Inwardly Rectifying and Voltage-gated Outward Potassium Channels Exhibit Low Sensitivity to Methylmercury

The concentration- and time-dependence of effects of methylmercury (MeHg) on voltage-gated outward K+ (Kv) channels, inwardly rectifying K+ (Kir) channels, voltage-gated Ca2+ channels and GABAA receptor activated channels were compared in cerebellar granule cells in culture using whole cell patch clamp recording techniques. The objective was to determine if MeHg equally affects different types of ion channels. Under similar experimental conditions, these four ion channel types displayed markedly different sensitivity to MeHg. At 0.1-1 μM, MeHg caused apparent inhibition of Ca2+-channel and GABAA receptor-mediated currents, but did not cause any significant effect on Kv or Kir channels. Among the four channel types examined, GABAA receptors appeared to be the most sensitive to MeHg. The Kv channels, particularly the delayed rectifiers (DRs), appeared to be relatively resistant to MeHg compared with GABAA receptors and Ca2+ channels. Kir channels were virtually unaffected by MeHg in the concentration range of 10-100 μM. The differential sensitivity of GABAA receptors and Kv channels to MeHg was also observed in granule and Purkinje cells in freshly isolated cerebellar slices of rat. The insensitivity of Kir channel to MeHg was also seen in Xenopus laevis oocytes expressing cloned Kir7.1 channels. Thus, these appear to be general properties of these channels as opposed to distinct effects associated with granule cells in culture. These results suggest that MeHg does preferentially affect certain types of ion channels. Hence, the effects of MeHg on membrane ion channels are not due simply to nonspecific actions on the membrane. Furthermore, at least certain types of Kir channels appear to be the most resistant type of ion channel reported to date to effects of MeHg.