Nucleotide receptors activate cation, potassium, and chloride currents in a liver cell line
Fitz, J.G.; Sostman, A.H.
American Journal of Physiology 266(4 Pt 1): G544-G553
1994
ISSN/ISBN: 0002-9513 PMID: 8178992 Document Number: 424177
By use of whole cell patch-clamp techniques, the effects of extracellular ATP on membrane ion currents of HTC cells from a rat liver tumor line were evaluated. ATP (500 mu-M) or the nonhydrolyzable analogue adenosine 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 consecutive 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 exhibited outward rectification of the current-voltage relation. All three currents were present in 39% of cells. Increasing intracellular Ca-2+ concentration ((Ca-2+)-i) by exposure to the 5'-nucleotide receptor agonist 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 ( gtoreq 5 mM) 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 activate currents; a P-2gamma-preferring agonist 2-methylthioadenosine triphosphate activated I-cat and I-K at concentrations of 500 mu-M but not 50 mu-M. In perforated patch recordings, ATP produced triphasic changes in membrane potential with initial depolarization due to I-cat, subsequent hyperpolarization due to I-K, and a later sustained depolarization 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 (Ca-2+)-i, and sustained activation of I-Cl through a separate Ca-2+-independent mechanism.