Metals and phosphate in the chloragosomes of Lumbricus terrestris and their possible physiological significance

Abstract

The chloragog cells of the earthworm Lumbricus terrestris contain numerous granules, chloragogomes, which were analyzed for metals and phosphate by histochemistry, by use of an electron microscope X-ray microprobe (EMMA) and by chemical analysis of chloragosome preparations. Inorganic and organic phosphate each accounts for about 3% of the chloragosome dry mass, Ca for 2--3%, Zn for 1--3% and Mg for 0.2--0.4%. Carbonate is not present in chloragosomes. The average molar ratios Ca:Mg:Zn:total PO4 are 1:0.1:0.3:1. The Ca:PO4 ratio is fairly constant (correlation coefficient 0.99), while the Zn:PO4 ratio varies considerably. It is concluded that Ca is bound in the form of inorganic CaHPO4 and organic R-OPO3Ca (or possibly Ca-polyphosphate complexes). Mg may also be phosphate-bound, while Zn probably is not. Chemical analysis of the calciferous glands revealed a high concentration of Ca, small amounts of Mg and phosphate, but no Zn. It is concluded that Zn is not excreted through the calciferous gland. Storage of Ca in the chloragocytes and excretion of CaCO3 by the calciferous gland may be physiologically linked. Regulation of the concentrations of Ca and HCO3- ions in the blood and coelomic fluid may assist in equalization of osmotic pressures during dehydration and rehydration. This regulation may be a major function of the chloragosomes. The chloragosomes were discussed in relation to the "spherites" of various arthropods and molluscs and to the "cytosomes" of anoxia-tolerating molluscs.