SYNERGISTIC ACTIVATION OF NON-RECTIFYING SMALL-CONDUCTANCE CHLORIDE CHANNELS BY FORSKOLIN AND PHORBOL ESTERS IN CELL-ATTACHED PATCHES OF THE HUMAN COLON-CARCINOMA CELL-LINE HT-29CL.19A

Cell-attached patch-clamp studies with the human colon carcinoma HT-29 cl.19A cells revealed a small chloride channel with a unitary conducta nce of 6.5 pS at 70 mV and 4.6 pS at -70 mV clamp potential after cAMP was increased by activation of adenylyl cyclase by forskolin. Usually channels inactivated upon patch excision, but in a few excised patche s the channels stayed active and displayed a linear I/V relation in sy mmetrical (150 mmol/l) chloride solutions with a conductance of 7.5 pS . A 16-fold increase in channel incidence was observed when forskolin and phorbol 12,13-dibutyrate (PDB) were present together. The open pro bability was voltage-independent and was not different in the presence of forskolin plus PDB or with forskolin alone. The conductance sequen ce of the channel as deduced from outward currents carried by five dif ferent anions including chloride was: Cl- > Br- > NO3- > gluconate ) I -. The permeability sequence deduced from the reversal potentials was NO(3)(-)greater than or equal to Br- > Cl- > I- > gluconate. With iodi de in the pipette the conductance decreased strongly. Moreover, the in ward current was reduced by 61%, indicating a strong inhibition of the chloride efflux by iodide. Similarly, the forskolin-induced increase of the short-circuit current (I-sc) in confluent filter-grown monolaye rs was strongly reduced by iodide in the apical perfusate. Iodide also increased the fractional resistance of the apical membrane and repola rized the membrane potential, indicating an inhibitory action on the f orskolin-induced increase of the apical chloride conductance. The PDB- induced I-sc was also reduced by iodide, suggesting that the same chlo ride conductance is involved in the forskolin and in the PDB response. The results suggest that forskolin via cAMP-dependent protein kinase and PDB via protein kinase C regulate the same non-rectifying small-co nductance chloride channels in the HT-29cl.19A cells.