Fluid absorption with and without sodium in isolated perfused snake proximal tubules

Net fluid absorption (Jv) was studied in isolated, perfused snake (Thamnophis spp.) proximal renal tubules. With standard (150 mmol/liter Na⁺) bicarbonate-buffered Ringer in perfusate and standard Ringer plus dextran (4 g/100 ml) in bath, Jv was about 0.9 nl min^-1 mm^-1. Removing dextran from bath reduced Jv by about 20%. When sodium in perfusate was replaced with choline. Jv approached zero. However, when sodium in bath as well as perfusate was replaced with choline, Jv returned to control level. Results were the same when sodium was replaced with tetramethylammonium, sodium chloride was replaced with sucrose or lactose, or chloride was replaced with methyl sulfate. In contrast, replacing sodium in perfusate or in both perfusate and bath with lithium did not reduce Jv. Fluid absorption was always isosmotic. Replacing bicarbonate with phosphate or Tris in sodium-containing media had no effect on Jv, but the presence of buffer in sodium-free or low-chloride media may have been important for Jv. Reducing temperature 10°C reduced Jv by about 35% with either sodium chloride or sucrose in both perfusate and bath. The results indicate that isosmotic fluid absoprtion can occur when lithium is substituted for sodium or when some other substitution is made for sodium, chloride, or both in perfusate and bath simultaneously.