In vitro effects of lead ions on peripheral benzodiazepine receptors and adenylyl cyclase activity in the mantle of Mytilus galloprovincialis

As an extension of our previous work, where the density of peripheral benzodiazepine receptors (PBR) increased in mantle mitochondria of the marine mollusk Mytilus galloprovincialis Lmk. under chronic exposure to lead, the present study investigates the in vitro effects of an exogenous source of lead ions on PBR and on adenylyl cyclase (AC) complex in mantle membranes of mussels collected from a non-polluted coastal area. PBR binding experiments used the specific isoquinoline carboxamide derivative [ 3H]PK 11195, and AC activity was measured using a modified procedure adapted to M. galloprovincialis. Lead ions (Pb 2+) dose-dependently decreased either the [ 3H]PK 11195 specific binding in mitochondria or basal AC velocity in plasma membranes of mussel mantle . The IC 50 values for lead ions were 10 μM with [ 3H]PK 11195 binding and 25 μM with AC activity, with maximal inhibition values of 60% and 70%, respectively. Moreover, lead behaved as a non-competitive inhibitor on [ 3H]PK 11195 binding and as a ‘mixed’ inhibitor on AC activity. The present results suggest that some of the early effects induced by lead in mussel cell metabolism consist in significant changes of the PBR density and cyclic AMP production in the mantle of M. galloprovincialis.