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Potassium transporters: inward current


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Potassium transporters: inward current

Potassium transporters: inward current

Most of potassium channels under physiological conditions provide outward potassiumcurrents that contribute to cell resting potential, action potential repolarization andhyperpolarization. Under certain conditions, these channels could mediate potassium flowinto the cell as well.

Unlike most voltage-gated channels, Hyperpolarization-activated cation channels of theHCN gene family, such as Hyperpolarization activated cyclic nucleotide-gated potassiumchannel 1 ( HCN1 ), are activated by hyperpolarizing voltage steps to negativepotentials to -60 mV, which is near the resting potentials of most cells. The HCNchannels are regulated by cyclic nucleotides and have unusual ion selectivity in thatthey conduct both Na+ and K+ ions but exclude Li+. The ratio of the K+ to Na+permeability of the channel ranges from 3:1 to 5:1, yielding values for the reversalpotential of -25 to -40 mV. As a result, activation of the channel at typical restingpotentials results in a net inward current carried largely by Na+. The latter depolarizesthe membrane toward threshold for firing an action potential. HCN channels contribute tospontaneous rhythmic activity in both heart and brain and Ih-current provided by thesechannels is often referred to as the pacemaker current. Ih also plays important role insensory transduction of visual, taste and olfactory stimuli [1], [2].

Cyclic-nucleotide gated ( CNG ) channels are nonselective cation channels thatare activated primarily by cAMP and cGMP. CNG channels are nonselective cationchannels, allowing the monovalent cations K+, Na+, Li+, Rb+, and Cs+ to conduct almostequally well. However, under physiological conditions, CNG channels carry aninward Na+ and Ca2+ current. CNG channels generate the primary electrical signalin photoreceptors and olfactory sensory neurons [2].

Amiloride-sensitive cation channel 1 ( BNaC1(ASIC2) ) and Amiloride-sensitivecation channel 2 ( BnaC2(ASIC1a) ) are activated by protons and are thereforeacid-sensing ion channels (ASICs). Typical ASIC current displays a Na+/K+ selectivityratio d10. ASICs show some permeability to Ca2+. These channels are thought to beinvolved in pain perception, especially following tissue acidosis [3].

Purinergic receptor P2X (P2X) family of ligand-gated ion channel receptors areactivated by extracellular ATP and are permeable to Na+, K+ and Ca2+. Opening of P2Xreceptors causes cell depolarization by cationic influx. P2X receptors play role insmooth muscle contraction, neuronal excitation, inflammation and cell death [4], [5].

Potassium ions could enter the cell not only through voltage- or ligand-gated ionchannels, but also through gap junctions. Gap junctions are specialized intercellularjunctions characterized by the apposition of the plasma membrane of apposing cells suchthat there is only a narrow 2-3 nm gap. They contain hydrophilic membrane channels thatmediate the passage of ions, metabolites, and small cell signaling molecules less than 1kDa in size. Gap junction is formed when two hemi channels (connexons) are joined end toend in the extracellular space, each connexon consisting of six protein subunits known asthe connexin, such as Gap junction protein gamma 1 ( Connexin 45 ). Gating of thegap junction channel is controlled by multiple factors such as calcium concentration, pH,transjunctional membrane potential and protein phosphorylation [6]