CALCIUM-ACTIVATED CHLORIDE CHANNELS IN CULTURED EMBRYONIC XENOPUS SPINAL NEURONS

1. Single-channel currents were recorded from Xenopus spinal neurons developing in vitro using the patch-clamp technique, to identify the channels underlying the large and small macroscopic Ca2+-activated Cl- currents (I(Cl(Ca))) present in these cells. 2. Channels of large (maxi-channels; 310 pS) and smaller conductance (mini-channels; 50-60 pS) are activated by elevation of cytoplasmic Ca2+ concentration. Channel activity is not altered by subsequent removal of Ca2+ from the bath, arguing against a direct ligand-type Ca2+ dependence. The much higher incidence of channel activation in cell-attached patches from cells permeabilized with the Ca2+ ionophore A23187 than in excised patches also suggests the involvement of some unidentified intracellular factor. 3. The reversal potential of maxi-Cl- channels is not altered by changes in Na+ concentration, but is shifted in the negative direction by the substitution of Cl- by methanesulfonate on the intracellular side of the patch, indicating their anionic selectivity. 4. Maxi-Cl-channels exhibited the presence of multiple probable subconductance states and showed marked voltage-dependent inactivation above and below +/-20 mV. 5. Examination of maxi-Cl- channels at early times in culture (6-9 h) and 24 h later did not reveal any developmental change in the characteristics described above. However, the mean open duration of the channel was found to increase twofold during this period of time. 6. The simultaneous presence of maxi-and mini-Cl- channels prevented detailed characterization of the latter. The anionic selectivity of mini-Cl- channels is suggested by their reversal potential that lies close to the Cl- equilibrium potential. 7. The observed physiological and developmental properties of Cl- channels do not allow clear correlation of the activity of each channel class with specific macroscopic I(Cl(Ca)). Both the diversity and developmental modification of I(Cl(Ca)) may result from changes in internal Ca2+ regulation, from changes in the distribution of Cl- relative to Ca2+ channels or to intracellular stores, or from some other factor. 8. Activation of these channels may contribute to the repolarization of Ca2+-dependent action potentials present at early stages of neuronal differentiation. The sustained activation of these channels after the removal of Ca2+ may suggest their involvement in some long-term regulation of neuronal excitability.