Substrate-induced modulation of ATP turnover in dog and rabbit proximal 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 Nai+ induced by binding of substrate to membrane transporters (and pump). We propose that a variation of the "apparent visibility" of Nai+ may occur, arising from either a compartmentation of Nai+ in dog cortical tubules or an inhomogeneous extracellular distribution of the shift reagent.