Oxidative effects of selenite on rat ventricular contractility and Ca movements

The study aimed at characterizing the effects of selenite, known for its reactivity with thiols, on cardiac contractility and excitation-contraction coupling.

The inotropic effects of selenite were studied on rat papillary muscles. Freshly isolated rat ventricular myocytes were used to determine the selenite-induced alterations in thiol contents, free Ca2+ levels (in fura-2 loaded cells), Ca2+ currents and contractile properties of skinned cells.

Selenite, at concentrations > or = 0.1 mM, affected muscle contractions 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 (respectively to 37 +/- 3 and 166 +/- 5% of their control values after 20 min exposure). These changes, irreversible on washout, could be reversed by the disulfonic reducing agent dithiothreitol (DTT, 1 mM). Lowering temperature from 35 degrees to 22 degrees C or preincubating the muscles with the disulfonic stilbene SITS (0.2 mM) completely prevented the selenite-induced transient positive inotropy and rise in resting tension. In isolated myocytes, 10 min exposure to 1 mM selenite induced a 40 +/- 9% decrease of total sulfhydryl content. At this concentration, selenite rapidly caused a rise of basal [Ca2+]i together with a diminution of the Ca2+ spike amplitude (respectively to 165 +/- 15 and 45 +/- 9% of their control values after 5 min exposure). In addition, selenite significantly enhanced at each Ca2+ concentration the force generated by skinned myocytes. Ca2+ currents, measured at 22 degrees C, decreased by 28 +/- 8% in the presence of 1 mM selenite. These effects were reversed by DTT.

The results demonstrate that selenite, 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. Extracellular negative effects of selenite are due to a partial reduction of Ca2+ current magnitude. Intracellular effects are mediated both by a diminution of Ca2+ handing by intracellular organelles and by a sensitization of the contractile to Ca2+ ions. The results further indicate that selenite uptake into the cardiac cells occurs mainly through the temperature-sensitive anion exchanger.