Characterization of the ATP-inhibited K+ current in canine coronary smooth muscle cells

Intracellular adenosine triphosphate (ATP)-inhibited K+ currents (I K, ATP ) in canine coronary artery smooth muscle cells were characterized in the wholecell 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 μM and a Hill coefficient of 2.3. In ATPi-free solutions, the large current at −40 mV was reduced by glibenclamide with aKi 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 μM 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 μM adenosine increased the current slope conductance by 36±15% and 73±10% respectively between −90 to −40 mV. TheI K, ATP although very small in these cells, was extremely effective in causing membrane potential hyperpolarization.