Endogenous inhibitor of Na-K-Cl cotransport system in inbred Dahl rats.

Dahl salt-sensitive (DS) rats seem characterized by a ubiquitous increase in Na-K-Cl cotransport activity. Here, an endogenous inhibitor of the Na-K-Cl cotransport system (cotransport inhibitory factor, CIF) was investigated in inbred Dahl salt-sensitive (DS) and salt-resistant (DR) rats. The animals were orally loaded for 10 days with 2% NaCl. Plasma from salt-loaded DS rats inhibited cotransport with a 50% inhibition concentration value (IC50) of 6.4 +/- 0.6% (% plasma concentration, vol/vol) vs. 24.2 +/- 2.2% in DR rats (P < 0.0001). In urine, IC50 for cotransport inhibition was constantly lower in DS before and all during the whole salt-loading period (after 10 days of salt loading, IC50 was 2.59 +/- 0.11% and 6.00 +/- 0.24% in DS and DR rats, respectively; P < 0.0001). After 3 days of salt loading, higher salt appetite in DS rats magnified the differences in urinary CIF excretion. In erythrocytes from DS rats, increased cotransport activity was strongly correlated with urinary CIF excretion (r = 0.967). In conclusion, DS rats present increased plasmatic and urinary CIF levels. This can be a compensatory phenomenon to reduce cotransport hyperactivity and increased NaCl reabsorption at the thick ascending limb of Henle's loop.