Dependence of water movement on sodium transport in kidney proximal tubule: A microperfusion study substituting lithium for sodium

Abstract

The relationship between water and sodium movements through the mammalian proximal convoluted tubule was investigated by substituting lithium for sodium. Proximal convoluted rat Kidney tubules were perfused in vivo with a Ringer solution containing 107 meq/liter lithium and 42 meq/liter sodium. Several micropunctures were made along the same nephron, and [3H] inulin, [14C] glucose, 22Na, osmolality, Na, Mg and Cl were determined on each sample. Measurements of 22Na showed that sodium and lithium diffusion rates were practically identical throughout the entire epithelium. A one- for-one exchange of sodium for lithium induced a negative transepithelial net flux of Na from plasma to lumen. However, despite this negative flux, a positive net water movement was measured from lumen to plasma. This movement was proportional both to glucose reabsorption and to the rise in the chloride concentration, two mechanisms known to be dependent on the transcellular movement of sodium. It was therefore concluded that the net water flux was a function of the unidirectional transcellular net flux of Na. Rabbit proximal convoluted tubules were perfused in vitro with solution containing 75 meq/liter Li and 75 meq/liter Na on both the luminal and peritubular sides. Under these conditions, the water reabsorption rate dropped to half its control value. Water movement was therefore a function of the external sodium concentration, which in turn probably regulates the intracellular Na concentration.

Lithium absorption in tight and leaky segments of intestine

“…We therefore reason that most of the Li § absorption in our experiments was probably passive, hence probably via the junctions. Corman et al (1981) reached a similar conclusion for Li + movements across rat proximal tubule.…”

“…Proximal segments of both these tubes exhibit numerous hallmarks of leaky epithelia, while distal segments exhibit hallmarks of tight epithelia (Diamond, 1977). In the present paper we extend these parallels by examining whether the gradient in lithium absorption established for the kidney (Thomsen, 1968;Thomsen & Schou, 1968;Hayslett & Kashgarian, 1979;Corman, Roinel & de Rouffignac, 1981) also characterizes the human intestine.…”

“…In these four other epithelia as in human jejunum, Na + is slightly more permeable than Li + (PNa/PLI is unknown in human ileum), yielding the permeability sequence PK > PNa > PLY The main point to be emphasized is that the permeabilities of these three ions in these four leaky epithelia are quite similar and within at most a factor of 2~ of each other, as in human jejunum and ileum. In another leaky epithelium, renal proximal tubule, Li + absorption is similar to Na + absorption (Corman et al, 1981), but the ratio PLi/PNa has not been specifically determined. As in our experiments on human intestine, the proximal tubule experiments do not specifically distinguish among possible modes of Li + transport.…”

Effect of Angiotensin II on Proximal Tubular Reabsorption in Normal Humans

“…This finding was rather unexpected, and we have no clear explanation for it. We can only hypothesize that modifications of FF influence significantly FELi since a substantial, even if not the whole amount of lithium reabsorption occurs passively via a paracellular route [ 18], whilst they do not affect FEP2M since the latter is reabsorbed exclusively and actively by endocytosis via a transcellular path way.…”

“…The response of the proxi mal tubule to All has been investigated by calculating the fractional excretion of lithium (FELi), which is thought to be a marker of the proximal sodium reabsorp tion [16][17][18], and of the p2 microglobulin (FEPyM), which is not a marker of sodium reabsorption but is actively reabsorbed only in the proximal tubule by a transcellular process [19].…”