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2-Aminoethoxydiphenyl Borate Directly Inhibits Channels Composed of Connexin26 and/or Connexin32

Department of Pharmacology, Zhongsan College of Medicine, Sun Yet-San University, Guangzhou, China (L.T.); and Department of Pharmacology and Physiology, New Jersey Medical School, Newark, New Jersey (A.L.H.)

2-Aminoethoxydiphenyl borate (2-APB), a commonly used blocker of IP₃-induced calcium ion release and of store-operated channels, inhibits gap junction conductance when applied to cultured cells. The character and pharmacology of this inhibition was explored using 1) hemichannels composed of connexin32 (Cx32) and/or connexin26 (Cx26) purified from native sources and from transfected HeLa cells in which the connexin had a cleavable C-terminal epitope tag and 2) the corresponding junctional channels. Using reconstituted hemichannels in a liposome-based transport-specific fractionation assay (TSF), 2-APB reversibly inhibited homomeric Cx32 and heteromeric Cx26/Cx32 channels from native tissue and their tagged forms from HeLa cells. The IC_50-TSF value of the inhibition was 47 µM at pH 6.5. 2-APB did not inhibit tagged homomeric Cx26 channels even after tag cleavage (leaving several amino acids at the carboxyl terminus). Protonated 2-APB is the inhibitory agent, but channel sensitivity to 2-APB also increases as pH is lowered. To help define the chemical requirements for inhibition, the effects of four structural analogs of 2-APB were determined. The inhibitory action of 2-APB was shown to be distinct from that of aminosulfonates. 2-APB and its analogs, except phenytoin, inhibited dye-coupling through junctional channels formed by all the tagged channel forms except Cx26, consistent with the TSF studies. However 2-APB significantly inhibited dye coupling between cells expressing untagged Cx26, suggesting that an unmodified C terminus is required for action on Cx26 channels. These results show that protonated 2-APB directly and reversibly inhibits connexin channels composed of Cx26 and/or Cx32 and suggest involvement of the carboxyl-terminal domain.

Address correspondence to: Dr. Liang Tao, Department of Pharmacology, Zhongsan College of Medicine, Sun Yet-San University, Guangzhou China 510080. E-mail: taol{at}mail.sysu.edu.cn

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