Cd2+ and Hg2+ affect glucose release and cAMP-dependent transduction pathway in isolated eel hepatocytes

Isolated hepatocytes of the European eel (Anguilla anguilla) have been used as experimental model to characterize the effects of Cd2+ and Hg2+ on either basal or epinephrine-stimulated glucose release. Cd2+ strongly reduced glucose output from cells perifused in BioGel P4 columns and challenged with epinephrine, with a maximum inhibition of 95% reached at 10 muM (IC50 0.04 muM). The epinephrine-stimulated glucose output was also reduced by Hg2+ although a significant inhibition of about 60% was achieved only at 10 muM (IC50 5 muM). The possible influence of Cd2+ and Hg2+ on adenylyl cyclase/cAMP transduction pathway has been investigated, since this system is known to play a pivotal role in the regulation of fish liver glycogen breakdown and consequent glucose release. Micromolar concentrations of both heavy metals significantly reduced the epinephrine-modulated cAMP levels in isolated eel hepatocytes, in good agreement with the reduction of glucose output. Cd2+ and Hg2+ also significantly reduced basal and epinephrine-stimulated adenylyl cyclase activity in liver membrane preparations. A competitive inhibition with respect to Mg2+ was shown by Cd2+ and Hg2+ which significantly reduced the affinity of the allosteric activator for the adenylyl cyclase system. Apparent Kin for Mg2+ was 4.35 mM in basal conditions, and increased to 9.1 and 7.1 mM in the presence of 10 muM Cd2+ and Hg2+, respectively. These results indicate that Cd2+ and Hg2+ may impair a crucial intracellular transduction pathway involved in the adrenergic control of glucose metabolism, but also in several other routes of hormonal regulation of liver functions. (C) 2003 Elsevier Science B.V. All rights reserved.