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.