IONIC TRANSPORT IN MACULA DENSA CELLS

Recent work has provided substantial insights into functional characte ristics of macula densa (MD) cells. Microelectrode and patch-clamp exp eriments on the rabbit isolated thick ascending limb (TAL)/glomerulus preparation have shown that MD cells possess a furosemide-sensitive Na :K:2Cl cotransporter, an apical 41-pS K+ channel, and a dominant basol ateral Cl- conductance. Increasing luminal fluid [NaCl] ([NaCl](L)) re sults in furosemide-sensitive cell depolarization due to a rise in int racellular [Cl-] that stimulates basolateral electrogenic Cl- efflux. Intracellular pH (pH(i)) measurements show the presence of an apical N a:H exchanger that couples transepithelial Na+ transport to pH(i). Exp erimental results and thermodynamic considerations allow estimation of intracellular [Na+] and [Cl-] ([Na+](i), [Cl-](i)) under different co nditions. When the Na:K:2Cl cotransporter is equilibrated (or in the p resence of furosemide), [Na+](i) and [Cl-](i) are low (similar to 6 to 7 mM); whereas when the cotransporter is fully activated, [Na+](i) an d [Cl-](i) increase substantially to approximately 70 and 20 mM, respe ctively. Finally, luminal addition of NH4+ produces cell acidification that depends on NH4+ apical transport rate through the Na:K:2Cl. Usin g a simple transport model for NH4+, the initial NH4+ influx rate in M D cells is comparable to the corresponding Aux in TAL. This challenges the idea that MD cells have a low transport activity but supports our findings about large changes in intracellular concentrations as a fun ction of [NaCl](L).