In many papers that uses proteoliposome uptake assay of proton-coupled transporters (proton symporters), people compare the results of "proton-dependent liposome assay using different pH in internal and external the buffers" with the "proton-independent counterflow liposome assay". In the counterflow liposome assay, there is no pH difference between the internal and external buffer, and the liposome is loaded with high concentration of unlabeled substrate, and the external buffer has low concentration of radioactive substrate.

Here are some examples:

www.pnas.org/cgi/doi/10.1073/pnas.1301079110

www.pnas.org/cgi/doi/10.1073/pnas.1710727114

But if the transporter is a proton symporter, how can it transport the radioactive substrate in the counterflow assay system when there is no pH gradient?

Also, the radioactive substrate needs to get transported into liposome uphill its concentration gradient in the counterflow assay system - so wouldn't it need even more energy?

I tried reading many papers but none of them explained how this system works in detail and i couldn't get my head around it

I would really appreciate it if anyone could help me understand how the proton-independent counterflow assay works for the proton-coupled transporters.

Thank you in advance!