger nematocyst discharge in response to physical contact.; Indirect evidence suggested that the NANA chemosensory pathway acts through the intracellular second-messenger, cyclic-AMP (cAMP). We now show that NANA dose-dependently increases in situ cAMP levels in the ectodermal. layer of tentacles from A. pallida, but has no effect on the endodermal cAMP content. In addition, NANA activates cAMP-dependent protein kinase (PKA) in whole tentacle homogenates.; High levels of extracellular Mg2+ are commonly used to anesthetize excised tentacles and to block discharge. We find that high levels of Mg2+ block the NANA stimulated cAMP increase. This supports the fact that high Mg2+ levels inhibit nematocyst discharge, but calls into question published findings in which NANA-induced changes in stereociliary bundle length of excised, Mg2+-anesthetized tentacles are attributed to cAMP.; We also find that NANA stimulates calcium influx into isolated tentacle ectodermal cells and that the influx is sensitive to various L-type calcium channel blockers, including dihydropyridines. The coincidence of the desensitization region of the nematocyst discharge curve with those NANA concentrations that most stimulate calcium influx, suggests a role of NANA-stimulated calcium influx in desensitization.