Characterization of ion channels and O2 sensitivity in gill neuroepithelial cells of the anoxia-tolerant goldfish (Carassius auratus)

The neuroepithelial cell (NEC) of the fish gill is an important model for O₂ sensing in vertebrates; however, a complete picture of the chemosensory mechanisms in NECs is lacking, and O₂ chemoreception in vertebrates that are tolerant to anoxia has not yet been explored. Using whole cell patch-clamp recording, we characterized four types of ion channels in NECs isolated from the anoxia-tolerant goldfish. A Ca²⁺-dependent K⁺ current (I_KCa) peaked at ~20 mV, was potentiated by increased intracellular Ca²⁺, and was reduced by 100 μM Cd²⁺. A voltage-dependent inward current in Ba²⁺ solution, with peak at 0 mV, confirmed the presence of Ca²⁺ channels. A voltage-dependent K⁺ current (I_KV) was inhibited by 20 mM tetraethylammonium and 5 mM 4-aminopyridine, revealing a background K⁺ current (I_KB) with open rectification. Mean resting membrane potential of –45.2 ± 11.6 mV did not change upon administration of hypoxia (Po₂ = 11 mmHg), nor were any of the K⁺ currents sensitive to changes in Po₂ during whole cell recording. By contrast, when the membrane and cytosol were left undisturbed during fura-2 or FM 1-43 imaging experiments, hypoxia increased intracellular Ca²⁺ concentration and initiated synaptic vesicle activity. 100 μM Cd²⁺ and 50 μM nifedipine eliminated uptake of FM 1-43. We conclude that Ca²⁺ influx via L-type Ca²⁺ channels is correlated with vesicular activity during hypoxic stimulation. In addition, we suggest that expression of I_KCa in gill NECs is species specific and, in goldfish, may contribute to an attenuated response to acute hypoxia.

NEW & NOTEWORTHY This study provides the first physiological characterization of oxygen chemoreceptors from an anoxia-tolerant vertebrate. Neuroepithelial cells (NECs) from the gills of goldfish displayed L-type Ca²⁺ channels and three types of K⁺ channels, one of which was dependent upon intracellular Ca²⁺. Although membrane currents were not inhibited by hypoxia during patch-clamp recording, this study is the first to show that NECs with an undisturbed cytosol responded to hypoxia with increased intracellular Ca²⁺ and synaptic vesicle activity.

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