The Involvement of the Intracellular Redox State and pH in the Metabolic Control of Stimulus–Response Coupling

This chapter discusses the consecutive stages involved in the operation of the stimulus–response–recovery (SRR) cycle. When a pharmacological agonist interacts with a receptor, usually situated on the outer cell membrane, it sets in motion a chain of events known as the “SRR” cycle. The cycle starts with a “receptor mechanism”; it consists of those signaling processes that translate the receptor–agonist interaction into a change in the intracellular concentration of the second messenger. The chapter discusses the question of whether the redox state controlling enzymes and their products are themselves obligatory parts of a receptor mechanism or whether they act in a regulatory manner by inhibiting or stimulating part of the receptor mechanism. The chapter focuses on response systems activated by pharmacomechanic coupling rather than those occurring by electromechanic coupling. The difference lies in the fact that electromechanic coupling occurs via a change in potential across the plasma membrane, whereas pharmacomechanic coupling does not.