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1. Effects of adenosine 5'-triphosphate (ATP) on ionic currents of dispersed smooth muscle cells of the rabbit portal vein were investigated using the voltage-clamp procedure. 2. ATP (greater than or equal to 300 microM) produced transient and maintained inward currents. The former was inactivated within a few seconds, but the latter lasted more than several minutes. The transient but not the maintained current was blocked by pre-treatment with alpha,beta-methylene adenosine 5'-triphosphate (AMP-CPP). The amplitude of the latter was increased by ATP in a concentration-dependent manner. The following investigations were made on the ionic mechanism of the ATP-induced maintained inward current. 3. In 2.5 mM-Ca(2+)-containing tetraethylammonium chloride (TEA-Cl) solution (2.5 mM-Ca(2+)-TEA+ solution), the reversal potential for the ATP-induced inward current was close to the Cl- equilibrium potential, and in 140 mM-Na+ (nominally Ca(2+)-free or 0.3 mM-EGTA-containing) solution, the reversal potential was coincident with the Na+ equilibrium potential. 4. In 2.5 mM-Ca(2+)-TEA+ solution but not in 140 mM-Na+ solution and in physiological salt solution (PSS), niflumic acid (10 microM), a Cl- channel blocker, and Cl(-)-deficient perfusate in the pipette markedly inhibited the ATP-induced inward current. These results imply that in 2.5 mM-Ca(2+)-TEA+ solution the ATP-activated ion channel may admit Ca2+ which then accelerates the Ca(2+)-dependent Cl- current, but in 140 mM-Na+ solution and in PSS this channel may admit only Na+. 5. Intracellular perfusion of guanosine 5'-O-(3-thio triphosphate (GTP gamma S) did not provoke the current, but significantly increased the amplitude of the ATP-induced inward current in 2.5 mM-Ca(2+)-TEA+, 140 mM-Na+ and 2.5 mM-Ba(2+)-containing TEA+ (2.5 mM-Ba(2+)-TEA+) solutions. On the other hand, intracellular perfusion of guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) reduced the amplitude of the ATP-induced inward current in the above solutions. 6. A low concentration of ATP (30 microM) transiently augmented the amplitude of the voltage-dependent Ca2+ current recorded in both 2.5 mM-Ca(2+)-TEA+ solution and PSS, but a high concentration of ATP (3 mM) consistently inhibited the voltage-dependent Ca2+ current in both solutions (4 mM-EGTA in the pipette). Such inhibition was partly prevented by application of 20 mM-EGTA in the pipette.(ABSTRACT TRUNCATED AT 400 WORDS)