VOLTAGE-GATED K+ CURRENTS REGULATE RESTING MEMBRANE-POTENTIAL AND [CA2+](I) IN PULMONARY ARTERIAL MYOCYTES

The membrane potential (E(m)) of pulmonary arterial smooth muscle cells (PASMCs) regulates pulmonary arterial tone by controlling voltage-gated Ca2+ channel activity, which is a major contributor to [Ca2+](i). The resting membrane is mainly permeable to K+; thus, the resting E(m) is controlled by K+ permeability through sarcolemmal K+ channels. At least three K+ currents, voltage-gated K+ (K-V) currents, Ca2+-activated K+ (K-Ca) currents, and ATP-sensitive (K-ATP) currents, have been identified in PASMCs. In this study, both patch-clamp and quantitative fluorescent microscopy techniques were used to determine which kind(s) of K+ channels (K-V, K-Ca, and/or K-ATP) is responsible for controlling E(m) and [Ca2+](i) under resting conditions in rat PASMCs. When the bath solution contained 1.8 mmol/L Ca2+ and the pipette solution included 0.1 mmol/L EGTA, depolarizations (-40 to +80 mV) elicited both K-Ca and K-V currents. Removal of extracellular Ca2+ and increase of intracellular EGTA concentration (to 10 mmol/L) eliminated the Ca2+ influx-dependent K-Ca current. 4-Aminopyridine (4-AP, 5 to 10 mmol/L) but not charybdotoxin (ChTX, 10 to 20 nmol/L) significantly reduced K-V current under these conditions. In current-clamp experiments, 4-AP decreased E(m) (depolarization) and induced Ca2+-dependent action potentials; this depolarization increased [Ca2+](i) in intact PASMCs. Neither ChTX nor the specific blocker of K-ATP channels, glibenclamide (2 to 10 mu mol/L), caused membrane depolarization and the increase in [Ca2+](i). However, pretreatment of PASMCs with ChTX enhanced the 4-AP-induced increase in [Ca2+](i). These results suggest that the 4-AP-sensitive K-V currents that are active in the resting state are the major contributors to regulation of E(m) and thus [Ca2+](i) in rat PASMCs.