Mechanisms of Hyposmotic Volume Regulation in Isolated Nematocytes of the Anthozoan <i>Aiptasia diaphana</i>

Abstract

The nature and role of potassium (K) and water transport mediating hyposmotically-induced regulatory volume decrease (RVD) were studied in nematocytes dissociated with 605 mM thiocyanate from acontia of the Anthozoan Aiptasia diaphana. Cell volume and hence RVD were calculated from the inverse ratios of the cross sectional areas of nematocytes (A/A_o) measured before (A_o) and after (A) challenge with 65% artificial sea water (ASW). To distinguish between K channels and K-Cl cotransport (KCC), external sodium (Na) and chloride (Cl) were replaced by K and nitrate (NO₃), respectively. Inhibitors were added to identify K channels (barium, Ba), and putative kinase (N-ethylmaleimide, NEM) and phosphatase (okadaic acid, OA) regulation of KCC. In 65% NaCl ASW, nematocytes displayed a biphasic change in A/A_o, peaking within 4 min due to osmotic water entry and thereafter declining within 6 min due to RVD. Changing NaCl to KCl or NaNO₃ ASW did not affect the osmotic phase but attenuated RVD, consistent with K channel and KCC mechanisms. Ba (3 mM) inhibited RVD. NEM and OA, applied separately, inhibited the osmotic phase and muted RVD suggesting primary action on water transport (aquaporins). NEM and OA together reduced the peak A/A_o ratio during the osmotic phase whereas RVD was inhibited when OA preceded NEM. Thus, both K channels and KCC partake in the nematocyte RVD, the extent of which is determined by functional thiols and dephosphorylation of putative aquaporins facilitating the preceding osmotic water shifts.

Evidence for Aquaporin-mediated Water Transport in Nematocytes of the Jellyfish <i>Pelagia noctiluca</i>

“…1), when exposed to reduced salinity, display two phases of cell volume response: first, swelling of their cytoplasm, and second, shrinking to the original cell volume prior to returning these cells to isosmotic media. Whereas the second phase due to RVD has been characterized to some extent in these cells by this laboratory [12,15] the mechanisms in charge of the first phase, termed osmotic phase (OP) are largely unknown. The novel finding of this study is that the OP appears to be mediated by a HgCl 2 -sensitive transport mechanism: 0.1-25 µM HgCl 2 completely abrogated the OP and consequently, since no cell swelling occurred, inhibited RVD (Figs.…”

“…1). La Spada and co-workers [12][13][14][15] have investigated the RVD mechanisms in isolated nematocytes, but studies on the OP after hyposmotic challenge are still missing.…”

Sea Water Acidification Affects Osmotic Swelling, Regulatory Volume Decrease and Discharge in Nematocytes of the JellyfishPelagia noctiluca

“…Both RVD and RVI have mostly been investigated in mammalian cells [10], but also in cells of lower vertebrates and invertebrates [11,12,13]. With regard to invertebrates, we have previously demonstrated cell volume regulation in response to both hyposmotic and hypertonic stress in isolated nematocytes, and suggested the involvement of transport systems for multiple ions (namely Cl - and K + for RVD and Na + for RVI) and aquaporins in this regulation [9,14,15,16,17]. …”

Cadmium affects osmotic phase and regulatory volume decrease in cultured human embryonic kidney cells

“…13,15 RVD mechanisms are highly conserved and common to many cells from evolutionary distant species. 14,[16][17][18] Similarly, the uptake of osmolytes, followed by gain of osmotically obliged water occurs under hypertonic shrinkage, leading to regulatory volume increase (RVI), a process exhibited by many cell types and mainly due to Na + influx. 13,14,19 Studies on the volume sensor transduction pathway for volume regulation 20,21 provide evidence for intracellular Ca 2+ involvement, protein phosphorylation, arachidonic acid and phosphoinositide turnover, as well as mechanical sensors, such as stretch-activated channels and N o n -c o m m e r c i a l u s e o n l y cytoskeleton.…”