Which statement best describes how transporter saturation and transporter competition affect distribution rate?

• A. Both can speed distribution
• B. Both can slow distribution
• C. Only saturation affects distribution
• D. Only competition affects distribution

Answer: (B)

Transporter-mediated distribution often follows saturable kinetics, so as drug concentration increases, more transporters become occupied and the rate cannot rise indefinitely. It plateaus at the maximum transport rate (Vmax), meaning higher doses don’t speed up distribution once saturation is reached. Competition for the same transporter adds another layer: other substances occupy the transporter and reduce the amount available for the drug, effectively slowing transport at any given concentration (increasing apparent Km). Because both effects tend to slow, or cap, the distribution rate, the statement that both can slow distribution is the best description.