Evidence for a Na/K pump in nematocytes isolated from sea anemones

Abstract

Nematocytes, the stinging cells of the primitive marine animals jellyfish and sea anemones respond with regulatory volume decrease (RVD) to osmotic swelling in 35% diluted artificial sea water (65% ASW, Marino et al., Cell Physiol Biochem 2010). Since RVD is attained through K+ and Cl‐efflux via channels and cotransporters, an active pump mechanism must be required to maintain the cellular K+ gradient and expel Na, which was here explored. Cells were isolated from acontia of the anemone Aiptasia mutabilis (Strait of Messina, Italy) by applying 605mM NaSCN plus 0.01mM Ca2+ and then exposed to isosmotic or 65% hyposmotic ASW in the presence and absence of ouabain, the specific Na/K pump inhibitor. Ouabain present at 10‐4 M, but not at 10‐6 and 10‐8M, induced significant cell swelling in isosmotic ASW suggesting Na/K pump inhibition and failure to extrude Na causing an osmotic influx of water. This effect was attenuated by replacing Na with choline in the ASW. Ouabain applied at 10‐4M prior to 65% ASW did not affect the initial osmotic swelling phase but inhibited RVD consistent with loss of K required for this process. Results suggest that nematocytes possess Na/K pumps with low ouabain affinity that actively extrude Na and maintain cellular K steady state levels, necessary to perform RVD.