NUCLEOTIDE RECEPTORS ACTIVATE CATION, POTASSIUM, AND CHLORIDE CURRENTS IN A LIVER-CELL LINE

By use of whole cell patch-clamp techniques, the effects of extracellu lar ATP on membrane ion currents of HTC cells from a rat liver tumor l ine were evaluated. ATP (500 mu M) or the nonhydrolyzable analogue ade nosine 5'-O-(3-thiotriphosphate) caused sequential activation of three currents: I-cat (-1,325 +/- 255 pA at -80 mV) occurred early, was due to increased Na+ and K+ permeability, was present in 56% of 64 consec utive cells, and rapidly inactivated; I-K (274 +/- 45 pA at 0 mV) was present in 59% of cells and also inactivated; and I-Cl (1,172 +/- 237 pA at + 60 mV) was present in 94% of studies, was sustained, and exhib ited outward rectification of the current-voltage relation. All three currents were present in 39% of cells. Increasing intracellular Ca2+ c oncentration ([Ca2+](i)) by exposure to the 5'-nucleotide receptor ago nist UTP (500 mu M) or to thapsigargin activated I-cat and I-K but not I-Cl, whereas increasing ethylene glycol-bis(beta-aminoethyl ether)-N ,N,N',N'-tetraacetic acid in the pipette (greater than or equal to 5 m M) inhibited ATP-dependent activation of I-cat and I-K but not I-Cl. A P-2x-preferring agonist alpha,beta-methylene ATP (500 mu M) did not a ctivate currents; a P-2y-preferring agonist 2-methylthio-adenosine tri phosphate activated I-cat and I-K at concentrations of 500 mu M but no t 50 mu M. In perforated patch recordings, ATP produced triphasic chan ges in membrane potential with initial depolarization due to I-cat, su bsequent hyperpolarization due to I-K, and a later sustained depolariz ation due to I-Cl. These findings indicate that ATP modulates HTC cell ion permeability through initial activation of I-cat and I-K mediated by 5'-nucleotide receptors which mobilize [Ca2+](i), and sustained ac tivation of Is through a separate Ca2+-independent mechanism.