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First published online August 28, 2009
Journal of Experimental Biology 212, 2977-2982 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.032177

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X-ray microanalysis of Rb⁺ entry into cricket Malpighian tubule cells via putative K⁺ channels

Alan T. Marshall^1,* and Peta L. Clode²

¹ Department of Zoology and Analytical Electron Microscopy Laboratory, Faculty of Science, Technology and Engineering, La Trobe University, Melbourne, VIC 3086, Australia
² Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Crawley, WA 6009, Australia

^* Author for correspondence (A.Marshall{at}latrobe.edu.au)

Accepted 16 June 2009

Elemental imaging by X-ray microanalysis of fully frozen-hydrated samples shows that when Malpighian tubules of the black field cricket (Teleogryllus oceanicus L.) are incubated in saline in which Rb⁺ has been substituted for K⁺, Rb⁺ replaces much of the cellular K⁺ in the main segment of control Malpighian tubules and this is prevented by incubation in saline containing Ba²⁺, a non-selective K⁺ channel blocker. Similarly the amount of cellular K⁺ is greatly reduced when tubules incubated in normal, i.e. K⁺ containing, saline are exposed to Ba²⁺. By considering the amounts of cellular K and Rb remaining in the main segments of tubules incubated in either K⁺ containing saline or Rb⁺ containing saline after Ba⁺ treatment, it is suggested that: (a) a major part (56%) of cellular K⁺ enters by Ba²⁺ sensitive K⁺ channels and that Rb⁺ can also enter by this route; (b) a smaller fraction (26%) of cellular K⁺ enters by a previously proposed Na⁺/K⁺/2Cl^– co-transporter, which can also transport Rb⁺; (c) a previously proposed Na⁺/K⁺-ATPase is responsible for maintaining a K⁺ pool (18%) in the tubule cells that is not exchangeable by Rb⁺; and (d) entry by this Na⁺/K⁺-ATPase is not available to Rb⁺.

Key words: Malpighian tubules, Teleogryllus oceanicus, X-ray microanalysis, barium, potassium channels, rubidium

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