Analysis of Ca(2+) uptake into the smooth endoplasmic reticulum of permeabilised sternal epithelial cells during the moulting cycle of the terrestrial isopod Porcellio scaber.

In terrestrial isopods, large amounts of Ca(2+) are transported across anterior sternal epithelial cells during moult-related deposition and resorption of CaCO(3) deposits. Because of its toxicity and function as a second messenger, resting cytosolic Ca(2+) levels must be maintained below critical concentrations during epithelial Ca(2+) transport, raising the possibility that organelles play a role during Ca(2+) transit. We therefore studied the uptake of Ca(2+) into Ca(2+)-sequestering organelles by monitoring the formation of birefringent calcium oxalate crystals in permeabilised anterior and posterior sternal epithelium cells of Porcellio scaber during Ca(2+)-transporting and non-transporting stages of the moulting cycle using polarised-light microscopy. The results indicate ATP-dependent uptake of Ca(2+) into organelles. Half-maximal crystal growth at a Ca(2+) activity, a(Ca), of 0.4 micromol l(-1) and blockade by cyclopiazonic acid suggest Ca(2+) uptake into the smooth endoplasmic reticulum by the smooth endoplasmic reticulum Ca(2+)-ATPase. Analytical electron microscopical techniques support this interpretation by revealing the accumulation of Ca(2+)-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 Ca(2+)-transporting stages of late premoult and intramoult. These results suggest a contribution of the smooth endoplasmic reticulum as a transient Ca(2+) store during intracellular Ca(2+) transit.