Phosphate, calcium and magnesium fluxes into the lumen of the rat proximal convoluted tubule.

In order to study fluxes of phosphate (Pi), Ca and Mg into the rat proximal tubule, a modification of the split-droplet microinjection technique was used. Injected fluids were isotonic solutions containing no Pi, Ca or Mg. The initial NaCl concentration of the injectates was either (a) 115 mM(l (which resulted in net fluid entry into the lumen), (b) 125 mM/l (no net fluid movement) or (c) 150 mM/l (net reabsorption of fluid). Injected droplets were subsequently collected from the nephron and their ionic concentrations determined using electron probe analysis. All 3 ions entered the tubular lumen. For the 115 mM and 125 mM NaCl injectates, Pi concentration increased for the first 15 s, then reached steady values of 2.07 mM/l and 2.30 mM/l respectively. Using 150 mM NaCl as injectate, Pi concentration increased for only 10 s, and then reached an average value of 2.04 mM/l. Ca and Mg concentrations in reaspirated droplets showed no correlation with time, indicating that entry into the lumen was almost immediate. The mean Ca concentration using 115 mM NaCl injectate was 1.63 mM/l, higher than with equilibrated or reabsorbed injectates (1.01 and 1.15 mM/l respectively). Mg concentration following injection of 115 mM NaCl solution (0.45 mM/l) was lower than with the other 2 injectates (0.92 and 0.85 mM/l). It is suggested that Pi and Mg enter the proximal tubular lumen from the tubular cells, while Ca entry may be transtubular and take place via intercellular pathways.