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 in hibition by cytoplasmic ATP. Recent evidence in vascular smooth muscle has suggested that these channels show weak sensitivity to intracellular ATP. H owever, it is not known whether these channels regulate the resting K+ cond uctance in vascular smooth muscles. Therefore, the aim of the present inves tigation was to characterize this current in rat aorta myocytes and to exam ine whether it contributes to setting the membrane potential. Methods: The conventional and nystatin-permeablised whole cell patch clamp techniques we re used to characterize the effect of glibenclamide on membrane potential a nd K+ current in enzymatically dispersed rat aorta myocytes, Results: The m ean resting potential measured in current clamp mode using the permeabilize d patch approach was -54+/-5 mV(n=8). Glibenclamide (10 mu M) caused a reve rsible 24-mV depolarization in these cells. In symmetrical K+ (135 mM) solu tion an inward glibenclamide-sensitive (10 mu M) current (-4.1+/-0.7 pA/pF; n=5), hereafter termed I-glib, was observed at a membrane potential of -80 mV when cells held at -60 mV were ramped from -80 to +80 mV. In the absenc e of any nucleotide in the pipette solution, I-glib measured by the convent ional 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 signi ficantly (P<0.01) increased I-glib (-44.8+/-8.4 pA/pF; n=13). Inclusion of 1 mM ATP in the GDP-containing pipette solution had no significant effect o n the current amplitude (-56.4+/-10.7 pA/pF; n=7). I-glib fell to -11.0+/-2 .9 pA/pF (n=10) if 1 mM GDP and 3 mM ATP were present. In symmetrical K+, t he I-glib observed in the presence of 1 mM ATP in the pipette was increased by more than two-fold in the presence of 10 mu M levcromakalim. In PSS con taining 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 I-glib ch annels in rat aorta myocytes differ from classical K-ATP channels, being re latively insensitive to intracellular ATP. I-glib therefore appears to have an important role in contributing to the maintenance of the resting potent ial in rat aortic smooth muscle. (C) 1999 Elsevier Science B.V. All rights reserved.