Detection and characterisation of a Na⁺/Li⁺ countertransporter in renal proximal tubule cells
This study aimed to determine whether a functionally appropriate cell type, renal proximal tubule (PT) cells, displayed Na+/Li+ countertransport activity, and if so, to characterise the exchange mechanism further and delineate any relationship with the NHE. Both freshly isolated rat PT cells and primary cultures of rabbit PT cells demonstrated SLC activity. Since Li+ can take the place of Na+ in various different exchange mechanisms, the contribution of the Na+/K+ ATPase and NHE to the measured SLC activity was shown to be minimal with the use of specific inhibitors for these transporters. Kinetic parameters could not be determined in these experiments, however, as Li+ efflux from cells was shown to be extremely rapid and initial rates of efflux could not be accurately determined. To further investigate the possibility of SLC and NHE being associated transporters, we wished to measure protein expression of NHE1 and NHE3 in rabbit PT cells and correlate this with erythrocyte and PT cell SLC activity. Failure to detect either NHE isoform in the rabbit PT cells, however, prevented completion of these experiments. Evidence to suggest a possible link between SLC and NHE activity arose through investigation into signalling mechanisms affecting SLC activity. Similar to previous published data on the inhibitory effect of cAMP upon NHE activity, human erythrocyte SLC Km and Vmax were reduced following stimulation of the cAMP pathway. Because of the inability to measure kinetic parameters for SLC activity in the rat PT cell model, only standard SLC activity could be measured and this was not affected by stimulation of cAMP pathway using cAMP derivatives or forskolin. This is the first study which has identified SLC activity in a renal cell type and further characterisation will hopefully lead to the identification of the protein responsible for SLC activity and contribute to the understanding of the pathophysiological role this transport mechanism.