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Biologists have long recognized that the transport of ions and of neutral species across cell membranes is central to physiological function. Cells rely on their biomembranes, which separate the cytoplasm from the extracellular medium, to maintain the two electrolytes at very different composition. Specialized molecules, essentially biological nanodevices, have evolved to selectively control the movement of all the major physiological species. As should be clear, there have to be at least two distinct modes of transport. To maintain the disequilibrium, there must be molecular assemblies that drive ions and other permeable species against their electrochemical potential gradients. Such devices require energy input, typically coupling a vectorial pump with a chemical reaction, the dephosphorylation of ATP (adenosine triphosphate). These enzymes () control highly concerted, and relatively slow, process, with turnovers of ≫ 100 s¡ .

Gramicidin channels are miniproteins in which two tryptophan-rich subunits associate by means of transbilayer dimerization to form the conducting channels. That is, in contrast to other ion channels, gramicidin channels do not open and close; they appear and disappear. Each subunit in the bilayer-spanning channel is tied to the bilayer/solution interface through hydrogen bonds that involve the indole NH groups as donors andwater or the phospholipid backbone as acceptors. The channel’s permeability characteristics are well-defined: gramicidin channels are selective for monovalent cations, with no measurable permeability to anions or polyvalent cations; ions and water move through a pore whose wall is formed by the peptide backbone; and the single-channel conductance and cation selectivity vary when the amino acid sequence is varied, even though the permeating ions make no contact with the amino acid side chains. Given the plethora of available experimental information—for not only the wild-type channels but also for channels formed by amino acid-substituted gramicidin analogues—gramicidin channels continue to provide important insights into the microphysics of ion permeation through bilayer-spanning channels. For similar reasons, gramicidin channels constitute a system of choice for evaluating computational strategies for obtaining mechanistic insights into ion permeation through the more complex channels formed by integral membrane proteins.

The bit of information in nerves is the action potential, a fast electrical transient in the transmembrane voltage that propagates along the nerve fiber. In the resting state, the membrane potential of the nerve fiber is about ¡ 60 mV (negative inside with respect to the extracellular solution). When the action potential is initiated, the membrane potential becomes less negative and even reverses sign (overshoot) within a millisecond and then goes back to the resting value in about 2 ms, frequently after becoming even more negative than the resting potential. In a landmark series of papers, Hodgkin and Huxley studied the ionic events underlying the action potential and were able to describe the conductances and currents quantitatively with their classical equations (Hodgkin and Huxley, 1952). The generation of the rising phase of the action potential was explained by a conductance to Na ions that increases as the membrane potential is made more positive. This is because, as the driving force for the permeating ions (Na) was in the inward direction, more Na ions come into the nerve and make the membrane more positive initiating a positive feedback that depolarizes the membrane even more. This positive feedback gets interrupted by the delayed opening of another voltage-dependent conductance that is K-selective. The driving force for K ions is in the opposite direction of Na ions, thus K outward flow repolarizes the membrane to its initial value. The identification and characterization of the voltage-dependent Na and K conductances was one of the major contributions of Hodgkin and Huxley. In their final paper of the series, they even proposed that the conductance was the result of increased permeability in discrete areas under the control of charges or dipoles that respond to the membrane electric field. This was an insightful prediction of ion channels and gating currents.

Potassium (K) channels are largely responsible for shaping the electrical behavior of cell membranes. K channel currents set the resting membrane potential, control action potential duration, control the rate of action potential firing, control the spread of excitation and Ca influx, and provide active opposition to excitation. To support these varied functions, there are a large number of K channel types, with a great deal of phenotypic diversity, whose properties can be modified by many different accessory proteins and biochemical modulators.

Among ion channels, the large-conductance Ca-activated K channel (BK channel) is in many ways unique. It has a very large single-channel conductance—ten times that of most vertebrate K channels—and yet it maintains strict K selectivity. It senses as little as 200 nM Ca, but it contains no consensus Ca-binding motifs, and it is the only channel to be activated by both intracellular Ca and membrane voltage. In fact, there is a synergy between these stimuli such that the higher the internal Ca concentration ([Ca]), the smaller the depolarization needed to activate the channel. Furthermore, the BK channel has its own brand of auxiliary subunits that profoundly affect gating. In this chapter, I will discuss what is understood about the origins of these properties in terms of allosteric models and channel structure. At the outset, however, I should say that there is not yet a crystal structure of the BK channel or any of its components, so much of the current thinking about BK-channel structure relies on analogy to other channels.

Voltage-gated sodium channels subserve regenerative excitation throughout the nervous system, as well as in skeletal and cardiac muscle. This excitation results from a voltage-dependent mechanism that increases regeneratively and selectively the sodium conductance of the channel e-fold for a 4–7 mV depolarization of the membrane with time constants in the range of tens of microseconds. Entry of Na into the cell without a companion anion depolarizes the cell. This depolarization, called the action potential, is propagated at rates of 1–20 meters/sec. In nerve it subserves rapid transmission of information and, in muscle cells, coordinates the trigger for contraction. Sodium-dependent action potentials depolarize the membrane to inside positive values of about 30–40 mV (approaching the electrochemical potential for the transmembrane sodium gradient). Repolarization to the resting potential (usually between –60 and –90 mV) occurs because of inactivation (closure) of sodium channels, which is assisted in different tissues by variable amounts of activation of voltage-gated potassium channels. This sequence results in all-or-nothing action potentials in nerve and fast skeletal muscle of 1–2 ms duration, and in heart muscle of 100–300 ms duration. Recovery of regenerative excitation, i.e., recovery of the ability of sodium channels to open, occurs after restoration of the resting potential with time constants of a few to several hundreds of milliseconds, depending on the channel isoform, and this rate controls the minimum interval for repetitive action potentials (refractory period).

Ion channels underlie the electrical activity of cells. Calcium channels have a unique functional role, because not only do they participate in this activity, they form the means bywhich electrical signals are converted to responses within the cell. Calcium concentrations in the cytoplasm of cells are maintained at a low level, and calcium channels activate quickly such that the opening of ion channels can rapidly change the cytoplasmic environment. Once inside the cell, calcium acts as a “second messenger” prompting responses by binding to a variety of calcium sensitive proteins. Calcium channels are known to play an important role in stimulating muscle contraction, in neurotransmitter secretion, gene regulation, activating other ion channels, controlling the shape and duration of action potentials and many other processes. Since calcium plays an integral role in cell function, and since excessive quantities can be toxic, its movement is tightly regulated and controlled through a large variety of mechanisms.

In the early 1980s, Chris Miller and colleagues described a curious “double-barreled” chloride channel from the electric organ of fish reconstituted in planar lipid bilayers (Miller and White, 1980). Single-channel openings occurred in “bursts” separated by long closures. A single burst was characterized by the presence of two open conductance levels of equal size and the gating (i.e., openings and closings) during a burst could be almost perfectly described as a superposition of two identical and independent conductances that switched between open and closed states with voltage-dependent rates α and β (Hanke and Miller, 1983) (Fig. 8.1).

Ligand-gated ion channels (LGICs) are fast-responding channels in which the receptor, which binds the activating molecule (the ligand), and the ion channel are part of the same nanomolecular protein complex. This chapter will describe the properties and functions of the nicotinic acetylcholine LGIC superfamily, which play a critical role in the fast chemical transmission of electrical signals between nerve cells at synapses and between nerve and muscle cells at endplates. All the processing functions of the brain and the resulting behavioral output depend on chemical transmission across such neuronal interconnections. To describe the properties of the channels of this LGIC superfamily,we will mainly use two examples of this family of channels: the excitatory nicotinic acetylcholine receptor (nAChR) and the inhibitory glycine receptor (GlyR) channels. In the chemical transmission of electrical signals, the arrival of an electrical signal at the synaptic terminal of a nerve causes the release of a chemical signal—a neurotransmitter molecule (the ligand, also referred to as the agonist). The neurotransmitter rapidly diffuses across the very narrow 20–40 nm synaptic gap between the cells and binds to the LGIC receptors in the membrane of the target (postsynaptic) cell and generates a new electrical signal in that cell (e.g., Kandel et al., 2000). How this chemical signal is converted into an electrical one depends on the fundamental properties of LGICs and the ionic composition of the postsynaptic cell and its external solution.

Living cells are exposed to a variety of mechanical stimuli acting throughout the biosphere. The range of the stimuli extends from thermal molecular agitation to potentially destructive cell swelling caused by osmotic pressure gradients. Cellular membranes present a major target for these stimuli. To detect mechanical forces acting upon them cell membranes are equipped with mechanosensitive (MS) ion channels. Functioning as molecular mechanoelectrical transducers of mechanical forces into electrical and/or chemical intracellular signals these channels play a critical role in the physiology of mechanotransduction. Studies of prokaryotic MS channels and recent work on MS channels of eukaryotes have significantly increased our understanding of their gating mechanism, physiological functions, and evolutionary origins as well as their role in the pathology of disease.