HYPOXIC AND METABOLIC-REGULATION OF VOLTAGE-GATED K+ CHANNELS IN RAT PULMONARY-ARTERY SMOOTH-MUSCLE CELLS

Inhibition of voltage-gated K+ (K-V) channels by 4-aminopyridine (4-AP) depolarizes pulmonary artery (PA) smooth muscle cells, induces Ca2+-dependent action potentials and increases [Ca2+](i). Neither charybdotoxin, which blocks Ca2+ activated K+ channels, nor glibenclamide, which blocks ATP-sensitive K+ channels, has such effects on membrane potential (E(m)) and [Ca2+](i). Hypoxia reversibly decreases the 4-AP-sensitive K-V currents (I-K(V)) in PA myocytes. The resulting membrane depolarization caused by decreased I-K(V) induces Ca2+-dependent action potentials and thereby raises [Ca2+](i). Thus, K-V channel activity plays a critical role in: (a) regulating E(m) and [Ca2+](i) under physiological conditions; and (b) sensing O-2 alteration and transducing the hypoxic stimulus to changes of E(m) and [Ca2+](i). The metabolic inhibitors 2-deoxy-D-glucose (2-DOG; 10 mM) and carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazone (FCCP; 3-5 mu M), the reducing agent reduced glutathione and inhibitors of cytochrome P-450, all mimic the effects of hypoxia on I-K(V) and E(m) in PA myocytes. Furthermore, hypoxia and 2-DOG negligibly affect I-K(V) and E(m) in mesenteric artery smooth muscle cells. These results suggest that hypoxia, perhaps via a localized reduction of ATP, triggers the block of K-V channels and depolarizes PA myocytes. This blockade may also be mediated by a change in cellular redox status, perhaps via a conformational change of a haem- (or metal-) containing regulatory moiety that is attached to the channel protein.