N-Acetyl Neuraminic Acid (NANA) Activates L-Type Calcium Channels on Isolated Tentacle Supporting Cells of the Sea Anemone (Aiptasia pallida).

AbstractSensory receptors control nematocyst discharge on sea anemone tentacles. Micromolar N-acetylated sugars (e.g., N-acetyl neuraminic acid [NANA]) bind chemoreceptors on ectodermal supporting cells and predispose adjacent nematocyst discharge in response to mechanical contact via a cyclic adenosine monophosphate (cAMP)-dependent sensitization pathway, while higher NANA levels dose-dependently desensitize. Recent evidence implicates L-type calcium channels in desensitizing the pathway in aconitate sea anemones Aiptasia pallida (also known as Exaiptasia diaphana). We, therefore, hypothesize that NANA activates calcium influx via L-type calcium channels. We demonstrate a dose-dependent, NANA-activated ⁴⁵Ca influx into dissociated ectodermal cells isolated from A. pallida tentacles, with maximal influx occurring at desensitizing concentrations of NANA. The L-type calcium channel inhibitors nifedipine, diltiazem, methoxyverapamil, and cadmium blocked NANA-stimulated ⁴⁵Ca influx. Elevated extracellular KCl levels dose-dependently increased nifedipine-sensitive ⁴⁵Ca influx to implicate voltage-gated calcium channels. Forskolin, 8-bromo-cAMP, and the protein kinase A inhibitor H-8 affect NANA-stimulated calcium influx in a manner consistent with activated cAMP-dependent pathway involvement. Because NANA chemoreceptors localize to supporting cells of cnidocyte supporting cell complexes, NANA activation of ⁴⁵Ca influx into isolated tentacle ectodermal cells suggests that L-type calcium channels and NANA chemoreceptors co-localize to supporting cells. Indeed, a fluorescent marker of L-type calcium channels localizes to the apical ectoderm adjacent to nematocysts of live tentacles. We conclude that supporting cell chemoreceptors activate co-localized L-type calcium channels via a cAMP-dependent mechanism in order to initiate desensitization. We suggest that pathway desensitization may conserve nematocysts from excessive discharge during prey capture.