Pulmonary artery tone is partly determined by the membrane potential (Em) of pulmonary artery smooth muscle cells (PASMCs). The Kv7 family of K+ channels, encoded by KCNQ genes, has been implicated in the regulation of Em in rat PASMCs, such that retigabine, an activator of Kv7 channels, promotes pulmonary vasodilation (Joshi et al, 2009). Zinc pyrithione (ZnPy) is a recently identified Kv7 channel activator with a site of action distinct from retigabine (Xiong et al, 2008). This study aimed to determine if ZnPy could activate pulmonary artery Kv7 channels. Its effects on Em and whole-cell K+ currents were investigated in acutely isolated rat PASMC using the whole-cell patch clamp technique. Data are given as mean ± s.e.m. of n cells and were compared using students unpaired t-test, p<0.05 considered significant. At 10 µM, ZnPy consistently hyperpolarized PASMCs by 11 ± 1 mV (n=12, p<0.001). Its effect on Em was unaffected by the presence of 10μM glibenclamide (11 ± 1 mV, n=60, p<0.001), indicating that it did not involve activation of KATP channels. In contrast, in the presence of 10 mM tetraethylammonium ions (TEA), ZnPy (10 μM) did not cause significant hyperpolarisation (n= 10), suggesting that it might involve TEA-sensitive Kv7 or BKCa channels. Involvement of the latter is unlikely, because in the presence of 50 nM iberiotoxin to selectively block BKCa channels, the hyperpolarisation induced by ZnPy remained at 14 ± 3mV (n= 5, p=0.01). Kv7 channels are more likely to underlie the response to ZnPy, because it was abolished by the Kv7 blocker XE991 (10µM, n=10). ZnPy (10 μM) was also found to activate K+ current, but with different pharmacology. The current activated at 0 mV, by brief (200ms) steps from a holding potential of -80 mV, was enhanced nearly 4-fold from 532 ± 84 pA to 2000 ± 331 pA (n= 33, p <0.001). This increase was unaltered by 50 nM iberiotoxin and 10 μM XE99, but was greatly reduced by 10 mM TEA: ZnPy increased current from 547 ± 183 pA to only 617 ± 171 pA (n=6). When cells were clamped at 0 mV for at least 5 min to inactivate delayed rectifier K+ channels, ZnPy (10 μM) still induced outward current, from 41± 6 pA to 261 ± 79 pA (n=31, p<0.01). This effect was prevented in the presence of either 10 mM TEA (n=5) or 10 μM XE991 (n=4). Thus ZnPy activated a current with Kv7-like properties, but also induced an inactivating current mediated by distinct, TEA-sensitive, but XE991-insensitive channels. Taken together, the results support the idea that ZnPy hyperpolarises PASMC by opening non-inactivating, Kv7 channels. The hyperpolarisation is expected to inhibit voltage-gated Ca2+ influx, leading to muscle relaxation and ultimately pulmonary artery dilation.