Na REABSORPTION AND K SECRETION IN THE DISTAL TUBULE OF NEWBORN DOGS

Abstract

Newborns do not excrete infused saline loads as efficiently as do adults. Proximal tubular Na reabsorption is similar in newborn and adult dogs under control and saline expansion (SE) conditions (A.J.P. 228:1403, 1975). Differences occur in more distal segments of the nephron. In order to evaluate Na reabsorption in the distal convoluted and cortical collecting tubule, Na reabsorption was measured in 26 newborn and 9 adult dogs before and during maximal inhibition with amiloride, a drug known to inhibit Na reabsorption and K secretion in these regions. Under control conditions 3.6±.3% of the filtered Na was reabsorbed in the amiloride sensitive region of the nephron (ASR) in the puppy compared to 2.3±.4% for the adult (p<.05). Without amiloride, SE increased total fractional Na excretion 8.0±2.3% in adults and 1.9±.4% in puppies. When the ASR was inhibited SE increased fractional Na excretion 7.2±1.0% in adults and 4.5±1.2% in puppies. During SE, fractional Na reabsorption in the ASR was higher (p<.05) in the puppy (5.3±.4%) than in the adult (2.8±1.2%). K secretion in the ASR was the same in the puppy (1.4±.2 meq/ml GFR) and the adult (1.4±.1) during control as well as during SE, (1.4±.2 for puppies and 1.5±.4 for adults). There was no correlation between Na re-absorbed and K secreted in the ASR for both puppies and adults during control or SE periods. These results support the conclusion that the ASR contributes to some but not all of the attenuated response of the newborn to saline expansion. In addition, Na reabsorption and K secretion in this region are dissociated.

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