Cell volume regulation in hepatopancreas of Mytilus galloprovincialis exposed to subletal concentrations of CdCl2

Our previous studies showed that the digestive cells of the Mytilus galloprovincialis hepatopancreas are able to exhibit a regulatory volume decrease (RVD) after the swelling produced by hypotonic stress. Preincubation of the isolated cells with CdCl2 (1 and 10 mM) inhibited this homeostatic response. In the present work the RVD response was studied in the digestive cells of Mytilus galloprovincialis exposed to Cd2+ (40 mg.l-1) for 21 days and was compared with that of control animals held in the same laboratory conditions but in the absence of the metal. During the exposition time no animal mortality was registered. However the isolated digestive cells of the treated animals, unlike those of the control ones, were not able to perform RVD. In the attempt to guess the possible mechanisms affected by Cd2+ and responsible of the impaired homeostatic response, we tested the involvement of two well known targets of the metal in the RVD response such as the cytoskeleton and the Na+- K+-ATPase. The observation that the cells of the control animals do not exhibit RVD when preincubated in the presence of 10-5 M cytocalasin D, know inhibitor of actin polymerization, and of 10-3 M ouabain, known inhibitor of the pump, suggests that: 1) the integrity of cytoskeleton is essential for the RVD response; 2) the ionic transport mechanisms responsible of the recovery of cell volume during the hypotonic stress are dependent on the operation of the Na+-K+-ATPase. These results led us to hypothesize that the inhibitory effect of RVD produced by Cd2+ could be due to its effect on the cytoskeleton and/or the pump. However a toxic effect on calcium homeostasis cannot be excluded since we showed the strong dependence of RVD by Ca2+. Other studies, by employing different techniques, will be necessary to shed light on the biochemical and molecular target of the heavy metal.