What is the speed of response for metabotropic receptors?

• A. Slower than ion channels (mediated by effectors and second messengers)
• B. Faster than ion channels
• C. Immediate
• D. The same as ionotropic receptors

Answer: (A)

Metabotropic receptor signaling is slower because it operates through a cascade rather than direct gating. When a ligand binds a metabotropic receptor (a GPCR), the receptor activates a G protein, which then modulates an effector enzyme or ion channel via second messengers (like cAMP, IP3, or DAG). These second messengers trigger phosphorylation events and other biochemical steps before the final effect—such as opening or closing a channel or altering cellular activity—occurs. Each step adds delay, so the overall response develops over tens to hundreds of milliseconds or longer. In contrast, ionotropic (ligand-gated) receptors form a channel that opens directly in response to ligand binding, producing rapid ion flow within a few milliseconds. Thus, the speed of response for metabotropic receptors is slower than that of ion channels.