Site of origin of an urinary Na-K-Cl cotransport inhibitor

We investigated the site of origin of a potent inhibitor of the Na-K-Cl cotransport system (CIF), which has been previously identified in urines from salt-loaded rats. Rats were given a 2% NaCl solution to drink for 15 and 60 days and the plasma was obtained and tested for cotransport inhibitory activity (on bumetanide-sensitive Li+ efflux in Li-loaded human erythrocytes) in dose-response curves. The IC50 for cotransport inhibition (vol:vol dilution reducing cotransport activity by 50%) was found to be 26.5 +/- 7.2 (mean +/- SEM, n = 5) and 6.9 +/- 0.7% (n = 10) on days 15 and 60, respectively (control samples, day 0, only inhibited about 20% cotransport activity at a 30% plasma vol:vol dilution, n = 7). Organ extracts from salt-loaded rats were prepared and tested for cotransport inhibitory activity. A statistically significant cotransport inhibitory activity was only found in pituitary extracts (-36 +/- 3 vs. -5 +/- 4% in control rats, n = 4, p < 0.01). Dissection of a large number of pituitary glands from salt-loaded rats revealed CIF activity only in the neurohypophysis (cotransport inhibition -46.5 +/- 5.2 vs. -0.5 +/- 16.4% in the anterior lobe, n = 4, p < 0.05). In conclusion, CIF is a new circulating endogenous factor, probably secreted by the neurohypophysis.