Macula densa cells sense luminal NaCl concentration via furosemide sensitive Na⁺2Cl⁻K⁺ cotransport

Abstract

The macula densa cells of the juxtaglomerular apparatus probably serve as the sensor cells for the signal which leads to the appropriate tubuloglomerular feedback response. The present study reports basolateral membrane voltage (PD_bl) measurements in macula densa cells. We isolated and perfused in vitro thick ascending limb segments with the glomerulus, and therefore the macula densa cells, and the early distal tubule still attached. Macula densa cells were impaled with microelectrodes under visual control. PD_bl was recorded in order to examine how these cells sense changes in luminal NaCl concentrations. The addition of furosemide, a specific inhibitor of the Na⁺2Cl⁻K⁺ cotransporter in the thick ascending limb, to the lumen of the perfused thick ascending limb hyperpolarized PD_bl from −55±5 mV to −79±4 mV (n=7). Reduction of NaCl in the lumen perfusate from 150 mmol/l to 30 mmol/l also hyperpolarized PD_bl from −48±3 mV to −66±5 mV (n=4). A Cl⁻ concentration step in the bath from 150 mmol/l to 30 mmol/l resulted in a 24±4 mV (n=4) depolarization of PD_bl. This depolarization of PD_bl was absent when furosemide was present during the Cl⁻ concentration step. These data suggest that the macula densa cells sense changes in luminal NaCl concentration via coupled uptake of Na⁺ and Cl⁻. The transport pathways for NaCl transport in macula densa cells are probably identical to those in the thick ascending limb: the (Na⁺+K⁺)-ATPase in the basolateral membrane drives Na⁺ and Cl⁻ uptake via the luminal Na⁺2Cl⁻K⁺ cotransport, Cl⁻ leaves the cell via basolateral Cl⁻ channels and K⁺ recycles across the apical membrane via K⁺ channels. Changes in intracellular Cl⁻ activity as a result of altered luminal NaCl uptake, and thus voltage changes of the basolateral membrane are probably the first signal in the tubuloglomerular feedback regulation.