Voltage-Gated K + Currents Regulate Resting Membrane Potential and [Ca 2+ ] i in Pulmonary Arterial Myocytes

Abstract

Abstract The membrane potential (E m ) of pulmonary arterial smooth muscle cells (PASMCs) regulates pulmonary arterial tone by controlling voltage-gated Ca 2+ channel activity, which is a major contributor to [Ca 2+ ] 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, Ca 2+ -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 [Ca 2+ ] i under resting conditions in rat PASMCs. When the bath solution contained 1.8 mmol/L Ca 2+ 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 Ca 2+ and increase of intracellular EGTA concentration (to 10 mmol/L) eliminated the Ca 2+ 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 Ca 2+ -dependent action potentials; this depolarization increased [Ca 2+ ] 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 [Ca 2+ ] i . However, pretreatment of PASMCs with ChTX enhanced the 4-AP–induced increase in [Ca 2+ ] 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 [Ca 2+ ] i in rat PASMCs.