Abstract
When equivalent I.V. saline loads were given to 30 newborn and 7 adult dogs, adults excreted 32% of the infused sodium after 2 hours compared to 10% for puppies (p<0.01). GFR increased equivalently (30%) for pupppies and adults but the fraction of filtered sodium excreted (FeNa) was 6.4% for adults and 2.4% for puppies (p<0.01). In order to determine whether inability to adequately decrease tubular sodium reabsorption in the newborn dog had a proximal or distal locus, sodium reabsorption beyond the proximal tubule was blocked (D.B.) in 27 additional puppies with ethacrynic acid and chlorothiazide. Following D.B. in 9 puppies FeNa remained constant for 180 minutes with no sodium load and when dosage of diuretics varied over a two fold range. In 11 puppies FeNa was 26% following D.B. but FeNa increased to 46% (p<0.01) when a saline load was imposed on top of the D.B. In 7 puppies FeNa was 1.5% following a saline load but increased to 49% (p<0.01) when D.B. was imposed on top of the saline load. Since saline loading produced an increase in FeNa during D.B., proximal tubular Na reabsorption must have decreased during saline loading. The inability of the puppy to respond to a saline load therefore is due to increased distal sodium reabsorption.
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