REGULATION OF APICAL CL- CONDUCTANCE AND BASOLATERAL K+ CONDUCTANCES BY PHORBOL ESTERS IN HT-29CL.19A CELLS

The effect of phorbol esters [4beta-phorbol 12,13-dibutyrate (PDB) and phorbol 12-myristate 13-acetate (PMA)] on potential differences and resistances was studied with the conventional microelectrode technique applied to confluent filter-grown monolayers of the human colon carcinoma cell line HT-29cl.19A. Phorbol esters (PDB or PMA from 10(-7) to 10(-6) M) evoked 1) a transient increase in the transepithelial potential difference (peak value 3.5 +/- 0.5 mV), 2) a depolarization of the cell potential by 23 +/- 2 mV at the peak of the transepithelial potential change and a continued decrease during the decline of the transepithelial potential, and 3) a decrease of the fractional resistance of the apical membrane consisting of two phases, a relative rapid one (time constant 1.2 +/- 0.2 min) and a much slower further decrease during the second phase (time constant 34 +/- 1 min). Ion replacements and electrical circuit analyses indicate that PDB activates an apical Cl- conductance and slowly inhibits the basal K+ conductance of the basolateral membrane. PDB reduced the transepithelial response to forskolin due to inhibition of the basal K+ conductance. The Ca2+ ionophore ionomycin accelerated the effect of PDB. Staurosporine inhibited the effects of PDB, suggesting that the PDB effects are mediated via activation of a protein kinase C.