We examined the actions of mercury (Hg2+) and zinc (Zn2+) on voltage-activated calcium channel currents of cultured rat dorsal root ganglion (DRG) neurons, using the whole-cell patch clamp technique. 2. Micromolar concentrations of both cations reduced voltage-activated calcium channel currents. Calcium channel currents elicited by voltage jumps from a holding potential of -80 to 0 mV (mainly L- and N-currents) were reduced by Hg2+ and Zn2+. The threshold concentration for Hg2+ effects was 0.1 microM and that for Zn2+ was 10 microM. Voltage-activated calcium channel currents were abolished (> 80%) with 5 microM Hg2+ or 200 microM Zn2+. The peak calcium current was reduced to 50% (IC50) by 1.1 microM Hg2+ or 69 microM Zn2+. While Zn2+ was much more effective in reducing the T-type calcium channel current--activated by jumping from -80 to -35 mV--Hg2+ showed some increased effectiveness in reducing this current. 3. The effects of both cations occurred rapidly and a steady state was reached within 1-3 min. While the action of Zn2+ was not dependent on an open channel state, Hg2+ effects depended partially on channel activation. 4. While both metal cations reduced the calcium channel currents over the whole voltage range, some charge screening effects were detected with Hg2+ and with higher concentrations (> 100 microM) of Zn2+. 5. As Zn2+ in the concentration range used had no influence on resting membrane currents, Hg2+ caused a clear inward current at concentrations > or 2 microM.(ABSTRACT TRUNCATED AT 250 WORDS)
