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First published online January 31, 2006
Journal of Experimental Biology 209, 677-688 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02052

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Suppression of Na⁺/K⁺-ATPase activity during estivation in the land snail Otala lactea

Christopher J. Ramnanan and Kenneth B. Storey^*

Institute of Biochemistry and Department of Biology, College of Natural Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6

^* Author for correspondence (e-mail: kenneth_storey{at}carleton.ca)

Accepted 20 December 2005

Entry into the hypometabolic state of estivation requires a coordinated suppression of the rate of cellular ATP turnover, including both ATP-generating and ATP-consuming reactions. As one of the largest consumers of cellular ATP, the plasma membrane Na⁺/K⁺-ATPase is a potentially key target for regulation during estivation. Na⁺/K⁺-ATPase was investigated in foot muscle and hepatopancreas of the land snail Otala lactea, comparing active and estivating states. In both tissues enzyme properties changed significantly during estivation: maximal activity was reduced by about one-third, affinity for Mg.ATP was reduced (K_m was 40% higher), and activation energy (derived from Arrhenius plots) was increased by 45%. Foot muscle Na⁺/K⁺-ATPase from estivated snails also showed an 80% increase in K_m Na⁺ and a 60% increase in K_a Mg²⁺ as compared with active snails, whereas hepatopancreas Na⁺/K⁺-ATPase showed a 70% increase in I₅₀ K⁺ during estivation. Western blotting with antibodies recognizing the alpha subunit of Na⁺/K⁺-ATPase showed no change in the amount of enzyme protein during estivation. Instead, the estivation-responsive change in Na⁺/K⁺-ATPase activity was linked to posttranslational modification. In vitro incubations manipulating endogenous kinase and phosphatase activities indicated that Na⁺/K⁺-ATPase from estivating snails was a high phosphate, low activity form, whereas dephosphorylation returned the enzyme to a high activity state characteristic of active snails. Treatment with protein kinases A, C or G could all mediate changes in enzyme properties in vitro that mimicked the effect of estivation, whereas treatments with protein phosphatase 1 or 2A had the opposite effect. Reversible phosphorylation control of Na⁺/K⁺-ATPase can provide the means of coordinating ATP use by this ion pump with the rates of ATP generation by catabolic pathways in estivating snails.

Key words: metabolic rate depression, reversible phosphorylation, hepatopancreas, foot muscle, regulation of ion pumps, land snail Otala lactea

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