Effects of hypotonic saline loading in hydrated dog: evidence for a saline-induced limit on distal tubular sodium transport.

We performed studies on dogs under hydrated conditions, utilizing the rate of free water formation (C(H2O)) as an index of the rate of distal tubular sodium transport. Since C(H2O) could be progressively increased with no evidence of a maximal rate during loading with hypotonic (2.5%) mannitol, it was concluded that there is no limit on distal tubular sodium transport during mannitol loading. In contrast, during hypotonic (0.45%) saline loading C(H2O) rose initially, but as urine flow (V) exceeded 25% of the filtered load C(H2O) attained maximal levels (up to 20% of the filtered load) and remained stable as V increased to 50% of the filtered load. It was concluded that saline loading progressively inhibits proximal sodium reabsorption. Initially, the distal tubule absorbes a large fraction of the proximal rejectate and sodium excretion rises slightly. Eventually, an alteration in distal sodium transport appears which culminates in a maximal rate or transport limit. This distal transport limit provoked by saline loading could not be characterized by a classical Tm as seen with glucose and does not seem to be consequent to high rates of flow through the distal tubule. Regardless of the precise nature of this limit, the major increment in sodium excretion develops during saline loading only after saline alters the capacity of the distal tubule to transport sodium.