Extracellular acidosis activates ASIC-like channels in freshly isolated cerebral artery smooth muscle cells

Abstract

Recent studies suggest that certain acid-sensing ion channels (ASIC) are expressed in vascular smooth muscle cells (VSMCs) and are required for VSMC functions. However, electrophysiological evidence of ASIC channels in VSMCs is lacking. The purpose of this study was to test the hypothesis that isolated cerebral artery VSMCs express ASIC-like channels. To address this hypothesis, we used RT-PCR, Western blotting, immunolabeling, and conventional whole cell patch-clamp technique. We found extracellular H+-induced inward currents in 46% of cells tested ( n = 58 of 126 VSMCs, pH 6.5–5.0). The percentage of responsive cells and the current amplitude increased as the external H+ concentration increased (pH6.0, n = 28/65 VSMCs responsive, mean current density = 8.1 ± 1.2 pA/pF). Extracellular acidosis (pH6.0) shifted the whole cell reversal potential toward the Nernst potential of Na+ ( n = 6) and substitution of extracellular Na+ by N-methyl-d-glucamine abolished the inward current ( n = 6), indicating that Na+ is a major charge carrier. The broad-spectrum ASIC blocker amiloride (20 μM) inhibited proton-induced currents to 16.5 ± 8.7% of control ( n = 6, pH6.0). Psalmotoxin 1 (PcTx1), an ASIC1a inhibitor and ASIC1b activator, had mixed effects: PcTx1 either 1) abolished H+-induced currents (11% of VSMCs, 5/45), 2) enhanced or promoted activation of H+-induced currents (76%, 34/45), or 3) failed to promote H+ activation in nonresponsive VSMCs (13%, 6/45). These findings suggest that freshly dissociated cerebral artery VSMCs express ASIC-like channels, which are predominantly formed by ASIC1b.