03.01.4.1 Introduction

The second target for drug binding is ion channels. Ion channels are protein pores (hole) in the lipid membrane that can be opened or closed, similar to a gate that can be opened or closed. When an ion channel is closed, there is no ion transfer across the membrane. In contrast, when the ion channel is open, the opening is small and only allows ions to flow. When the channel is open, ions flow down their concentration gradient. The movement of ions underlies many physiological processes. Ion channels are specific for a species of ions (Na⁺ channels, K⁺ channels, Ca²⁺ channels), and there are also subtypes of ion channels. Ion channels have binding sites on them that drugs can use to inhibit or enhance the movement of ions, to alter the physiological function of the ion channel.