SMALL AND INTERMEDIATE CONDUCTANCE CHLORIDE CHANNELS IN HT29 CELLS

Recently, it has been shown that intermediate conductance outwardly re ctifying 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 H T29 cells. In the present study, this effect of C. I. was characterize d further. Heat treated human placental cytosol was extracted in organ ic solvents and dissolved in different electrolyte solutions. It is sh own that the reduction of baseline conductance (g(o)) is caused by inh ibition of small non-resolvable channels, which are impermeable to Na and SO42-, but permeable to Cl-. The regulation of these small Cl--co nducting channels (g(o)) and of ICOR was examined further. First, no a ctivating 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 w as reduced by 22% in a Ca2+-free solution. g(o) was insensitive to cha nges 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 compa red with the effects of corresponding control cytosols, and no signifi cant differences between CF and control cytosols were found. We conclu de that the excised patches of HT29 cells contain ICOR and small non-r esolvable Cl--conducting channels which are similarly inhibited by C. I. Apart from a weak effect of Ca2+ ion the ICOR, g(o) and the ICOR do not seem to be directly controlled by Ca2+ or PKA. C. I. of normal an d CF epithelia have a similar inhibitory potency on Cl- channels.