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The Journal of Experimental Biology 205, 1935-1942 (2002)
© 2002 The Company of Biologists Limited

Analysis of Ca²⁺ uptake into the smooth endoplasmic reticulum of permeabilised sternal epithelial cells during the moulting cycle of the terrestrial isopod Porcellio scaber

Monica Hagedorn and Andreas Ziegler^*

Zentrale Einrichtung Elektronenmikroskopie, Universität Ulm, 89069 Ulm, Germany

^* Author for correspondence (e-mail: andreas.ziegler{at}medizin.uni-ulm.de )

Accepted 18 April 2002

In terrestrial isopods, large amounts of Ca²⁺ are transported across anterior sternal epithelial cells during moult-related deposition and resorption of CaCO₃ deposits. Because of its toxicity and function as a second messenger, resting cytosolic Ca²⁺ levels must be maintained below critical concentrations during epithelial Ca²⁺ transport, raising the possibility that organelles play a role during Ca²⁺ transit. We therefore studied the uptake of Ca²⁺ into Ca²⁺-sequestering organelles by monitoring the formation of birefringent calcium oxalate crystals in permeabilised anterior and posterior sternal epithelium cells of Porcellio scaber during Ca²⁺-transporting and non-transporting stages of the moulting cycle using polarised-light microscopy. The results indicate ATP-dependent uptake of Ca²⁺ into organelles. Half-maximal crystal growth at a Ca²⁺ activity, a_Ca, of 0.4 µmol l^-1 and blockade by cyclopiazonic acid suggest Ca²⁺ uptake into the smooth endoplasmic reticulum by the smooth endoplasmic reticulum Ca²⁺-ATPase. Analytical electron microscopical techniques support this interpretation by revealing the accumulation of Ca²⁺-containing crystals in smooth membranous intracellular compartments. A comparison of different moulting stages demonstrated a virtual lack of crystal formation in the early premoult stage and a significant fivefold increase between mid premoult and the Ca²⁺-transporting stages of late premoult and intramoult. These results suggest a contribution of the smooth endoplasmic reticulum as a transient Ca²⁺ store during intracellular Ca²⁺ transit.

Key words: calcium oxalate, Crustacea, epithelial Ca²⁺ transport, biomineralisation, smooth endoplasmic reticulum Ca²⁺-ATPase, SERCA, sequestration, Ca²⁺-ATPase, isopod, Porcellio scaber

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