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Acid-sensing ion channels (ASICs) are proton-activated sodium channels of the nervous system. Mammals express four ASICs, and orthologs of these genes have been found in all chordates examined to date. Despite a high degree of sequence conservation of all ASICs across species, the response to a given increase in external proton concentration varies markedly: from large and slowly inactivating inward currents to no detectable currents. The underlying bases of this functional variability and whether it stems from differences in proton-binding sites or in structures that translate conformational changes have not been determined yet. We show here that the ASIC1 ortholog of an early vertebrate, lamprey ASIC1, does not respond to protons; however, only two amino acid substitutions for the corresponding ones in rat ASIC1, Q77L and T85L, convert lamprey ASIC1 into a highly sensitive proton-activated channel with apparent [H.sup.+] affinity of [pH.sub.50] 7.2. Addition of C73H increases the magnitude of the currents by fivefold, and W64R confers desensitization similar to that of the mammalian counterpart. Most amino acid substitutions in these four positions increase the rates of opening and closing the pore, whereas only few, namely, the ones in rat ASIC1, slow the rates. The four residues are located in a contiguous segment made by the [beta]1-[beta]2-linker, [beta]1-strand, and the external segment of the first transmembrane helix. We conclude that the segment thus defined modulates the kinetics of opening and closing the pore and that fast kinetics of desensitization rather than lack of acid sensor accounts for the absence of proton-induced currents in the parent lamprey ASIC1. epithelial sodium channel/degenerin; proton sensor; proton-sensing ion channel; channel kinetics; activation by low pH; acid-sensing ion channel doi: 10.1152/ajpcell.00100.2010.

Voltage-gated [Na.sup.+]] channel currents ([I.sub.Na]) are expressed in several types of smooth muscle cells. The purpose of this study was to evaluate the expression of [I.sub.Na], its functional role, pathophysiology in cultured human (hASMCs) and rabbit aortic smooth muscle cells (rASMCs), and its association with vascular intimal hyperplasia. In whole cell voltage clamp, [I.sub.Na] was observed at potential positive to -40 mV, was blocked by tetrodotoxin (TTX), and replacing extracellular [Na.sub.+] with N-methyl-D-glucamine in cultured hASMCs. In contrast to native aorta, cultured hASMCs strongly expressed SCN9A encoding [Na.sub.v]1.7, as determined by quantitative RT-PCR. [I.sub.Na] was abolished by the treatment with SCN9A small-interfering (si)RNA (P < 0.01). TTX and SCN9A siRNA significantly inhibited cell migration (P < 0.01, respectively) and horseradish peroxidase uptake (P < 0.01, respectively). TTX also significantly reduced the secretion of matrix metalloproteinase-2 6 and 12 h after the treatment (P < 0.01 and P < 0.05, respectively). However, neither TTX nor siRNA had any effect on cell proliferation. L-type [Ca.sup.2+] channel current was recorded, and [I.sub.Na] was not observed in freshly isolated rASMCs, whereas TTX-sensitive [I.sub.Na] was recorded in cultured rASMCs. Quantitative RT-PCR and immunostaining for [Na.sub.v]1.7 revealed the prominent expression of SCN9A in cultured rASMCs and aorta 48 h after balloon injury but not in native aorta. In conclusion, these studies show that [I.sub.Na] is expressed in cultured and diseased conditions but not in normal aorta. The [Na.sub.v]l.7 plays an important role in cell migration, endocytosis, and secretion. [Na.sub.v]1.7 is also expressed in aorta after balloon injury, suggesting a potential role for [Na.sub.v]1.7 in the progression of intimal hyperplasia. vascular smooth muscle; sodium channel; SCN9A; balloon injury

This study determined whether nucleotides that bind to purinergic receptors (P2R) regulate the expression or function of serum- and glucocorticoid-inducible kinase-1 (SGK1) in mouse renal inner medullar collecting duct cells (mIMCD-3). The SGK1 protein was detected by Western blotting. A significant reduction of cytosolic SGK1 expression was observed in the cells pretreated with P2R agonist adenosine 5'-O-(3-thiotriphosphate) (ATP[gamma]S), and the reduction could be reversed by P2R antagonists. This reduction was also observed in cells that were pretreated with agonists for P2R subtypes. Using ELISA, we observed a reduced SGK1 kinase activity in ATP[gamma]S-pretreated cells. This effect was reversed by P2R antagonists. Furthermore, an increase of SGK1 kinase activity in aldosterone-pretreated cells was suppressed by ATP[gamma]S. These studies demonstrate for the first time that SGK1 can be downregulated by nucleotides in renal collecting duct epithelial cells, likely via the activation of P2R, and suggest that activation of renal purinergic signaling regulates a SGK1-dependent pathway that is known to modulate ion transport in the renal collecting duct. purinergic signaling; phosphatidylinositol 4,5-bisphosphate; epithelial sodium channels; mineralocorticoid receptor; renal ion transport; serum- and glucocorticoid-inducible kinase-1