The mechanisms underlying the relaxant effect of methyl and ethyl gallates in the guinea pig trachea in vitro: contribution of potassium channels.

The relaxant response and the possible contribution of K+ channels to the relaxation caused by both methyl and ethyl gallates, two compounds isolated from the Brazilian medicinal plant Phyllanthus urinaria, were investigated in the guinea pig trachea in vitro. Both methyl and ethyl gallate (0.01-30 microM) caused graded and complete relaxation of the guinea pig trachea without epithelium, pre-contracted by histamine, with mean EC50 values of 1.8 (1.2-2.2) microM and 0.7 (0.6-0.8) microM, respectively, and Emax of both 100+/-0%. Response to ethyl, but not methyl gallate, was significantly shifted to the right, with no change in the maximum effect when the epithelium was removed. The increase in K+ concentration in the medium to 80 mM completely abolished the relaxant response caused by both methyl and ethyl gallate. In addition, tetraethylammonium (10 mM) reduced by 50+/-6% and 43+/-4% the relaxation caused by methyl and ethyl gallates. In contrast, glibenclamide (3 microM) shifted (by about two- and fourfold) the concentration-response curves for both methyl and ethyl gallates, with no changes in the maximum effect. Charybdotoxin (100 nM), but not apamin (100 nM), significantly blocked by 54+/-5% and 59+/-4% the relaxation of both methyl and ethyl gallates. In contrast, SQ 22536 (10 microM; a selective adenylyl cyclase inhibitor), methylene blue (10 microM) or ODQ (1 microM; a guanylyl cyclase inhibitor) did not significantly affect the relaxant response caused by either of the compounds. These results provide evidence that the relaxation caused by both methyl and ethyl gallates in the guinea pig trachea in vitro may involve the activation of large-conductance Ca2+-activated K+ channels, and, to a lesser extent, ATP-sensitive K+ channels. Such results extend our previous observations and are consistent with the notion that methyl and ethyl gallates are mainly responsible for the relaxant action previously demonstrated in the extract of this plant.