Flavonoids belong to the large group of plant polyphenols, which are currently consumed in large amounts in the daily diet. They are also present in many medicinal plants, and herbal medicines containing flavonoids have been used in traditional folk remedies for centuries (Di Carlo et al. 1999). Recent studies have shown that several flavonoids stimulate Cl^– secretion across rat colonic mucosa (Cermak et al. 2001, 2002). Our laboratory has also shown that baicalein, a major flavonoid derived from Scutellariae radix, evoked Cl^– secretion across rat colonic mucosa, possibly via a cAMP-dependent pathway (Ko et al. 2002). This study aims to examine the effect of baicalein on Cl^– secretion in human colonic epithelial cells and its interaction with Ca²⁺-dependent secretagogues.

In this study, a technique which allows us to monitor short-circuit current (I_SC) and [Ca²⁺]_i concurrently in a polarized epithelium was employed (Wong & Ko, 2002). T84 cells were grown on Transwell-Col membranes for 9 days using standard culture techniques. Confluent monolayers were clamped in a modified Ussing chamber and perfused bilaterally in normal Krebs solution. Data are means ± S.E.M. of n observations and statistical analyses were performed using Student’s unpaired t test.

Basolateral application of baicalein evoked a concentration-dependent increase in I_SC with an EC₅₀ of about 40 µM. The increase in I_SC was mainly due to chloride secretion and was not accompanied by any discernible increase in [Ca²⁺]_i. The maximal increase in I_SC, however, was relatively small when compared to its effect on rat colonic mucosa (Ko et al. 2002). We therefore explored whether baicalein acted synergistically with Ca²⁺-dependent secretagogues. Basolateral application of the acetylcholine analogue carbachol (10 µM) stimulated an increase in I_SC (ΔI_SC = 6.85 ± 0.54 µA cm^-2, n = 21) and [Ca²⁺]_i (Δ ratio = 0.30 ± 0.01). In the presence of baicalein (100 µM), the carbachol-evoked increase in I_SC was markedly potentiated (ΔI_SC = 14.71 ± 3.63 µA cm^-2, n = 11, P < 0.01) while the increase in [Ca²⁺]_i was not significantly enhanced by baicalein (Δ ratio = 0.29 ± 0.02, P > 0.01). A similar potentiation effect was also observed when histamine (basolateral, 100 µM) was used as another Ca²⁺-dependent secretagogue.

In conclusion, our data demonstrate that baicalein stimulated a Ca²⁺-independent Cl^– secretion across human colonic T84 epithelia. Although the baicalein-evoked I_SC was relatively small, it potentiated the Cl^– secretory response to Ca²⁺-dependent secretagogues.

This work was supported by Earmarked Grant for Research, Research Grant Councils of Hong Kong (CUHK 4171/02M).