Plasma membrane ion channels

Normal cellular physiology depends on the ability of ions to move across cell membranes. Ion channels are highly specialized transport proteins that facilitate this movement by forming regulated ion-permeable routes through the membranes of nearly all excitable and non-excitable cells. They are found in plasma membranes and in the membranes of intracellular organelles. Plasma membrane ion channels display significant structural and functional diversity (Ashcroft 2006) and... 

Hool, L.C., 2006. Reactive oxygen speeies in cardiae signalling from mitoehondria to plasma membrane ion channels. Clinical andExperimental Pharmacology Physiology 33(1-2), 146-151. 

In smooth muscle, the sarcoplasmic reticulum (SR) plays an important role in regulating cell excitability by communicating intimately with ion channels in the surface membrane. In most cases, Ca2+ release from ryanodine-sensitive Ca2+ release channels (RyRs) in the SR leads to a paradoxical decrease in smooth muscle cell excitability due to activation of plasma membrane K+ channels (Fig. 1 Nelson et al 1995). This is in stark contrast to cardiac muscle, where Ca2+ release from RyRs supplies the majority (> 90%) of Ca2+ required for contraction (Cheng et al 1993, Cannell et al 1995). In this paper, we will briefly review the basic... 

Ca2+ levels, with one of the intracellular calcium-chelating, fluorescent probes, quin-2, fura-2 or indo-1, demonstrates that, in both cases, there is a rapid rise in intracellular Ca2+ as this ion is released from intracellular stores. Analysis of stimulated B lymphocytes, using the probe fura-2, indicates that if Ca2+ in the external medium is removed the intracellular Ca2+ level returns to basal levels in 5 to 7 minutes, but if there is Ca2+ present in the external media a sustained increase in intracellular Ca2+ is detected [36]. Such analysis suggests the opening of a plasma membrane calcium channel but the nature of the channel or mechanism of its opening are not presently known. It is possible that the opening of this channel could be stimulated by one of the inositol phosphates. 

In addition to activating protein kinases, cyclic AMP and cyclic GMP also bind to and directly regulate the activity of plasma membrane cation channels referred to as cyclic nucleotide—gated (CNG) channels. CNG ion channels have been found in kidney, testis, heart, and the CNS. These channels open in response to direct binding of intracellular cyclic nucleotides and contribute to cellular control of the membrane potential and intracellular Ca levels. The CNG ion channels are multisubunit pore-forming channels that share structural similarity with the voltage-gated K channels. 

Ballatori N, Shi C, Boyer JL (1988) Altered plasma membrane ion permeability in mercury-induced cell injury studies in hepatocytes of elasmobranch Raja erinacea. Toxicol Appl Pharmacol 95 279-291 Benndorf K, Nilius B (1988) Different blocking effects of Cd and Hg on the early outward current in myocardial mouse cells. Gen Physiol Biophys 7 345-352 Blazka ME, Shaikh ZA (1991) Differences in cadmium and mercury uptakes by hepatocytes role of calcium channels. Toxicol Appl Pharmacol 110 355-363 Brunder DG, Dettbarn C, Palade P (1988) heavy metal-induced Ca " release from sarcoplasmic reticulum. J Biol Chem 263 18785-18792 Butler JN (1964) Introduction to complex formation equilibria. Ionic equilibrium, a mathematical approach. Addison-Wesley, Reading, Massachusetts Palo Alto London, p 261... 

Dmg receptors are chemical entities which are typically, but not exclusively, small molecules that interact with cellular components, frequently at the plasma membrane level (1,2). There are many types of receptors heat, light, immune, hormone, ion channel, toxin, and vims are but a few that can excite a cell. The receptor concept can be appHed generally to signal recognition processes where a chemical or physical signal is recognized. This recognition is translated into response (Fig. 3) and the process can be seen as a flow of information. 

Ion Channels. The excitable cell maintains an asymmetric distribution across both the plasma membrane, defining the extracellular and intracellular environments, as well as the intracellular membranes which define the cellular organelles. This maintained a symmetric distribution of ions serves two principal objectives. It contributes to the generation and maintenance of a potential gradient and the subsequent generation of electrical currents following appropriate stimulation. Moreover, it permits the ions themselves to serve as cellular messengers to link membrane excitation and cellular... 

Ion channels are proteins which span the plasma membrane and can be opened by transmembrane voltage changes (voltage-dependent ion currents) or by binding of a neurotransmitter. Ion channels which are selective for Na+or Ca2+ ions cause excitation, ion channels with selectivity for Cl- or K+ usually cause inhibition of cells. 

At the level of a single channel, addition of ACh is followed by transient openings of the channel. The current i flowing through an open channel is 4 pA at a membrane potential Voi-l 00 mV. Since one ampere (A) represents the flow of 6.24-1018 charges per second, 2.5-107 Na+ ions per second flow through an open channel. The conductance g of a plasma membrane channel is the measure of the ease of flow of cuirent between the extracellular space and the cytosol or vice... 

Plasma membrane channels. The most common mechanism for the movement of into smooth muscle cells Ifom the extracellular space is the electrodiffusion of Ca " ions through highly selective channels. This movement can be significant in two quite different ways. First, Ca ions carry two positive charges and, in fact, most of the inward charge movement across the plasma membrane of smooth muscle myocytes is carried by Ca. Most smooth muscle action potentials are known to be Ca " action potentials. And second, the concentration of intracellular free calcium, the second messenger, is increased by inward calcium movement. 

Calcium channels are members of the large family of proteins, including Na and channels, which become incorporated into plasma membranes, and which form intermittent aqueous pathways through which ions can move. The channels open and close. As is the case generally for membrane spanning proteins, a Ca channel is formed by a set of helical units, in this case seven, which associate to form the channel. 

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