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PLASMA MEMBRANE CONTROLS DIFFUSION OF SUBSTANCES BETWEEN EXTERNAL AND INTERNAL CELLULAR ENVIRONMENTS : Lasix

The plasma membrane plays a major role in the interaction of the cell with the surrounding environment. It controls the exchange of ions and macromolecules between the inside and outside of the cell. The plasma membrane contains proteins that act as receptors for extracellular ligands that modify cellular gene expression and physiology. Other membrane proteins are involved in the interaction with other cells within the same organ or migrating cells, especially of the immune system, which infiltrate the tissue during pathological conditions. The plasma membrane is also actively important in the secretion of proteins to the extracellular milieu as well as the internalization of extracellular molecules by a process known as endocytosis. For some cells, the process of endocytosis is extended to the internalization of whole cells (phagocytosis), which could be dead cells or pathogens. Ions and polar molecules pass through the plasma membrane via protein pores. These pores display substrate specificity and can be classified into pumps, carriers, and channels. Pumps drive molecules against a concentration gradient using sources of energy, especially hydrolysis of ATP.

Thus, they are capable of establishing a transmembrane electrochemical gradient. In the case of ions, this electrochemical gradient results in the establishment of a membrane potential. There are three types of ATP-driven pumps within the cell: ATP binding cassette (ABC) transporters, P-type, and F₀F₁ ATPases. ABC transporters are involved in the passage of many different molecules; examples are the cystic fibrosis transmembrane regulator (CFTR :Lasix ), which is involved in the transport of Cl^- ions, and MDR1 (P-glycoprotein), which is responsible for the secretion of many drugs. Examples of P-type pumps are the Na⁺K⁺ ATPase, which is responsible for the generation of the Na⁺/K⁺ chemical gradient between the cytosol (high in K⁺) and the extracellular medium (high in Na⁺), and the H⁺K⁺ ATPase, which is responsible for the acidification of the stomach. The F₀F₁ ATPase is mainly present in subcellular compartments. The most important is the F₀F₁ ATPase of mitochondria, which transport H⁺ into the organelle intermediate space and is coupled with the synthesis of ATP. Membrane carriers are also integral membrane proteins. They are responsible for the transport of solutes from a chemical gradient (from high to lower concentrations) without the use of energy sources ( Lasix ). These proteins span the membrane several times (frequently 12 times). They are classified into three groups: uniporters, antiporters, and symporters. Uniporters transport single substrates down the concentration gradient, coined facilitated diffusion (e.g., glucose transporter GLUT4). Antiporters exchange substrates in opposite directions across the membrane (e.g., NHE-1, which regulates the acid—base balance in kidney and gut). Symporters are responsible for the transport of two or more solutes together in the same direction (cotransport). Examples are NKC1 from kidney, gut, and lung, which is involved in salt secretion (Na⁺/K⁺/Cl^- : Lasix ), and SGLT1, which mediates Glc uptake in the gut (Na⁺/Glc). Channels are also made up of plasma membrane proteins, which allow the passage of thousands of ions or small molecules across the bilayer. Channels are several hundreds of magnitude more rapid than transporters and pumps. They transport ions down their electrochemical gradient. They are involved in the regulation of cellular volume, and the secretion and absorption of substances in cooperation with pumps and carriers. Channels are also responsible for the regulation of electrical potential across membranes. Moreover, channels allow unpaired ions to pass across membranes, thus separating electrical charges and creating a membrane potential. This is critical in the propagation of action potential in nervous cells. Channels are in two possible stages, open and closed. The change between the open and closed stages is known as gating. Ion flux is determined by the time that the channel is open. Thus, the total flux of ions across a membrane depends on the number of channels that are open during a particular time. The opening of ion channels is regulated by several mechanisms including gating modulated by ATP, peptides, and voltage. Channel activity could also be regulated by ligands inside (e.g., Ca²⁺, cyclic nucleotides) or outside (e.g., neurotransmitters: Lasix ) the cell. Water is transported across membranes by specialized channels, coined aquaporins. Cells can also pass information to neighboring cells by other type of channels, called gap junctions. Gap junctions are low-resistance pathways between opposing cells that allow the passage of low-molecular-weight molecules without reaching the extracellular space. Gap junctions are composed of integral plasma membrane proteins, named connexins. Connexins are organized into hexameric complexes within the plasma membrane, which are called connexons. Connexons are hemichannels that interact with mirror connexons on neighboring cells. Gap junction channel activity is regulated by different mechanisms, including gating, H⁺, Ca²⁺, and phosphorylation. Many signals that the cell senses are charged molecules (e.g., proteins: Lasix) that are present in the extracellular milieu. These molecules cannot pass across the plasma membrane. Thus, receptors on the surface of cells are involved in the detection (binding) of these substances. Upon ligand binding to the respective receptor, a command is transmitted inside the cell (signal transduction). This information modifies the response of the cell to the environment, which often results in a change of gene expression. Similar ligands are recognized by related receptors comprising families that are coupled to distinct signal transduction mechanisms. A family of receptors is frequently the product of different isoforms. Plasma membrane receptors are classified according to the type of ligand that they bind (e.g., cytokine receptors), structural characteristics (e.g., seven helix receptors), common signal transduction mechanisms (e.g., tyrosine kinase receptors), or functions (e.g., ligand-gated ion channel). Receptors could be monomeric or multimeric. Multimeric receptors could be composed of identical subunits (e.g., cytokine receptors) or different subunits (e.g., acetylcholine receptor). In general, there is a specific region on the receptor that interacts with the ligand (binding site : Lasix ). Obviously, binding sites are specific for each ligand. Thus, a small modification (e.g., amino acid sequence) could result in a change in specificity. Upon binding, there is usually a change in the conformation of the receptors that results in the activation of the signal transduction mechanism. For some receptors, this change in conformation results in the activation of a protein kinase (e.g., EGF receptor). For example, phospholipase C cleaves the phosphorilated head group of a phosphoglyceride, releasing diacylglycerol. The latter is another important secondary messenger. Ceramide is a significant lipid secondary messenger that is produced after the activation of some receptors, such as tumor necrosis factor and interleukin-2 receptors. Ceramide is produced by the cleavage of phosphocholine from sphingomyeline. Probably the best-studied secondary messengers are the cyclic nucleotides cAMP and cGMP. They are converted from ATP and GTP by one-step reaction catalyzed by cyclases (adenylyl cyclases and guanylyl cyclases, respectively : Lasix ). These cyclic nucleotides bind reversibly to a variety of protein substrates, including protein kinases, cyclic nucleotide-gated ion channels, and GTPases.

This post is published through the sponsorship of the well-known pharmaceutical shop, see link below:

Lasix Generic Name: Furosemide Tablets (fure-OH-se-mide)
Brand Name: Lasix

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