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 α.

Carbachol, Cl- channels, Cystic fibrosis, Forskolin, Intestinal secretion, Ionomycin, K+ channels, PDB, Protein kinase C, T84 cell line,
Pfluegers Archiv: European journal of physiology
Department of Urology

Bajnath, R.B, van den Berghe, N, de Jonge, H.R, & Groot, J.A. (1993). Activation of ion transport by combined effects of ionomycin, forskolin and phorbol ester on cultured HT-29cl.19A human colonocytes. Pfluegers Archiv: European journal of physiology, 425(1-2), 90–99. doi:10.1007/BF00374508