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 mildly 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 clump 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 μmol/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.