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

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 (pH(6.0), n = 28/65 VSMCs responsive, mean current density = 8.1 +/- 1.2 pA/pF). Extracellular acidosis (pH(6.0)) 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 microM) inhibited proton-induced currents to 16.5 +/- 8.7% of control (n = 6, pH(6.0)). 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.

Altered whole kidney blood flow autoregulation in a mouse model of reduced beta-ENaC.

Renal blood flow (RBF) autoregulation is mediated by at least two mechanisms, the fast acting myogenic response (approximately 5 s) and slow acting tubuloglomerular feedback (TGF; approximately 25 s). Previous studies suggest epithelial Na(+) channel (ENaC) family proteins, beta-ENaC in particular, mediate myogenic constriction in isolated renal interlobar arteries. However, it is unknown whether beta-ENaC-mediated myogenic constriction contributes to RBF autoregulation in vivo. Therefore, the goal of this investigation was to determine whether the myogenic mediated RBF autoregulation is inhibited in a mouse model of reduced beta-ENaC (m/m). To address this goal, we evaluated the temporal response of RBF and renal vascular resistance (RVR) to a 2-min step increase in mean arterial pressure (MAP). Pressure-induced changes in RBF and RVR at 0-5, 6-25, and 110-120 s after step increase in MAP were used to assess the contribution of myogenic and TGF mechanisms and steady-state autoregulation, respectively. The rate of the initial increase in RVR, attributed to the myogenic mechanism, was reduced by approximately 50% in m/m mice, indicating the speed of the myogenic response was inhibited. Steady-state autoregulation was similar between beta-ENaC +/+ and m/m mice. Although the rate of the secondary increase in RVR, attributed to TGF, was similar in beta-ENaC +/+ and m/m mice, however, it occurred over a longer period (+10 s), which may have allowed TGF to compensate for a loss in myogenic autoregulation. Our findings suggest beta-ENaC is an important mediator of renal myogenic constriction-mediated RBF autoregulation in vivo.