THE CYTOSOLIC CHLORIDE CONCENTRATION IN MACULA DENSA AND CORTICAL THICK ASCENDING LIMB CELLS

It is believed that chloride transport through the macula densa (MD) cells is a factor involved in the tubuloglomerular feedback (TGF) mechanism and in MD-mediated renin release. In this study isolated and perfused rabbit kidney cortical thick ascending limb (cTAL) segments containing MD plaques and attached glomeruli were loaded with chloride (CL-sensitive) 6 methoxy-1-fluorophore (sulphanate-propyl) quinolinium (SPQ). MD and cTAL intracellular chloride concentration ([Cl-]i) was determined by using image-intensified video microscopy and digital image-processing for measuring the intensity of the emitted SPQ fluorescence. With 150 mm NaCl in lumen and bath the [Cl-]i in MD and cTAL cells was 58.8 +/- 7.2 mm (n = 20) and 68.7 +/- 9.8 mm (n = 14), respectively. When the presumed luminal Na+-2Cl-K+ co-transporter was blocked by adding 10(-4) M furosemide, the [Cl-], was reduced in both, MD and cTAL cells from 55.5 +/- 11.9 to 28.6 +/- 10.0 mm (n = 10) and from 43.8 +/- 2.6 to 13.1 +/- 4.5 mm (n = 5), respectively. A reduction in luminal NaCl from 150 to 30 mm also decreased both, MD and cTAL [Cl-]i from 69.4 +/- 9.1 to 36.5 +/- 5.1 mm (n = 9) and from 82.9 +/- 14.5 to 49.4 +/- 8.0 mm (n = 8), respectively. Basolateral addition of the Cl--channel blocker NPPB increased MD [Cl-], from 31.1 +/- 2.0 to 100.7 +/- 17.0 mm (n = 5) and cTAL [Cl-]i from 44.4 +/- 12.9 to 89.7 +/- 11.7 mm (n = 5). These results show the existence of a luminal Na+-2Cl--K+ cotransporter and a basolateral Cl- conductance, and that chloride transport is directed from the luminal to the basolateral side during symmetrical conditions. They also indicate that sensing of luminal NaCl by the MD cells could be of importance in the TGF mechanism and MD mediated renin release.