Intracellular adenosine triphosphate (ATP)-inhibited K+ currents (IK, ATP) in canine coronary artery smooth muscle cells were characterized in the whole-cell configuration using the suction pipette method. Cells dialysed internally with solutions containing 5 mM ATP (ATPi) showed little detectable whole-cell current at potentials more negative than -30 mV. However, cells dialysed with ATPi-free solutions developed a time- and voltage-independent current which reached a maximum of 132 +/- 25 pA at -40 mV about 10 min following patch rupture. After "run-up", the current showed little "run-down". Concentration-dependent inhibition by ATPi yielded an inhibition constant (Ki) of 350 microM and a Hill coefficient of 2.3. In ATPi-free solutions, the large current at -40 mV was reduced by glibenclamide with a Ki of 20 nM and a Hill coefficient of 0.95. Conversely, in 1 mM ATPi solutions, the small current at -40 mV was increased by P-1075 from 8 +/- 2 pA to 143 +/- 33 pA, with a dissociation constant (Kd) of 0.16 microM and a Hill coefficient of 1.7. The effect of P-1075 was antagonized by glibenclamide. Maximal current density elicited by either ATPi depletion or external application of the channel opener P-1075 was similar with slope conductances of 81 +/- 10 pS/pF and 76 +/- 13 pS/pF respectively in the potential range of -90 to -40 mV. External Ca2+ had no effect on this current. Finally, in 1 mM ATPi, 20 and 50 microM adenosine increased the current slope conductance by 36 +/- 15% and 73 +/- 10% respectively between -90 to -40 mV. The IK, ATP, although very small in these cells, was extremely effective in causing membrane potential hyperpolarization.