The Roles of Ca2+ and cAMP in the Nematocyst Discharge of the Sea Anemone Tentacle

<p>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.</p> <p>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.</p> <p>This project explores the chemoreceptor signaling pathway controlling nematocyst discharge from tentacles of the sea anemone, <em>Aiptasia pallida</em>. 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.</p> <p>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<em> in situ</em> cAMP levels in the ectodermal layer of tentacles from <em>A. pallida</em>, but has no effect on the endodermal cAMP content. In addition, NANA activates cAMP-dependent protein kinase (PKA) in whole tentacle homogenates. </p> <p>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 NANAstimulated cAMP increase. This supports the fact that high Mg²⁺ levels inhibit nematocyst discharge, but calls into question published findings in which NANAinduced changes in stereociliary bundle length of excised, Mg²⁺ -anesthetized tentacles are attributed to cAMP.</p> <p>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.</p>

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physiology

Year

2000

Author dc:creator

• Ozacmak, Veysel Haktan

Contributors dc:contributor

• David A. Hessinger
• John Buchholz
• William H. Fletcher
• Glyne U. Thorington
• Lubo Zhang

• Physiology
• Sea anemones -- physiology; Nematocysts; Spatial behavior -- physiology; Stress -- metabolism; Predatory Behavior; Calcium Channel Blockers -- pharmacology

Statement dc:rights

• This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.

Language dc:language

English

Repository record dc:identifier

https://scholarsrepository.llu.edu/etd/691

OAI identifier oai:identifier

oai:scholarsrepository.llu.edu:etd-1805