Effects of CdCl 2 on electrophysiological parameters in the intestine of the teleost fish, Anguilla anguilla

Abstract

In the present study the action of cadmium, a well known environmental pollutant, on the salt absorptive function of the eel, Anguilla anguilla, intestine was evaluated by estimating the rate of net Cl − absorption expressed as short circuit current ( I sc, μA cm −2). In tissues mounted on Ussing chamber, I sc and transepithelial potential difference ( V t, mV) responded in a concentration dependent manner to the addition of CdCl 2 to the serosal or mucosal bathing solutions. The dose response curve indicated that the maximal inhibition of I sc and V t (90% inhibition) after 1 h of incubation was obtained at a concentration of 100 μM from the serosal side and 5 mM from the mucosal one. To understand the nature of I sc inhibition induced by cadmium, the response of electrical parameters to the heavy metal exposure was examined in tissues exposed to different substances known to block Cl − transport at different cellular levels. Hence, bumetanide, which is known to inhibit Cl − transport by blocking the luminal Na +–K +–2Cl − cotransporter and Cl −-conductive channels at the basolateral membrane of the enterocyte, added to the mucosal (10 μM) or the serosal solution (100 μM), abolished the response of I sc and V t to CdCl 2 treatment. Similar results were obtained when tissues were pre-treated with ouabain (serosal, 100 μM) or when Na + or Cl − were omitted from the bathing media. Furthermore, cadmium alters the permselectivity of the tight junctions since the magnitude of the diffusion potential evoked by an imposed serosa–mucosa NaCl gradient (2:1) was strongly reduced by addition of CdCl 2 to either serosal or mucosal solution. Microelectrode experiments point to a relative impermeability of the luminal membrane to the heavy metal since the luminal membrane potential ( V m) remains unchanged during perfusion with CdCl 2. The results suggest that cadmium acts on the transport process responsible for Cl −-absorption in eel intestine. Therefore, one of the factors contributing to the toxic effect of cadmium on fish could be related to the ion-balance disturbances of body fluids subsequent to the altered osmoregulatory function of the intestine in marine teleosts.