You are talking about 2 different experimental measurements. In an uptake experiment, you are measuring how much of a substance is accumulated by the oocytes in a given span of time. By comparing the amounts taken up by control oocytes lacking the transporter and oocytes expressing the transporter, you can determine whether the substance is a substrate of the transporter: more of the substance will be taken up by the oocytes expressing the transporter.

In an inhibition measurement, you are measuring the effect of substance I on the uptake of a known substrate S of the transporter. In this case, you measure the accumulation of S in the oocytes in the presence of a range of concentrations of I, plot a curve of S accumulation versus I concentration, fit the data to an IC50 curve equation, and compute the concentration of I that inhibits the accumulation of S by 50%. There is no upper bound on IC50 that qualifies something as an inhibitor. It could have an IC50 of 1000 µM and be considered as a rather weak inhibitor. However, it is not a good practice to extrapolate inhibition curves to 50% if the highest concentration tested gives less than 50% inhibition. Suppose you get 40% inhibition at the top concentration tested of 100 µM. You should report the IC50 as >100 µM. You might then choose to repeat the experiment with a higher range of inhibitor concentrations to get an unbounded value for IC50.

There is not always a clear distinction between a transport substrate and and inhibitor of transport. A substance can be both of those things, which is why it is necessary to do 2 different experiments.

Hi, Adam B Shapiro, thanks for your answer.

IC50 is a common index for us to assess the bioactivity of a compound. When I have IC50 data, I can classify them with a threshold such as 100 uMol.

However, in some literature, only "uptake" data are available. And I've no idea how to judge whether I should categorize it as a inhibitor or a noninhibitor.

More complicatedly, in some case a substrate mat have an uptake of less than 50% to the control while for another substrate, it may higher than 50%. And authors always use "significance" (p-value) to show whether it is a inhibitor. This makes me confusing about "how to classify" since IC50 is not available. Does the "uptake less than 50%" equals to "inhibition > 50%"?

P-value is a statistical parameter to decide whether a difference between 2 measurements may have occurred by chance at some threshold level of certainty. It may be used to decide whether the effect observed is likely to be real, or just random variation. If the experiment was designed to find out whether something was an inhibitor, the P-value allows you to state the likelihood that the effect observed was not due to chance, i.e. that there really was inhibition. The P-value, however, is not a measurement of the potency of inhibition.

If the experiment being performed was an inhibition measurement (i.e. there were 2 substances involved: a transport substrate and a potential inhibitor of the transporter), then the statement "uptake less than 50%" (referring to the transport substrate's uptake) indicates that inhibition was >50%. Therefore, ignoring any scatter in the data, the IC50 of the inhibitor would be less than the concentration of the inhibitor used in the experiment. However, an IC50 should be calculated based on the effect of several concentrations of inhibitor, not just one.

Agree with@Adam B Shapiro.

regards