REDOX AGENTS AS A LINK BETWEEN HYPOXIA AND THE RESPONSES OF IONIC CHANNELS IN RABBIT PULMONARY VASCULAR SMOOTH-MUSCLE

Ca2+-activated K+ currents (I-K(Ca)) and voltage-dependent Ca2+-insens itive K+ currents (I-K(V)) were recorded using the patch clamp techniq ue to study the pulmonary (PASMC) and ear arterial smooth muscle cells (EASMC) of the rabbit and the possible regulatory mechanisms related to hypoxia. When a hypoxic solution (1 mM Na2S2O4, gassed with 100% N- 2) was superfused, the activity of Ca2+ activated K+ channels (K-Ca ch annels) recorded at a pipette potential of -70 mV in cell-attached mod e was decreased to 49 +/- 7% in PASMC, whereas EASMC K-Ca channels did not respond to hypoxia. In inside-out patches (bathed symmetrically i n 150 mM KCl), reducing agents such as dithiothreitol (DTT; 5 mM), red uced glutathione (GSH; 5 mM) and NADH (2 mM) decreased K-Ca channel ac tivity in PASMC, but they did not affect the EASMC K-Ca channel. Howev er, oxidizing agents such as 5,5'-dithio-bis (2-nitrobenzoic acid) (DT NB; 1 mM), oxidized GSH (GSSG; 5 mM) and NAD (2 mM) increased K-Ca cha nnel activity in both PASMC and EASMC. In the whole-cell configuration , using a pipette solution containing a high concentration of 1,2-bis( 2-aminophenoxy)ethane-N,N,N,N'-tetraacetic acid (BAPTA; 10 mM), PASMC I-K(V) were activated by depolarizing step pulses to voltages more pos itive than -30 mV (holding potential, -80 mV). I-K(V) was increased by application of a membrane-permeable oxidizing agent, 2,2'-dithio-bis( 5-nitropyridine) (DTBNP; 200 mu M), whereas it was decreased by applic ation of DTT (5 mM). From these results, it could be suggested that hy poxic pulmonary vasoconstriction is attributable, at least in part, to a change of cellular redox state, which decreases outward K+ currents . This hypothesis is further supported by the observation that the bas al redox state of EASMC K-Ca channels is more reduced than that of PAS MC K-Ca channels. The distinct responses to hypoxia of pulmonary and s ystemic arterial smooth muscle could be explained by this difference.