X-ray microprobe analysis of epithelial calcium transport

Abstract

The sternal epithelium of Porcellio scaber was used as a novel model to study the subcellular elemental distribution in control and Ca 2+-transporting stages in situ. The anterior sternal epithelium (ASE) is specialized for transport of cuticular Ca to sternal CaCO 3 deposits during premolt, and from these deposits during intramolt. The less specialized posterior sternal epithelium transports Ca 2+ to and from the cuticle. In the ASE cells basal [Na], [Cl], and [Mg] are higher than in the apical side. The basal [Na] increases from 105 to 173 mmol/kg dry mass between control and Ca 2+-transporting stages, accompanied by a decrease in [Cl] and [K]. The [Mg] increases, suggesting transepithelial Mg 2+-transport. Cytosolic [Ca] varied insignificantly between 4.5 and 5.7 mmol/kg dry mass, however, the number of Ca hot-spots with concentrations between 15 and 50 mmol/kg dry mass increased during transport. Mitochondrial [Ca] decreased in the ASE from 3.3 in the control to 1.0 in the late premolt and to 2.0 mmol/kg dry mass in the intramolt stage. The results suggest Na +-dependent mechanisms for transcellular Ca 2+-transport and the presence of Ca 2+-binding proteins. Organelles, probably the smooth endoplasmic reticulum, sequester Ca 2+ during intracellular Ca 2+-transport. A role of mitochondria as a storage site for cuticular Ca is excluded.