The activation of an apical Cl<SUP>-</SUP> conductance by extracellular ATP is potentiated by genistein in Necturus gallbladder epithelium

Necturus gallbladder epithelium (NGE) expresses a CFTR-homologous apical Cl^- conductance (G_a,Cl) which can be activated either by elevation of intracellular cAMP or by extracellular ATP. Here we show by microelectrode experiments and impedance analysis that genistein (50 µM), which is known to potentiate the stimulation of G_a,Cl in several cell culture models, also potentiates the stimulation of G_a,Cl by low doses of forskolin in NGE. Moreover, we show that genistein also potentiates the stimulation of G_a,Cl by ATP. In addition genistein renders gallbladders that initially do not respond to ATP sensitive to this stimulant, and it delays the conductance inactivation after ATP removal. Under control conditions G_a,Cl inactivates within <5 min, but in the presence of genistein a significant G_a,Cl persists even after 60 min. These effects of genistein are not related to inhibition of protein tyrosine kinases, since structurally different inhibitors of the tyrphostin family do not mimic the genistein effects. The data support our conclusion that stimulation of G_a,Cl by ATP is mediated by activation of the cAMP pathway and involves a CFTR-homologous protein. They also favour the view that genistein acts via inhibition of protein phosphatases which dephosphorylate CFTR, but cannot exclude the possibility of a direct interaction with CFTR.