Light induces changes in activities of Na+/K+-ATPase, H+/K+-ATPase and glutamine synthetase in tissues involved directly or indirectly in light-enhanced calcification in the giant clam, Tridacna squamosa

Abstract

The objective of this study was to determine the effects of 12 h of exposure to light, as compared with 12 h of exposure to darkness (control), on enzymatic activities of transporters involved in the transport of NH+4 or H+, and activities of enzymes involved in converting NH+4 to glutamate/glutamine in inner mantle, outer mantle, and ctenidia of the giant clam, Tridacna squamosa. Exposure to light resulted in a significant increase in the effectiveness of NH+4 in substitution for K+ to activate Na+/K+-ATPase (NKA), manifested as a significant increase in the Na+/NH+4-activated-NKA activity in the inner mantle. However, similar phenomena were not observed in the extensible outer mantle, which contained abundant symbiotic zooxanthellae. Hence, during light-enhanced calcification, H+ released from CaCO3 deposition could react with NH3 to form NH+4 in the extrapallial fluid, and NH+4 could probably be transported into the shell-facing inner mantle epithelium through NKA. Light also induced an increase in the activity of glutamine synthetase, which converts NH+4 and glutamate to glutamine, in the inner mantle. Taken together, these results explained observations reported elsewhere that light induced a significant increase in pH and a significant decrease in ammonia concentration in the extrapallial fluid, as well as a significant increase in the glutamine concentration in the inner mantle, of T. squamosa. Exposure of T. squamosa to light also led to a significant decrease in the N-ethylmaleimide (NEM)-sensitive-V-H+-ATPase (VATPase) in the inner mantle, and significant increases in the Na+/K+-activated-NKA, H+/NH+4-activated-H+/K+-ATPase, and NEM-sensitive-VATPase activities in ctenidia, indicating that light-enhanced calcification might perturb Na+ homeostasis and acid/base balance in the hemolymph, and might involve the active uptake of NH+4 from the environment. This is the first report on light having direct enhancing effects on activities of certain transporters/enzymes related to light-enhanced calcification in the inner mantle and ctenidia of T. squamosa.

Light-dependent expression of a Na+/H+exchanger 3-like transporter in the ctenidium of the giant clam,Tridacna squamosa, can be related to increased H+excretion during light-enhanced calcification

“…Rather, when taken together with information in the literature, our results indicate that the H + produced in the extrapallial fluid of T. squamosa during light‐enhanced calcification can be transported into the inner mantle tentatively through an unknown NH 4 + transporter (Ip et al. , ) or plasma membrane Ca 2+ ‐ATPase (Sano et al. ), and the H + is then shuttled through the hemolymph to the ctenidia where it is excreted by the apical TSNHE3 in exchange for Na + .…”

“…Alternatively, the symbiotic zooxanthellae residing in the extensible outer mantle or the ctenidia may act as “light‐sensing” elements and produce some signaling molecules for the clam host as proposed previously (Ip et al. ). These signaling molecules, after being released to the extracellular fluid of the clam host, may activate the transcription and translation of TSNHE3 /TSNHE3 in the epithelial cells of the ctenidia.…”

“…The ctenidium is nonpigmented and contains much lower quantities of symbiotic zooxanthellae as compared with the extensible outer mantle (Ip et al. ). A pair of ctenidia is positioned inside the mantle cavity shielded from direct illumination.…”

“…They were maintained in 350 L of recirculating seawater in a glass tank (L90 cm x W62 cm x H60 cm) under a 12 h light:12 h dark regime, with water conditions and light intensity as described previously (Ip et al. ), and acclimatized to laboratory conditions for at least 1 month before experiments. All applicable international, national, and/or institutional (National University of Singapore) guidelines for the care and use of animals were followed.…”

“…Ip et al. (, ) proposed that, during light‐enhanced calcification, H + released from CaCO 3 deposition could react with NH 3 to form NH 4 + in the extrapallial fluid, and NH 4 + could be transported subsequently into the shell‐facing epithelial cells of the inner mantle. There is also indirect evidence which suggests the involvement of a Ca 2+ ‐ATPase in light‐enhanced calcification in Tridacna derasa (Sano et al.…”