Gating of ion channels can be controlled by various factors, such as membrane voltage, extracellular agonists or antagonists, mitochondria, and water channels (aquaporins). These factors can regulate the gating of a cation-selective channel in the motor end plate of skeletal muscle cells. Intracellular messengers like Ca++, ATP, and cyclic guanosine monophosphate, as well as mechanical stretch of the plasma membrane, can also influence ion channel gating. The number of channels in the membrane and gating of the channels play a role in their regulation.

Solute carriers (SLCs) are a diverse group of membrane transporters categorized into over 50 families, with nearly 400 specific transporters identified so far. Uniporters, or facilitated transporters, transport a single molecule across the membrane. An important member of this group is the glucose transporter 1 (GLUT-1), also known as SLC2A1, which facilitates the entry of glucose into the cell. Symporters, or cotransporters, transport two or more molecules/ions across the membrane in the same direction. The Na+,K+,2Cl− (NKCC) symporter, specifically NKCC2 or SLC12A1, found in the kidney, is crucial for urine dilution and concentration. Antiporters, or exchange transporters, transport two or more molecules/ions across the membrane in opposite directions. The Na+-H+ antiporter, a member of this group of solute carriers, regulates intracellular pH and is present in all cells.

ATP-dependent transporters utilize ATP energy to drive the movement of molecules/ions across the membrane. There are two groups of ATP-dependent transporters: ATPase ion transporters and ATP-binding cassette (ABC) transporters. ATPase ion transporters can be further categorized into P-type ATPases and V-type ATPases. P-type ATPases undergo phosphorylation during the transport cycle. Na+,K+-ATPase is a notable P-type ATPase that transports three Na+ ions out of the cell and two K+ ions into the cell upon ATP hydrolysis. It plays a critical role in establishing cellular ion and electrical gradients, as well as maintaining cell volume. V-type H+-ATPases, also known as vacuolar H+-ATPases, are found in the membranes of various intracellular organelles such as endosomes and lysosomes. Another type of ATPase, F-type ATPases, is responsible for ATP synthesis in mitochondria but is not discussed in this chapter.
Fig. 1.4 ). Factors that can control gating include membrane voltage, extracellular agonists or antagonists (e.g., acetylcholine is an extracellular agonist mitochondria,andwaterchannels(i.e.,aquaporins)thatfunctionas that controls the gating of a cation-selective channel in the apore.
ATP,adenosinetriphosphate.motor end plate of skeletal muscle cells; see
Chapter 6), intracellular messengers (e.g., Ca++ , ATP, cyclic guanosine
CHAPTER 1 Principles of Cell and Membrane Function •Fig. 1.4 Recordingofcurrentflowthroughasingleionchannel.Thechannelspontaneouslyfluctuatesbetweenanopenstateandaclosedstate.Theamplitudeofthecurrentisapproximately2pA(2× 10−12amps);thatis,12.5millionions/secondcrossthemembrane.
monophosphate), and mechanical stretch of the plasma membrane. Ion channels can be regulated by a change in the number of channels in the membrane or by gating of the channels.
Solute carriers (denoted SLCs by the HUGO Gene Nomenclature Committee) represent a large group of membrane transporters categorized into more than 50 families; almost 400 specific transporters have been identified to date. These carriers can be divided into three groups according to their mode of transport. One group, uniporters (or facilitated transporters), transports a single molecule across the membrane. The transporter that brings glucose into the cell (glucose transporter 1 [GLUT-1], or SLC2A1) is an important member of this group. The second group, symporters (or cotransporters), couples the movement of two or more molecules/ions across the membrane. As the name implies, the molecules/ions are transported in the same direction. The Na+,K+,2Cl− (NKCC) symporter found in the kidney (NKCC2, or SLC12A1), which is crucial for diluting and concentrating the urine (see ), is a member of this group. The third group, antiporters (or exchange transporters), also couples the movement of two or more molecules/ions across the membrane; in this case, however, the molecules/ions are transported in opposite directions. The Na+-H+ antiporter is a member of this group of solute carriers. One isoform of this antiporter (NHE-1, or SLC9A1) is found in all cells and plays an important role in regulating intracellular pH.
The ATP-dependent transporters, as their name implies, use the energy in ATP to drive the movement of molecules/ ions across the membrane. There are two groups of ATP-dependent transporters: the ATPase ion transporters and the ATP-binding cassette (ABC) transporters. The ATPase ion transporters are subdivided into P-type ATPases and V-type ATPases.
The P-type ATPases are phosphorylated during the transport cycle. Na+,K+-ATPase is an important example of a P-type ATPase. With the hydrolysis of each ATP molecule, it transports three Na+ ions out of the cell and two K+ ions into the cell. Na+,K+-ATPase is present in all cells and plays a critical role in establishing cellular ion and electrical gradients, as well as maintaining cell volume (see
V-type H+-ATPases are found in the membranes of several intracellular organelles (e.g., endosomes, lysosomes); as a result, they are also referred to as vacuolar H+-ATPases. The aAnother type of ATPases, F-type ATPases, is found in the mitochondria, and they are responsible for ATP synthesis. They are not considered in this chapter.