FIGURE 2.

Apical and basolateral transporters expressed by thick ascending limbs thought to be important in blood pressure regulation. Predominant directions of ion movement are depicted by arrows. However, all except the Na⁺-K⁺-ATPase can move ions in either direction. The apical transporters are Na⁺-K⁺-2Cl⁻ cotransporter type 2 (NKCC2), renal outer medullary K⁺ channel (ROMK), and Na⁺/H⁺ exchanger type 3 (NHE). Multiple ROMK isoforms are expressed by thick ascending limbs. Multiple NHE isoforms are expressed by thick ascending limbs, but NHE3 is the most abundant. The basolateral transporters are Na⁺-K⁺-ATPase, KCl cotransporter, Cl⁻ channels, K⁺ channels, and aquaporin 1 (AQP). The Na⁺-K⁺-ATPase ultimately provides the driving force for all NaCl movement. Three subunits comprise the Na⁺-K⁺-ATPase. Although there are several α-, β-, and γ-subunits, α₁, β₁, γ_A/γ_B splice variants predominate. At least two different Cl⁻ channels are expressed by thick ascending limbs. At least 2 different K⁺ channels are expressed in this segment. AQP is expressed by thick ascending limbs, but other water channels may also be present.