VARIABLE NMR VISIBILITY OF INTRACELLULAR SODIUM INDUCED BY NA+-SUBSTRATE COTRANSPORT IN DOG CORTICAL TUBULES

The intracellular sodium concentration ([Na +]i) of dog kidney cortical tubules was monitored by flame photometry and 23Na NMR using dysprosium tripolyphosphate as shift reagent. Upon addition of substrates cotransported with sodium, flame photometry showed an increase in [Na+]i while no change (glutamine, glucose) or even a decrease (lactate) in the Na i+ NMR signal was observed. This discrepancy could not be explained by a lack of ATP prior to the addition of substrates or by a decrease of NMR visibility of Na i+ induced by binding of substrate to membrane transporters (and pump). We propose that a variation of the “apparent visibility” of Na i+ may occur, arising from either a compartmentation of Na i+ in dog cortical tubules or an inhomogeneous extracellular distribution of the shift reagent. © 1990 Academic Press, Inc. Copyright © 1990 Wiley‐Liss, Inc., A Wiley Company