Ion channels: structure, function and pharmacology
Excitable cells respond to a variety of informational inputs, chemical and physical, including neurotransmitters, hormones, pheromones, heat, light and pressure. These informational inputs are coupled to cellular response through transduction systems that include enzyme activation, substrate internalization and ion channel opening and closing (Figure 7.1). Ion channels are one class of biological effectors. They function to permeate ions, including the physiological cations Na+, K+, Ca2+ and Mg2+ and the anion Cl−, in response to diverse cell stimuli. The resultant ionic current may itself be the end consequence as in the maintenance of membrane potential or the discharge of potential in electric fish. More commonly, the ion current is coupled to other events, including the alteration of cellular sensitivity to other stimuli and the major generic processes of excitation-contraction coupling and stimulus-secretion coupling. In the latter examples, there is a dual function for calcium since it both carries current and serves as a cellular messenger coupling cell excitation to the calcium-dependent events of contraction and secretion.