Voltage clamp characterization of a calcium-dependent chloride conductance in a putative invertebrate motoneuron

Abstract

1. The properties of a Ca-activated Cl-current I_Cl(Ca) were investigated with the two electrode voltage clamp technique in the AL1 cell of the leechHaementeria ghilianii. 2. I_Cl(Ca) was revealed after Cl-loading of the cells, with outward K-currents eliminated by replacement of intra- and extracellular K with Cs, and with Na currents blocked by 1μM TTX. CsCl-containing electrodes were used for recording and current passing. 3. In response to depolarizing voltage steps the cells exhibited sustained Cl-dependent currents the size and polarity of which varied with [Cl]₀. The reversal potentials of tail currents of this conductance varied with [Cl]₀ as predicted for the equilibrium potential of Cl by the Nernst equation and were unaffected by changes in extracellular cation concentration. 4. The decay of the Cl-dependent tail-currents followed a process which could be described by the sum of two exponentials with time constantsτ₁ andτ₂ on the order of about 100 ms and 800 ms, respectively. No voltage-dependence of the time constants was apparent; however,τ₁ varied with the amount of Ca-influx. 5. The Cl-current required Ca for its activation. All current flow was abolished in Mn-containing Ringer solutions and when Ba was substituted for Ca. However, Sr could partially substitute for Ca as an activator of the current. 6. The activation curve for Cl-dependent tail currents was U-shaped directly paralleling the amount of Ca-influx, and no Cl-current flow could be induced at depolarizations below the activation threshold or beyond the apparent reversal potential for the Ca-current. 7. I_Cl(Ca) was impermeable to large anions such as proprionate and acetate, but was permeable to both Br and Cl ions.