OXIDATIVE EFFECTS OF SELENITE ON RAT VENTRICULAR CONTRACTILITY AND CAMOVEMENTS

Objective: The study aimed at characterizing the effects of selenite, known for its reactivity with thiols, on cardiac contractility and exc itation-contraction coupling. Methods: The inotropic effects of seleni te were studied on rat papillary muscles. Freshly isolated rat ventric ular myocytes were used to determine the selenite-induced alterations in thiol contents, free Ca2+ levels (in fura-2 loaded cells), Ca2+ cur rents and contractile properties of skinned cells. Results: Selenite, at concentrations greater than or equal to 0.1 mM, affected muscle con tractions by inducing a transient positive inotropic effect (up to 120 +/- 3% of control in 1 mM selenite) followed by a gradual decline of developed tension together with an increase in resting tension (respec tively to 37 +/- 3 and 166 +/- 5% of their control values after 20 min exposure). These changes, irreversible on washout, could be reversed by the disulfide reducing agent dithiothreitol (DTT, 1 mM). Lowering t emperature from 35 degrees to 22 degrees C or preincubating the muscle s with the disulfonic stilbene SITS (0.2 mM) completely prevented the selenite-induced transient positive inotropy and rise in resting tensi on, In isolated myocytes, 10 min exposure to 1 mM selenite induced a 4 0 +/- 9% decrease of total sulfhydryl content. At this concentration, selenite rapidly caused a rise of basal [Ca2+](i) together with a dimi nution of the Ca2+ spike amplitude (respectively to 165 +/- 15 and 45 +/- 9% of their control values after 5 min exposure). In addition, sel enite significantly enhanced at each Ca2+ concentration the force gene rated by skinned myocytes. Ca2+ currents, measured at 22 degrees C, de creased by 28 +/- 8% in the presence of 1 mM selenite. These effects w ere reversed by DTT, Conclusions: The results demonstrate that selenit e, through alterations of cellular thiol redox status, induced a dual action on muscle contraction that can be imputed to a combined action on Ca2+ channels, Ca2+ transporters and contractile proteins, Extracel lular negative effects of selenite are due to a partial reduction of C a2+ current magnitude. Intracellular effects are mediated both by a di minution of Ca2+ handing by intracellular organelles and by a sensitiz ation of the contractile to Ca2+ ions. The results further indicate th at selenite uptake into the cardiac cells occurs mainly through the te mperature-sensitive anion exchanger.