ATP-activated cation currents in single guinea-pig hepatocytes

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Abstract
Responses of single guinea-pig liver cells to the application of external ATP were studied using the whole-cell voltage clamp technique. When the cells were loaded with 5 mm EGTA in the absence of K+ and Cl− in both internal and external solutions, application of ATP (0.03-100 μm) elicited a large cation-selective inward current at negative holding potentials. The current densities at the peak of the response to 100 μm ATP were 4.5 ± 0.5 pA pF−1 (mean ± s.e.m., n = 18) in the presence of Na+ and Ca2+ in the external medium and 3.3 ± 0.7 pA pF−1 (n = 6) with Ca2+ as the major permeant ion. Divalent cations, when added during the response to ATP in the presence of Na+ and Ca2+, exerted different effects: CdSO4 (2 mm) totally and NiSO4 (2 mm) partially blocked the inward current whereas MnSO4 (2 mm) did not block it. The ATP-activated conductance was permeable to all the divalent cations tested in this study, i.e. Ca2+, Cd2+, Ni2+, Mn2+ and Mg2+. No response to ATP was observed in the absence of external cations. The activation of the inward current was not maintained in the continuous presence of ATP. The effect of Ca2+ ions on the desensitization of the response was studied in different external solutions. The decline in the amplitude of the inward current after the peak was fitted with a single exponential with a time constant of about 2 s for pure Ca2+, Cd2+ or Ni2+ currents, 3 s for Mg2+ or Mn2+ and 4 s in the presence of both Na+ and Ca2+. Under more physiological conditions, the entry of Ca2+ evoked after the stimulation of P2X purinoceptors was associated with an increase in fluo-3 fluorescence and a marked reduction in the delay before the mobilization of internal Ca2+ stores following the activation of P2Y purinoceptors.