Sensory 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.

中文翻译：

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N-乙酰神经氨酸 (NANA) 激活海葵 (Aiptasia pallida) 分离的触手支持细胞上的 L 型钙通道

感觉受体控制海葵触须上的刺丝囊排出。微摩尔N-乙酰化糖（例如，N-乙酰神经氨酸 [NANA]）与外胚层支持细胞上的化学感受器结合，并通过依赖于环磷酸腺苷 (cAMP) 的致敏途径响应机械接触，使相邻的刺丝囊放电，而较高的 NANA 水平剂量依赖性脱敏。最近的证据表明 L 型钙通道对乌头海葵Aiptasia pallida（也称为Exaiptasia diaphana）的通路脱敏。因此，我们假设 NANA通过L型钙通道。我们证明了剂量依赖性、NANA 激活的⁴⁵ Ca 流入从A. pallida触手分离的分离的外胚层细胞中，在 NANA 的脱敏浓度下发生最大流入。L 型钙通道抑制剂硝苯地平、地尔硫卓、甲氧维拉帕米和镉阻断了 NANA 刺激的⁴⁵ Ca 流入。升高的细胞外 KCl 水平呈剂量依赖性增加，对硝苯地平敏感⁴⁵Ca 流入牵连电压门控钙通道。毛喉素、8-溴-cAMP 和蛋白激酶 A 抑制剂 H-8 以与激活的 cAMP 依赖性途径参与一致的方式影响 NANA 刺激的钙内流。因为 NANA 化学感受器定位于支持细胞复合物的支持细胞，NANA 激活⁴⁵ Ca 流入分离的触手外胚层细胞表明 L 型钙通道和 NANA 化学感受器共同定位于支持细胞。事实上，L 型钙通道的荧光标记定位于与活触手的刺胞相邻的顶端外胚层。我们得出结论，支持细胞化学感受器通过激活共定位 L 型钙通道一种依赖 cAMP 的机制，以启动脱敏。我们建议通路脱敏可以保护线虫在捕获猎物期间免于过度放电。