Introduction to Ion Transport Across Biological Membranes

The movement of inorganic ions across biological membranes of animals plays a central role in the perception and integration of, and reaction to, environmental signals by the organism. Examples include vision, the integration and processing of this information (brain function), and the reaction of the organism to this information, for instance, muscle contraction (Fig. 1). Cells have the ability to hydrolyze adenosine triphosphate (ATP). The energy thus released is used to transport sodium and potassium ions across the cell membrane against a concentration gradient. This process establishes the transmembrane voltage (V_m) of the cell membrane. The transmembrane voltage is perturbed by the movement of inorganic ions (generally Na⁺, K⁺, Cl⁻, and Ca²⁺) along the concentration gradient and voltage difference across the cell membrane that occurs in signal transmission between cells. There are many different transmembrane channel-forming proteins, which are activated by (1) a concentration gradient of inorganic ions across the membrane, (2) the transmembrane voltage, (3) the binding of specific ligands to a channel-forming protein. (4) Some proteins use the energy liberated by the hydrolysis of ATP to transport inorganic ions against a concentration gradient. Only one example of each of these various proteins will be mentioned. In each case, the protein chosen is the one about which we have the most information. The proteins that facilitate inorganic ion transport across biological membranes are discussed in an order that illustrates their function in the life of an organism.

Figure 1 An environmental stimulus, for instance, light, activates a protein-mediated reaction in the eye, leading to the transmembrane flux of inorganic ions, a change in the transmembrane voltage, and neurotransmitter release. The neurotransmitter diffuses across a junction between the nerve terminal of the axon and the cell body of the adjacent cell about 20–40 nm in length, called synapse. The neurotransmitter binds to receptors in the membrane of the postsynaptic cell. Excitatory neurotransmitters () activate receptors that form cation-specific transmembrane channels. Inhibitory neurotransmitters () activate receptors that form anion-specific transmembrane channels. Once the transmembrane voltage of the cell is changed by a critical amplitude and sign (by ~ +20 mV), an all-or-none process occurs. Transmembrane Na+ and K+ channels in the axonal membrane open transiently, resulting in an electrical signal that travels down the axon and neurotransmitter is again secreted. This process repeats itself and is terminated when the neurotransmitter is released adjacent to receptors on the surface of muscle cells. In the case of muscle cells, the receptor is the muscle nicotinic acetylcholine receptor and the neurotransmitter acetylcholine. The voltage change in the muscle cell membrane initiates muscle contraction. (From Hess, G. P., and Grewer, C. (1998). “Methods in Enzymology” (G. Marriott, ed.), Vol. 291, pp. 443–474, Academic Press, New York.) The resulting flow of inorganic ions through the membrane of the muscle cell results in a change of its transmembrane voltage Vm and muscle contraction.