During the down regulation process, what occurs in response to continuous exposure to a ligand?

• A. The number of receptors increases
• B. The cell becomes unresponsive
• C. The affinity of receptors decreases
• D. Hormone production is stimulated

Answer: (B)

During the down-regulation process, continuous exposure to a ligand—such as a hormone or neurotransmitter—leads to a decrease in the responsiveness of the cell to that ligand. This typically occurs because prolonged stimulation can cause the cell to reduce the number of available receptors on its surface or decrease the sensitivity of those receptors. As a result, the cell may become unresponsive to the ligand. This mechanism serves as a protective response to prevent overstimulation and maintain cellular homeostasis.

In the context of the other options, the increase in the number of receptors would represent an up-regulation, where cells adapt to low levels of ligand by making more receptors. A decrease in the affinity of receptors could also occur, but the primary manifestation of down-regulation is the reduction in responsiveness, not necessarily just a decrease in affinity. Finally, while hormone production can be influenced by receptor activity, down-regulation typically does not stimulate more hormone production directly; rather, it signifies a need for the cell to adjust its sensitivity to the existing ligand levels.