Chapter 4: Pharmacodynamics: Target Tissue Receptors

INTRODUCTION

A receptor is a component on or within a cell that a substance can bind to.¹ Most drugs exert their effect by binding to and activating one of these receptors.¹ The receptors can be any cellular macromolecule, but many receptors have been identified as proteins or protein complexes that are located on or within the cell.^1,2 When a drug binds to a receptor, it initiates a chain of biochemical events that brings about some change in the physiological function of the cell.

This chapter will help you understand how drugs interact with specific receptors, and how these interactions cause changes in cell function. Drug–receptor interactions at the cellular level ultimately cause the physiological changes that we observe in our patients in response to drug therapy. Likewise, abnormal drug responses can sometimes be explained by changes in the way that drugs interact with their receptors. Hence, this chapter will help you understand how drugs addressed in subsequent chapters act on specific cells, and why these drugs cause various responses in patients.

RECEPTORS LOCATED ON THE CELL’S SURFACE

Receptors that recognize endogenous and exogenous compounds often have a binding site for these compounds that is located on the outer surface of the cell membrane.¹ By placing the binding site of the receptor on its outer surface, the cell is able to differentiate and respond to specific substances that approach the cell, without actually allowing these substances to enter. The receptor can then transmit a message through the cell membrane to cause some change in the activity within the cell. Hence, many surface receptors are “transmembrane” proteins that span the width of the cell membrane and can relay information from the binding site of the outer surface to some intracellular mechanism that causes a change within the cell.

Surface receptors are primarily responsive to specific amino acid, peptide, or amine compounds. These receptors typically affect cell function in one of three ways:

• By acting as an ion channel and directly altering membrane permeability

• By acting enzymatically to directly influence function within the cell

• By being linked to regulatory proteins that control other chemical and enzymatic processes within the cell

Each of these is addressed here.

Surface Receptors Linked Directly to Ion Channels

Membrane receptors may be involved directly in the cellular response to the drug by acting as an ion pore, which changes the membrane permeability.³ Perhaps the most well-known example is the acetylcholine receptor located on the postsynaptic membrane of the neuromuscular junction^2,4 (Fig. 4-1). When bound by acetylcholine molecules, the receptor activates and opens a pore through the cell membrane, thereby increasing the permeability of the muscle cell to sodium.^5,6 This action results in depolarization and excitation of the cell because of sodium influx. Another ...