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

1. The properties of a Ca-activated Cl-current ICl(Ca) were investigated with the two electrode voltage clamp technique in the AL1 cell of the leech Haementeria ghilianii. 2. ICl(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 1 microM 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 constants tau 1 and tau 2 on the order of about 100 ms and 800 ms, respectively. No voltage-dependence of the time constants was apparent; however, tau 1 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 a depolarizations below the activation threshold or beyond the apparent reversal potential for the Ca-current.(ABSTRACT TRUNCATED AT 250 WORDS)