Regulation of potassium channel Kir 1.1 (ROMK) abundance in the thick ascending limb of Henle's loop

The renal outer medullary potassium channel (ROMK) of the thick ascending limb (TAL) is a critical component of the counter-current multiplication mechanism. In this study, two new antibodies raised to ROMK were used to investigate changes in the renal abundance of ROMK with treatments known to strongly promote TAL function. These antibodies specifically recognized protein of the predicted size of 45 kD in immunoblots of rat kidney or COS cells transfected with ROMK cDNA. Infusion of 1-deamino-(8-D-arginine)-vasopressin (dDAVP), a vasopressin V2 receptor-selective agonist, for 7 d into Brattleboro rats resulted in dramatic increases in apical membrane labeling of ROMK in the TAL of dDAVP-treated rats, as assessed by immunocytochemical analyses. Using immunoblotting, a more than threefold increase in immunoreactive ROMK levels was observed in the outer medulla after dDAVP infusion. Restriction of water intake to increase vasopressin levels also significantly increased TAL ROMK immunolabeling and abundance in immunoblots. In addition, dietary Na⁺ levels were varied to determine whether ROMK abundance was also affected under other conditions known to alter TAL transport. Rats fed higher levels of sodium, as either NaC1 or NaHCO₃ (8 mEq/250 g body wt per d), exhibited significantly increased density of the 45-kD band, compared with the respective control animals. Moreover, in rats fed a low-NaC1 diet (0.25 mEq/250 g body wt per d), a 50% decrease in band density for the 45-kD band was observed (relative to control rats fed 2.75 mEq/250 g body wt per d of NaC1). These results demonstrate that long-term adaptive changes in ROMK abundance occur in the TAL with stimuli that enhance transport by this segment.