Small and intermediate conductance chloride channels in HT29 cells

Recently, it has been shown that intermediate conductance outwardly rectifying chloride channels (ICOR) are blocked by cytosolic inhibitor (C. I.) found in the cytosol of human placenta and epithelial cells. C. I. also reduced the baseline current in excised membrane patches of HT₂₉ cells. In the present study, this effect of C. I. was characterized further. Heat treated human placental cytosol was extracted in organic solvents and dissolved in different electrolyte solutions. It is shown that the reduction of baseline conductance (g_o) is caused by inhibition of small non-resolvable channels, which are impermeable to Na⁺ and SO₄^2–, but permeable to Cl^–. The regulation of these small Cl^–-conducting channels (g_o) and of ICOR was examined further. First, no activating effects of protein kinase A (PKA) on the open probability (P_o) of the ICOR or on the g_o) were observed. The P_o of the ICOR was reduced by 22% in a Ca²⁺-free solution. g_o was insensitive to changes in the Ca²⁺ activity. The effects of C. I. from a cystic fibrosis (CF) placenta and the CF pancreatic duct cell line CFPAC-1 were compared with the effects of corresponding control cytosols, and no significant differences between CF and control cytosols were found. We conclude that the excised patches of HT₂₉ cells contain ICOR and small non-resolvable Cl^–-conducting channels which are similarly inhibited by C. I. Apart from a weak effect of Ca²⁺ on the ICOR, g_o and the ICOR do not seem to be directly controlled by Ca²⁺ or PKA. C. I. of normal and CF epithelia have a similar inhibitory potency on Cl^– channels.