A role for a glibenclamide-sensitive, relatively ATP-insensitive K+ current in regulating membrane potential and current in rat aorta.

OBJECTIVE: ATP-sensitive K+ channels have been classified based on their inhibition by cytoplasmic ATP. Recent evidence in vascular smooth muscle has suggested that these channels show weak sensitivity to intracellular ATP. However, it is not known whether these channels regulate the resting K+ conductance in vascular smooth muscles. Therefore, the aim of the present investigation was to characterize this current in rat aorta myocytes and to examine whether it contributes to setting the membrane potential.
METHODS: The conventional and nystatin-permeablised whole cell patch clamp techniques were used to characterize the effect of glibenclamide on membrane potential and K+ current in enzymatically dispersed rat aorta myocytes.
RESULTS: The mean resting potential measured in current clamp mode using the permeabilized patch approach was -54 +/- 5 mV (n = 8). Glibenclamide (10 microM) caused a reversible 24-mV depolarization in these cells. In symmetrical K+ (135 mM) solution an inward glibenclamide-sensitive (10 microM) current (-4.1 +/- 0.7 pA/pF; n = 5), hereafter termed Iglib, was observed at a membrane potential of -80 mV when cells held at -60 mV were ramped from -80 to +80 mV. In the absence of any nucleotide in the pipette solution, Iglib measured by the conventional whole-cell method was -23.69 +/- 4.65 pA/pF (n = 9). With 1 and 3 mM ATP in the pipette, the average current density was -25 +/- 6.3 pA/pF (n = 8), and -9.4 +/- 2.7 pA/pF (n = 9), respectively. In the absence of ATP, 1 mM GDP significantly (P < 0.01) increased Iglib (-44.8 +/- 8.4 pA/pF; n = 13). Inclusion of 1 mM ATP in the GDP-containing pipette solution had no significant effect on the current amplitude (-56.4 +/- 10.7 pA/pF; n = 7). Iglib fell to -11.0 +/- 2.9 pA/pF (n = 10) if 1 mM GDP and 3 mM ATP were present. In symmetrical K+, the Iglib observed in the presence of 1 mM ATP in the pipette was increased by more than two-fold in the presence of 10 microM levcromakalim. In PSS containing 5 mM K+, a significant glibenclamide-sensitive current was observed at -45 mV membrane potential when cells dialyzed with 1 mM ATP were ramped between -80 to 30 mV.
CONCLUSION: These results demonstrate that Iglib channels in rat aorta myocytes differ from classical KATP channels, being relatively insensitive to intracellular ATP. Iglib therefore appears to have an important role in contributing to the maintenance of the resting potential in rat aortic smooth muscle.