Forskolin (i.e., cAMP)-modulation of ion transport pathways in filter-grown monolayers of the Cl(-)-secreting subclone (19A) of the human colon carcinoma cell line HT29 was studied by combined Ussing chamber and microimpalement experiments. Changes in electrophysiological parameters provoked by serosal addition of 10(-5) M forskolin included: (i) a sustained increase in the transepithelial potential difference (3.9 +/- 0.4 mV), (ii) a transient decrease in transepithelial resistance with 26 +/- 3 omega.cm2 from a mean value of 138 +/- 13 omega.cm2 before forskolin addition, (iii) a depolarization of the cell membrane potential by 24 +/- 1 mV from a resting value of -50 +/- 1 mV and (iv) a decrease in the fractional resistance of the apical membrane from 0.80 +/- 0.02 to 0.22 +/- 0.01. Both, the changes in cell potential and the fractional resistance, persisted for at least 10 min and were dependent on the presence of Cl- in the medium. Subsequent addition of bumetanide (10(-4) M), an inhibitor of Na/K/2Cl cotransport, reduced the transepithelial potential, induced a repolarization of the cell potential and provoked a small increase of the transepithelial resistance and fractional apical resistance. Serosal Ba2+ (1 mM), a known inhibitor of basolateral K+ conductance, strongly reduced the electrical effects of forskolin. No evidence was found for a forskolin (cAMP)-induced modulation of basolateral K+ conductance. The results suggest that forskolin-induced Cl- secretion in the HT-29 cl.19A colonic cell line results mainly from a cAMP-provoked increase in the Cl- conductance of the apical membrane but does not affect K+ or Cl- conductance pathways at the basolateral pole of the cell. The sustained potential changes indicate that the capacity of the basolateral transport mechanism for Cl- and the basal Ba(2+)-sensitive K+ conductance are sufficiently large to maintain the Cl- efflux across the apical membrane. Furthermore, evidence is presented for an anomalous inhibitory action of the putative Cl- channel blockers NPPB and DPC on basolateral conductance rather than apical Cl- conductance.