AUTHOR=Ip Yuen K. , Ching Biyun , Hiong Kum C. , Choo Celine Y. L. , Boo Mel V. , Wong Wai P. , Chew Shit F. TITLE=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 JOURNAL=Frontiers in Physiology VOLUME=6 YEAR=2015 URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2015.00068 DOI=10.3389/fphys.2015.00068 ISSN=1664-042X 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.