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A handful of biological proton-selective ion channels exist. Some open at positive or negative membrane potentials, others open at low or high pH, and some are light activated. This review focuses on common features that result from the unique properties of protons. Proton conduction through water or proteins differs qualitatively from that of all other ions. Extraordinary proton selectivity is needed to ensure that protons permeate and other ions do not. Proton selectivity arises from a proton pathway comprising a hydrogen-bonded chain that typically includes at least one titratable amino acid side chain. The enormously diverse functions of proton channels in disparate regions of the phylogenetic tree can be summarized by considering the chemical and electrical consequences of proton flux across membranes. This review discusses examples of cells in which proton efflux serves to increase pHi, decrease pHo, control the membrane potential, generate action potentials, or compensate transmembrane movement of electrical charge. [ABSTRACT FROM AUTHOR]

Voltage‐gated ion channels, whose first identified function was to generate action potentials, are divided into subfamilies with numerous members. The family of voltage‐gated proton channels (HV) is tiny. To date, all species found to express HV have exclusively one gene that codes for this unique ion channel. Here we report the discovery and characterization of three proton channel genes in the classical model system of neural plasticity, Aplysia californica. The three channels (AcHV1, AcHV2, and AcHV3) are distributed throughout the whole animal. Patch‐clamp analysis confirmed proton selectivity of these channels but they all differed markedly in gating. AcHV1 gating resembled HV in mammalian cells where it is responsible for proton extrusion and charge compensation. AcHV2 activates more negatively and conducts extensive inward proton current, properties likely to acidify the cytosol. AcHV3, which differs from AcHV1 and AcHV2 in lacking the first arginine in the S4 helix, exhibits proton selective leak currents and weak voltage dependence. We report the expansion of the proton channel family, demonstrating for the first time the expression of three functionally distinct proton channels in a single species. [ABSTRACT FROM AUTHOR]

Aspartate key to proton selectivity Voltage-gated proton channels are considered completely selective for protons -- no evidence exists for the permeation of ions other than H.sup.+. A study of the underlying mechanism of the high selectivity of the human voltage-gated proton channel reveals an Asp112 residue as the selectivity filter. When this residue was changed to a neutral amino acid, the mutant channel lost proton selectivity and either became anion-selective or did not conduct any ions. The ion selectivity of pumps and channels is central to their ability to perform a multitude of functions. Here we investigate the mechanism of the extraordinary selectivity of the human voltage-gated proton channel.sup.1, H.sub.V1 (also known as HVCN1). This selectivity is essential to its ability to regulate reactive oxygen species production by leukocytes.sup.2,3,4, histamine secretion by basophils.sup.5, sperm capacitation.sup.6, and airway pH.sup.7. The most selective ion channel known, H.sub.V1 shows no detectable permeability to other ions.sup.1. Opposing classes of selectivity mechanisms postulate that (1) a titratable amino acid residue in the permeation pathway imparts proton selectivity.sup.1,8,9,10,11, or (2) water molecules 'frozen' in a narrow pore conduct protons while excluding other ions.sup.12. Here we identify aspartate 112 as a crucial component of the selectivity filter of H.sub.V1. When a neutral amino acid replaced Asp 112, the mutant channel lost proton specificity and became anion-selective or did not conduct. Only the glutamate mutant remained proton-specific. Mutation of the nearby Asp 185 did not impair proton selectivity, indicating that Asp 112 has a unique role. Although histidine shuttles protons in other proteins, when histidine or lysine replaced Asp 112, the mutant channel was still anion-permeable. Evidently, the proton specificity of H.sub.V1 requires an acidic group at the selectivity filter., Author(s): Boris Musset [sup.1] , Susan M. E. Smith [sup.2] , Sindhu Rajan [sup.3] , Deri Morgan [sup.1] , Vladimir V. Cherny [sup.1] , Thomas E. DeCoursey [sup.1] Author Affiliations: [...]

Electrophysiological events are of central importance during the phagocyte respiratory burst, because NADPH oxidase is electrogenic and voltage sensitive. We investigated the recent suggestion that large-conductance, calcium-activated [K.sup.+] (BK) channels, rather than proton channels, play an essential role in innate immunity (Ahluwalia,J., A. Tinker, L.H. Clapp, M.R. Duchen, A.Y. Abramov, S. Page, M. Nobles, and A.W. Segal. 2004. Nature. 427:853-858). In PMA-stimulated human neutrophils or eosinophils, we did not detect BK currents, and neither of the BK channel inhibitors iberiotoxin or paxilline nor DPI inhibited any component of outward current. BK inhibitors did not inhibit the killing of bacteria, nor did they affect NADPH oxidase-dependent degradation of bacterial phospholipids by extracellular gIIA-PL[A.sub.2]) or the production of superoxide anion ([O.sub.2.sup..-]). Moreover, an antibody against the BK channel did not detect immunoreactive protein in human neutrophils. A required role for voltage-gated proton channels is demonstrated by [Zn.sup.2+] inhibition of NADPH oxidase activity assessed by [H.sub.2][O.sub.2] production, thus validating previous studies showing that [Zn.sup.2+] inhibited [O.sub.2.sup..-] production when assessed by cytochrome creduction. In conclusion, BK channels were not detected in human neutrophils or eosinophils, and BK inhibitors did not impair antimicrobial activity. In contrast, we present additional evidence that voltage-gated proton channels serve the essential role of charge compensation during the respiratory burst.

Voltage-gated proton channels are expressed highly in rat alveolar epithelial cells. Here we investigated whether these channels contribute to pH regulation. The intracellular pH ([pH.sub.i]) was monitored using BCECF in cultured alveolar epithelial cell monolayers and found to be 7.13 in nominally HC[O.sub.3]-free solutions [at external pH ([pH.sub.o]) 7.4]. Cells were acid-loaded by the N[H.sup.+.sub.4] prepulse technique, and the recovery was observed. Under conditions designed to eliminate the contribution of other transporters that alter pH, addition of 10 [micro]M Zn[Cl.sub.2], a proton channel inhibitor, slowed recovery about twofold. In addition, the [pH.sub.i] minimum was lower, and the time to nadir was increased. Slowing of recovery by Zn[Cl.sub.2] was observed at [pH.sub.o] 7.4 and [pH.sub.o] 8.0 and in normal and high-[K.sup.+] Ringer solutions. The observed rate of [Zn.sup.2+]-sensitive [pH.sub.i] recovery required activation of a small fraction of the available proton conductance. We conclude that proton channels contribute to [pH.sub.i] recovery after an acid load in rat alveolar epithelial cells. Addition of Zn[Cl.sub.2] had no effect on [pH.sub.i] in unchallenged cells, consistent with the expectation that proton channels are not open in resting cells. After inhibition of all known pH regulators, slow [pH.sub.i] recovery persisted, suggesting the existence of a yet-undefined acid extrusion mechanism in these cells. proton conductance; pH regulation; hydrogen ion; acid load; 2',7'bis(2-carboxyethyl)-5(6)-carboxyfluorescein; intracellular pH

Author(s): Thomas E. DeCoursey (corresponding author); Deri Morgan; Vladimir V. Cherny The enzyme NADPH oxidase in phagocytes is important in the body's defence against microbes: it produces superoxide anions (O[sub.2 [...]

Regulation of intracellular pH (pHi) in cardiomyocytes is crucial for cardiac function; however, currently known mechanisms for direct or indirect extrusion of acid from cardiomyocytes seem insufficient for energetically efficient extrusion of the massive H+ loads generated under in vivo conditions. In cardiomyocytes, voltage‐sensitive H+ channel activity mediated by the HVCN1 proton channel would be a highly efficient means of disposing of H+, while avoiding Na+ loading, as occurs during direct acid extrusion via Na+/H+ exchange or indirect acid extrusion via Na+–HCO3− cotransport. PCR and immunoblotting demonstrated expression of HVCN1 mRNA and protein in canine heart. Patch clamp analysis of canine ventricular myocytes revealed a voltage‐gated H+ current that was highly H+‐selective. The current was blocked by external Zn2+ and the HVCN1 blocker 5‐chloro‐2‐guanidinobenzimidazole. Both the gating and Zn2+ blockade of the current were strongly influenced by the pH gradient across the membrane. All characteristics of the observed current were consistent with the known hallmarks of HVCN1‐mediated H+ current. Inhibition of HVCN1 and the NHE1 Na+/H+ exchanger, singly and in combination, showed that either mechanism is largely sufficient to maintain pHi in beating cardiomyocytes, but that inhibition of both activities causes rapid acidification. These results show that HVCN1 is expressed in canine ventricular myocytes and provides a major H+ extrusion activity, with a capacity similar to that of NHE1. In the beating heart in vivo, this activity would allow Na+‐independent extrusion of H+ during each action potential and, when functionally coupled with anion transport mechanisms, could facilitate transport‐mediated CO2 disposal. Key points: Intracellular pH (pHi) regulation is crucial for cardiac function, as acidification depresses contractility and causes arrhythmias. H+ ions are generated in cardiomyocytes from metabolic processes and particularly from CO2 hydration, which has been shown to facilitate CO2 venting from mitochondria.Currently, the NHE1 Na+/H+ exchanger is viewed as the dominant H+ extrusion mechanism in cardiac muscle.We show that the HVCN1 voltage‐gated proton channel is present and functional in canine ventricular myocytes, and that HVCN1 and NHE1 both contribute to pHi regulation.HVCN1 provides an energetically efficient mechanism of H+ extrusion that would not cause Na+ loading, which can cause pathology, and that could contribute to transport‐mediated CO2 disposal.These results provide a major advance in our understanding of pHi regulation in cardiac muscle. [ABSTRACT FROM AUTHOR]