EXPRESSION OF THE NA-K-2CL COTRANSPORTER BY MACULA DENSA AND THICK ASCENDING LIMB CELLS OF RAT AND RABBIT NEPHRON

Sodium and chloride transport by the macula densa and thick ascending limb of Henle's loop participates importantly in extracellular fluid v olume homeostasis, urinary concentration and dilution, control of glom erular filtration, and control of renal hemodynamics. Transepithelial Na and Cl transport across the apical membrane of thick ascending limb (TALH) cells is mediated predominantly by a loop diuretic sensitive N a-K-2Cl cotransport pathway. The corresponding transport protein has r ecently been cloned. Functional studies suggest that the cotransporter is expressed by macula densa cells as well as by TALH cells. The curr ent studies were designed to identify sites of Na-K-2Cl cotransporter expression along distal nephron in rabbit and rat. Non-isotopic high-r esolution in situ hybridization, using an antisense probe for the apic al form of the Na-K-2Cl cotransporter identified expression throughout the TALH, from the junction between inner and outer medulla to the tr ansition to distal convoluted tubule. Expression by macula densa cells was confirmed by colocalization using markers specific for macula den sa cells. First, Na-K-2Cl cotransporter mRNA was detected in macula de nsa cells that did not stain with anti-Tamm-Horsfall protein antibodie s. Second, Na-K-2Cl cotransporter mRNA was detected in macula densa ce lls that show positive NADPH-diaphorase reaction, indicating high leve ls of constitutive nitric oxide synthase activity. In rat, levels of N a-K-2Cl cotransporter mRNA expression were similar in TALH and macula densa cells. In rabbit, expression levels were higher in macula densa cells than in surrounding TALH cells. The present data provide morphol ogical support for a previously established functional concept that Na -K-2Cl cotransport at the TALH is accomplished by the expression of a well-defined cotransporter. At the macula densa, this transporter may establish a crucial link between tubular salt load and glomerular vasc ular regulation.