Increased Expression of Sodium Transport Proteins and NA,K-ATPase Activity in the Outer Medulla of Kangaroo Rat is Related to its Greater Urine Concentrating Ability Compared to Munich Wistar and Sprague-Dawley Rat

Abstract

The kangaroo rat (Dipodomys merriami), a desert rodent, is able to concentrate its urine to more than 6,000 mosmol/kg water, nearly twice that of the SD rat, allowing the species to adapt in a dry environment. We hypothesize that active sodium reabsorption in the thick ascending limb (TAL) of the kangaroo rat outer medulla (OM) is greater than that of the SD rat, and plays a critical role in generating a steeper corticopapillary osmotic gradient. The OM was dissected and prepared for semi-quantitative western blotting. We measured a four-fold greater abundance of Na,K-ATPase α-1 subunit protein (NKA- α1) (n=5, p<0.0001) and nearly two-fold greater abundance of NKCC2 cotransporter and NHE3 protein expression in OM homogenates of kangaroo rat compared to SD rat (n=3, p<0.01). Three-day water restriction in SD rat (water reduced to 40% of normal consumption) did not significantly affect NKA- α1, NKCC2 and NHE3 expression (n=3, p>0.1). Na,K-ATPase (NKA) activity was measured as the difference in ATP hydrolyzed in the absence and presence of ouabain, as determined by measuring release of inorganic phosphate (Pi). OM NKA activity in kangaroo rat was significantly higher than in control SD rat, 7778 ± 973 and 4886 ± 296 nmoles ATP hydrolyzed/mg protein/30 min respectively (n=3). However, the volume of mitochondria per unit inner stripe TAL cell volume is identical in both species. We conclude that the greater urine concentrating ability in kangaroo rat compared to the SD rat is associated with greater expression of active sodium transport proteins and higher NKA activity, which increases sodium reabsorption in the TAL of the OM.