Hypoxia reduces potassium currents in cultured rat pulmonary but not mesenteric arterial myocytes

To explore possible mechanisms underlying hypoxia-induced pulmonary vasoconstriction, the effect of hypoxia on outward K⁺ current (I(out)) was evaluated in primary cultured rat pulmonary (PA) and mesenteric (MA) arterial smooth muscle cells using the whole cell patch-clamp technique. When the cells were bathed in standard physiological salt solution and the patch pipettes contained Ca²⁺-free media with 10 mM ethylene glycol-bis(β- aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), virtually all of the I(out), including both the rapidly inactivating component (I(rt)) and the steady-state (noninactivating) component (I(ss)), was mediated by voltage- gated K⁺ channels. Reduction of O₂ tension in the bath solution from 155 Torr to <74 Torr with sodium dithionite reversibly inhibited both I(rt) and I(ss) in PA myocytes, but not in MA myocytes. The hypoxia-sensitive I(ss) was activated at about -50 mV; thus, some of the channels responsible for this current may be open at the resting membrane potential (-40 ± 1 mV) of PA cells used in this study. Hypoxia also significantly depolarized PA cells bathed in PSS (1.8 mM Ca²⁺) from -40.7 ± 1.3 to -24.0 ± 2.4 mV, and PA cells bathed in Ca²⁺-free PSS (0.1 mM EGTA) from -38.4 ± 1.3 to -26.1 ± 3.9 mV. The hypoxia-induced inhibition of I(out) in PA cells was accompanied by an apparent increase in inward Ca²⁺ current. Removal of extracellular Ca²⁺ and addition of 2 mM EGTA to the bath solution while maintaining a Ca²⁺-free intracellular solution with 10 mM EGTA in the pipette (to prevent Ca²⁺-activated K⁺ channels from opening) did not preclude the hypoxia- induced inhibition of I(out) in PA cells. These data indicate that hypoxia attenuates voltage-gated K⁺ channel activity in PA cells but not in MA cells. The mechanism by which hypoxia inhibits I(out) is not known, but might be related to inhibition of oxidative metabolism. This inhibition of I(out) depolarizes the PA cells. By secondarily opening voltage-gated Ca²⁺ channels and promoting Ca²⁺ entry, the block of these K⁺ channels might be responsible for initiating hypoxia-induced pulmonary vasoconstriction.