Vascular
ATP-sensitive
potassium (
KATP) channels have an important role in hypoxic vasodilation. Because
KATP channel activity depends on intracellular
nucleotide concentration, one hypothesis is that
hypoxia activates channels by reducing cellular
ATP production. However, this has not been rigorously tested. In this study we measured KATP current in response to
hypoxia and modulators of cellular metabolism in single smooth muscle cells from the rat femoral artery by using the whole cell patch-clamp technique. KATP current was not activated by exposure of cells to hypoxic solutions (Po2 approximately 35 mmHg). In contrast, voltage-dependent
calcium current and the depolarization-induced rise in intracellular
calcium concentration ([Ca2+]i) was inhibited by
hypoxia. Blocking mitochondrial
ATP production by using the
ATP synthase inhibitor
oligomycin B (3 microM) did not activate current. Blocking glycolytic
ATP production by using
2-deoxy-D-glucose (5 mM) also did not activate current. The protonophore
carbonyl cyanide m-chlorophenylhydrazone (1 microM) depolarized the mitochondrial membrane potential and activated KATP current. This activation was reversed by
oligomycin B, suggesting it occurred as a consequence of mitochondrial
ATP consumption by
ATP synthase working in reverse mode. Finally,
anoxia induced by
dithionite (0.5 mM) also depolarized the mitochondrial membrane potential and activated KATP current. Our data show that: 1)
anoxia but not
hypoxia activates KATP current in femoral artery myocytes; and 2) inhibition of cellular energy production is insufficient to activate KATP current and that energy consumption is required for current activation. These results suggest that vascular
KATP channels are not activated during
hypoxia via changes in cell metabolism. Furthermore, part of the relaxant effect of
hypoxia on rat femoral artery may be mediated by changes in [Ca2+]i through modulation of
calcium channel activity.