Activation of ion transport by combined effects of ionomycin,forskolin and phorbol ester on cultured HT-29cl.19A human colonocytes

The differentiated clone 19A of the HT-29 human colon carcinoma cell line was used as a model to study the intracellular electrophysiological effects of interaction of the cAMP, the protein kinase C (PKC) and the Ca²⁺ pathways, (a) A synergistic effect between ionomycin and forskolin was observed. From intracellular responses it was concluded that the synergistic effect is caused by activation of an apical Cl^– conductance by protein kinase A and a basolateral K⁺ conductance by Ca²⁺. (b) A transient synergistic effect of ionomycin and the phorbol ester phorbol dibutyrate (PDB) was found. The decrease of the response appeared to be due to PKC-dependent inactivation of the basolateral K⁺ conductance. The synergism is caused by PKC-dependent increase of the apical Cl^– conductance and Ca²⁺-dependent increase of the basolateral K⁺ conductance. (c) The effects of carbachol and PDB were not fully additive presumably because of their convergence on PKC activation, (d) Forskolin and PDB, when added in this order, had a less than additive effect. Results of cell-attached patch-clamp studies, presented in the accompanying paper, showed a synergistic effect of forskolin and PDB on non-rectifying small-conductance Cl^– channels. Assuming that these channels are involved in the transepithelial responses it is suggested that forskolin and PDB induce a modulatory, synergistic increase of the apical Cl^– conductance when both pathways are activated simultaneously. (e) The HT-29cl.19A cells differ from T₈₄ cells in that the latter did not respond with an increase of the short-circuit current to addition of phorbol ester. This may be due to a very low expression of PKC.