Endothelin and vasopressin activate low conductance chloride channels in aortic smooth muscle cells

The non-contractile aortic smooth muscle cell line A7r5 was used to study the membrane events involved in the effect of vasoconstrictor peptides. Wholecell voltage-clamp and membrane potential recording techniques were used to demonstrate the contribution of an increased Cl^– conductance to the late depolarization induced by endothelin-1 and vasopressin. During cell-attached patch recording with N-methyl-d-glucamine in the pipette, bath application of endothelin or vasopressin induced single-channel inward currents in the following minutes. The current/potential (I/V) curve of the most frequently observed channel type — a small conductance Cl^– (SCl) channel — reversed near the cell membrane potential and showed a single-channel conductance of 1.8 pS for inward currents. After patch excision in an extracellular solution containing CaCl₂ (2 mM), the frequency of SCl channel openings increased. Patch excision in the absence of peptide stimulation also produced this channel activity. Replacement of CaCl₂ by a Ca²⁺ chelator on the intracellular face of a patch reversibly inhibited the channel activity, indicating that these SCl channels are Ca²⁺-activated Cl^– channels. The single-channel I/V characteristic showed outward rectification above +50 mV. An analysis of the gating kinetics of the SCl channel is given. Another channel type was recorded less frequently after peptide stimulation. It had a lower conductance (1.0–1.3 pS) and slower kinetics and was designated a very small conductance Cl^}-channel. It is concluded that activation of two types of Cl^– channels (at least one of which is Ca²⁺ dependent) is involved in the late depolarization produced by vasoconstrictor peptides in vascular smooth muscle cells of the aortic cell line A7r5.