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The present study was designed to investigate the electrophysiological characteristics of rat conduit pulmonary arterysmooth muscle cells (PASMCs) and the response to acute hypoxia. PASMCs of the 1st to 2nd order branches in the conduit pulmonaryarteries were obtained by enzymatic isolation. The PASMCs were divided into acute hypoxia preconditioned group and normoxiagroup. Hypoxia solutions were achieved by bubbling with 5% CO2 plus 95% N2 for at least 30 min before cell perfusion. Potassiumcurrents were compared between these two groups using whole-cell patch clamp technique. The total outward current of PASMCs wasmeasured under normoxia condition when iBTX [specific blocking agent of large conductance Ca-activated K+ (BKCa) channel] and4-AP [specific blocking agent of delayed rectifier K+ (KDR) channel] were added consequently into bath solution. PASMCs wereclassified into three types according to their size, shape and electrophysiological characteristics. Type I cells are the smallest withspindle shape, smooth surface and discrete perinuclear bulge. Type II cells show the biggest size with banana-like appearance. Type IIIcells have the similar size with type I, and present intermediary shape between type I and type II. iBTX had little effect on the totaloutward current in type I cells, while 4-AP almost completely blocked it. Most of the total outward current in type II cells wasinhibited by iBTX, and the remaining was sensitive to 4-AP. In type III cells, the total outward current was sensitive to both iBTX and4-AP. Acute hypoxia reduced the current in all three types of cells: (1 614.8±62.5) pA to (892.4±33.6) pA for type I cells (P<0.01);(438.3±42.8) pA to (277.5±44.7) pA for type II cells (P<0.01); (1 042.0±37.2) pA to (613.6±23.8) pA for type III (P<0.01), and raisedthe resting membrane potentials (Em) in all these three types of cells: (–41.6±1.6) mV to (–18.6±1.5) mV (P<0.01), (–42.3±3.8) mV to (–30.6±3.0) mV (P<0.01), (–43.3±1.6) mV to (–28.4±1.4) mV (P<0.01), for type I, II, III cells, respectively. These results suggest thatacute hypoxia suppresses the potassium current and improves the Em in PASMCs. These effects may be involved in the modulation ofconstriction/relaxation of conduit artery under acute hypoxia. Different distribution of KDR and BKCa channels in these three types ofPASMCs might account for their different constriction/relaxation response to acute hypoxia.