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J. Biol. Chem., Vol. 279, Issue 38, 39310-39316, September 17, 2004

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Cellular Mechanism for Potentiation of Ca²+-mediated Cl^– Secretion by the Flavonoid Baicalein in Intestinal Epithelia^*

Grace Gar-Lee Yue, Tiffany Wai-Nga Yip, Yu Huang, and Wing-Hung Ko

From the Department of Physiology, The Chinese University of Hong Kong, Shatin, N.T. Hong Kong, China

Flavonoids belong to a large group of plant polyphenols that are consumed daily in large amounts. Our previous findings have shown that baicalein, a major flavonoid derived from the medicinal herb Scutellariae radix, induces Cl^– secretion across rat colonic mucosa. The current study examines the effect of baicalein on Cl^– secretion in human colonic epithelial (T84) cells and its interaction with Ca²⁺- and cAMP-dependent secretagogues. We have employed a technique that allows concurrent monitoring of short-circuit current (I_SC) and [Ca²⁺]_i in polarized epithelium. Basolateral application of baicalein induced a concentration-dependent increase in I_SC. The increase in I_SC was because of Cl^– secretion and was not accompanied by any discernible increase in [Ca²⁺]_i. Baicalein acted synergistically with Ca²⁺- but not cAMP-dependent secretagogues. In the presence of baicalein, the carbachol and histamine induced increases in I_SC that were markedly potentiated while increases in [Ca²⁺]_i were not significantly enhanced. Baicalein treatment uncoupled Cl^– secretion from inhibitory effects normally generated by muscarinic activation. Baicalein treatment also resulted in increased cAMP content and activated PKA activity. Nystatin permeabilization studies revealed that baicalein stimulated an apical Cl^– current but did not activate any basolateral K⁺ current. These data suggest that baicalein potentiates Ca²⁺-mediated Cl^– secretion through a signaling pathway involving cAMP and protein kinase A, most likely through the cystic fibrosis transmembrane conductance regulator in the apical membrane.

Received for publication, June 17, 2004 , and in revised form, July 2, 2004.

^* This work was supported by Research Grants Council of the Hong Kong Special Administrative Region Project number CUHK4171/02M and a direct grant for research from The Chinese University of Hong Kong (Ref. No. 2003.1.070) (to W.-H. K.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: Dept. of Physiology, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China. Tel.: 852-2609-6781; Fax: 852-2603-5022; E-mail: whko{at}cuhk.edu.hk.

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