Ca2+-activated Cl− channel in plasmalemma ofNitellopsis obtusa

Abstract

The mechanisms of Cl−-channel activation in the plasmalemma ofNitellopsis obtusa was studied by measuring both the transient inward current under voltage clamp and Cl− efflux during the action potential. 9-anthracenecarboxylic acid (A-9-C) at 1.0mm inhibited both the transient inward current and the Cl− efflux, but did not uncouple the sudden cessation of the cytoplasmic streaming. Since this excitation-cessation coupling is caused by a transient increase in the cytoplasmic Ca2+ concentration, these results suggest that A-9-C inhibited not the Ca2+ channel but specifically the Cl− channel. The following results were found between the Ca2+-channel activation and the Cl−-channel activation: (1) The Ca2+-channel blocker La3+ uncoupled the excitation-cessation coupling and inhibited both the transient inward current and the Cl− efflux, although the Cl−-channel blocker A-9-C did not affect the excitation-cessation coupling. (2) The Cl− efflux was greatly reduced by depletion of Ca2+ from the external solution and restored by an increase in the external Ca2+ concentration. (3) An increase in the external ionic, strength which increases Ca2+ entry (T. Shiina & M. Tazawa,J. Membrane Biol.96:263–276, 1987) enhanced the Cl− efflux. (4) Mg2+, which cannot pass through the Ca2+ channel, reduced both the transient inward current and the Cl− efflux. (5) Although Sr2+ can pass through the plasmalemma Ca2+ channel, Cl−-channel activation by Sr2+ was only partial. These findings support the hypothesis that voltage-dependent Ca2+-channel activation, which increases the free Ca2+ concentration in the cytoplasm, is necessary for the subsequent Cl−-channel activation.