Phosphate, calcium and magnesium transfers in proximal tubules and loops of henle, as measured by single nephron microperfusion experiments in the rat

Summary

Oil blocked superficial proximal tubules were microperfused in situ (30 nl/min) with physiological solutions labelled with⁴⁵Ca,³²P _i and³H-inulin containing either 2 mM (normal P _i ) or 0.6 mM (low P _i ) as phosphate concentrations. Fluid was successively collected from the first distal, the last proximal and one or two other proximal convolutions of each perfused nephron.

³H,⁴⁵Ca and³²P radioactivities were measured by liquid scintillation with appropriate energy discrimination. P _i , Ca and Mg concentrations were determined by electron probe analysis.

Net and unidirectional fluxes were calculated for each tubule from the data provided by paired samples.

The results indicate that:

• 1. Proximal calcium net absorption involves large bidirectional fluxes and is not affected by the P _i concentration in the perfusate; calcium absorption along the loop probably includes two different processes, the first, involving bidirectional fluxes, might take place in thepars recta, the second, involving low Ca backflux and active transport against a chemical gradient, might take place along the ascending limb.

• 2. Magnesium net absorption was very low or even absent in the perfused proximal segments; Magnesium net absorption in the loop resulted in an Mg concentration in the early distal fluid lower than that in plasma ultrafiltrate, indicating active transport out of the ascending limb; Mg and Ca net absorptions in the loop were highly correlated.

• 3. Proximal net phosphate absorption was present with normal P _i solutions, but not with low P _i solutions, suggesting that proximal phosphate transport is concentration-limited. Unidirectional lumen-to-plasma P _i flux was almost equal to the net flux with both kinds of solutions, indicating that phosphate reabsorption involves very low permeability of the proximal tubule to phosphate back diffusion. In the loop, net phosphate absorption was very limited (normal P _i ) or nil (low P _i ), and the absence of any drop in³²P specific activity indicates that no bidirectional P _i exchanges took place along the loop of Henle.

Mean values for unidirectional and net fluxes of P _i and calcium in the proximal are given in the tables as pmoles per min per mm of tubule, as well as mean values for the net absorption flux of Ca, Mg andP _i within the loops (pmoles per min per loop).