CADMIUM METABOLISM IN THE CLAM RUDITAPES DECUSSATA - THE ROLE OF METALLOTHIONEINS

Cadmium uptake, storage and metabolism was studied in the whole soft t issues, digestive gland and gills of the bivalve Ruditapes (Venerupis) decussata. Accumulation of cadmium was linear with time in the whole soft tissues. Much of the metal occurred in the cytosol, bound to meta llothionein, and this also increased with time of exposure. Measuremen ts of metallothionein, by differential pulse polarography, showed that in controls and clams exposed to cadmium (400 mu g 1(-l)) concentrati ons of the protein were present according to the sequence: digestive g land> gills> remaining tissues. When clams were exposed to sublethal c admium concentrations, metallothionein concentrations measured in the heat-treated cytosolic extracts of the whole soft tissues increased by small but significant amounts. Increases in metallothionein concentra tions were also observed in the digestive gland and, most notably, in gills. X-ray microanalysis of gills and digestive gland revealed that cadmium was not accumulated within structured lysosomes, mitochondria or other cell organelles. The low levels of significant de novo synthe sis of metallothionein in response to cadmium exposure in the whole so ft tissues and digestive gland of R. decussata reduce the value of the se tissue preparations as a means of detecting sublethal responses to metals: gills appear to be a more suitable tissue for detecting such r esponses.