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.
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.
authors: Department of Biological Sciences, National University of Singapore, Singapore Singapore ; The Tropical Marine Science Institute, National University of Singapore, Singapore Singapore.
authors: Yuen K
authors: YK
authors: Ip
authors: Department of Biological Sciences, National University of Singapore, Singapore Singapore ; The Tropical Marine Science Institute, National University of Singapore, Singapore Singapore.
authors: Biyun
authors: B
authors: Ching
authors: Department of Biological Sciences, National University of Singapore, Singapore Singapore ; The Tropical Marine Science Institute, National University of Singapore, Singapore Singapore.
authors: Kum C
authors: KC
authors: Hiong
authors: Department of Biological Sciences, National University of Singapore, Singapore Singapore ; The Tropical Marine Science Institute, National University of Singapore, Singapore Singapore.
authors: Celine Y L
authors: CY
authors: Choo
authors: Department of Biological Sciences, National University of Singapore, Singapore Singapore ; The Tropical Marine Science Institute, National University of Singapore, Singapore Singapore.
authors: Mel V
authors: MV
authors: Boo
authors: Department of Biological Sciences, National University of Singapore, Singapore Singapore.
authors: Wai P
authors: WP
authors: Wong
authors: Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, Singapore Singapore.
authors: Shit F
authors: SF
authors: Chew
keywords: Ca2+-ATPase; H+-ATPase; ammonia; mantle; tridacnid; zooxanthellae