Zinc current in Helix soma membrane

Abstract

1. Characteristics of zinc inward current ( I Z n) were investigated in respect to its kinetics, voltage-dependence and pharmacological properties in the identified Heliix neurones using a suction pipette technique which allows internal perfusion and voltage clamp. 2. After complete blockage of Na + and K + currents, Zn 2+ evoked all-or-none actiona potentials with a prolonged plateau phase lasting for more than 30 sec. The action potentials were not affected in the presence of TTX but abolished by adding Ca 2+-blockers such as Co 2+ and verapamil. The overshoot was increased by 26 mV for a ten-fold increase in the external Zn 2+ concentration ([Zn 2+] 0). 3. Under voltage clamp conditions, the zinc inward current ( I Z n ) appeared around -20 mV and the maximum peak I Z n evoked at about +30mV. At more high voltage steps, I Z n became smaller and flattened, but did not reverse. 4. In spite of the presence of or absence of Mg 2+, increases of [Zn 2+] 0 induced a hyperbolic increase of I Z n and also shifted the threshold and peak potentials in the I- V curves along the voltage axis to the more positive direction. 5. I Z n was depressed dose- and time-dependently by externally and internally applied organic (verapamil) and inorganic (Co 2+) Ca 2+-blockers. Internal perfusion of Zn 2+ also depressed I Z n time-dependently without shifting the I- V curve and changing the leakage current. 6. Internal application of F − inhibited C n and I Z n time-depently and completely abolished both currents within 10 min. It is concluded that the voltage-dependent Ca channel in the identified Helix nerve cell bodies is permeable to Zn 2+. 7. Both activation ( τ m ) and inactivation ( τ h ) phases of I Z n were fitted by a single exponential function. Time constants for τ m and τ h for I Z n and I C a were voltage-dependent. The voltage dependency was greater for I Z n than for I C a , especially at high voltage steps. τ m and τ h values for I Z n were larger than those for I C a . 8. The steady-state inactivation value, h ∞, of I Z n showed dependence on either the voltage or duration of ‘conditioning’ prepulses. However, complete inactivation of I Z n at any given test pulses was not observed even in the prepulses duration of more than 12 sec. 9. Small amounts of Ca 2+ strongly reduced I Z n . The interaction of Ca 2+ on I Z n was 30 to 100 times stronger than that of Mg 2+. 10. Lower critical size which limits the ionic permeation through the Ca channel may exist just below of 0.74 Å which is the size of Zn 2+.