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

Abstract

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)

Calcium-activated chloride channels in the retina

“…A number of studies have proposed the exclusive regulation of Cl(Ca) channel activity by [Ca 2+ ] i in neurons. [5][6][7][8][9][10][11][12][13][14] In cone photoreceptors, the steep dependence of Cl(Ca) channels on [Ca 2+ ] i is linked to Ca 2+ influx through voltage-gated Ca channels as the non-specific Ca channel blocker, Cd 2+ , inhibited Cl(Ca) channel activation. 1,10,15 Such relationships between Ca 2+ entry through voltage-gated Ca channels and I Cl(Ca) activation have been observed in different systems including mammalian neurons and smooth muscles.…”

“…A number of studies have reported that Ba 2+ is ineffective in inducing Ca 2+ -activated Cltail currents in neurons. [6][7][8][9]16,18,[26][27][28][29][30] While it is likely that Ba 2+ ions do not activate Cl(Ca) channels, other explanations may account for the inhibited Ca 2+ -activated Cltail current. For example, Ba 2+ ions could be activating Cl(Ca) channels, but the diffusion of Ba 2+ from the submembrane area may be faster than that of Ca 2+ , which may account for the lack of tail currents.…”