The marine secondary metabolites 2,4-dibromophenol and 2,4,6-tribromophenol differentially modulate voltage dependent ion currents in neuroendocrine (PC12) cells

Abstract

2,4-Dibromophenol (2,4-DBP) and 2,4,6-tribromophenol (2,4,6-TBP) are marine secondary metabolites, with 2,4,6-tribromophenol playing an important role as industrially produced flame retardant and pesticide. Both substances disturb cellular calcium signals in neuroendocrine cells as previously shown by Hassenklöver et al. (2006) [Hassenklöver, T., Predehl, S., Pilli, J., Ledwolorz, J., Assmann, M., Bickmeyer, U., 2006. Bromophenols, both present in marine organisms and in industrial flame retardants, disturb cellular Ca 2+ signaling in neuroendocrine cells (PC12). Aquat. Toxicol. 76, 37–45]. We investigated calcium channel currents in detail and outward membrane currents as potential cellular targets of both bromophenols. In this electrophysiological approach, 2,4-DBP reduced voltage dependent calcium channel currents with a half-maximal concentration of 45 ± 32 μM (S.D.) and a Hill coefficient of 0.87 ± 0.49 (S.D.). 2,4,6-TBP reduced calcium channel currents with a half-maximal concentration of 28 ± 19 μM (S.D.) and a Hill coefficient of 0.79 ± 0.31 (S.D.). The major contribution to calcium channel currents was mediated by L-type (67%) and N-type channels (30%) in PC12 cells; both bromophenols modulated both current types. Whole cell outward currents, mainly carried by potassium ions, were reduced by 2,4-DBP with a half-maximal concentration of 41 ± 9 μM (S.D.) showing a Hill coefficient of 1.71 ± 0.31 (S.D.). 2,4,6-TBP showed a weak reduction of outward currents at high concentrations of 300 μM. 2,4,6-TBP selectively decreased calcium entry via calcium channels as revealed in whole cell patch clamp experiments, whereas 2,4-DBP reduced both in- and outward currents.