title: Effect of ions on binding of the thiazide-type diuretic metolazone to kidney membrane.
abstract: The effect of a number of ions on the binding of the thiazide-type diuretic metolazone (MTZ) to rat renal cortical membranes was studied to elucidate the mechanism of NaCl transport in the kidney distal tubule. Among the cations tested, Na+ significantly stimulated the binding up to 2.4-fold over control. The effective concentration of Na+ that produced half-maximal stimulation was 2-17 mM. Li+, K+, NH4+, Rb+, and Cs+ produced little stimulation of binding of MTZ. Several anions including Cl- inhibited binding. The inhibition of binding of MTZ by Cl- was enhanced by Na+ and Li+. Scatchard analyses revealed that 50 mM Na+ increased the affinity for binding of MTZ from a Kd = 3.56 +/- 0.15 nM to Kd = 1.32 +/- 0.11 nM. Chloride, in the presence of 50 mM Na+, competitively inhibited binding of MTZ by suppressing the affinity to Kd = 9.27 +/- 1.11 nM without changing the maximal number of binding sites (0.733 +/- 0.049 pmol/mg). A mechanism for the MTZ-sensitive NaCl transport is proposed, in which the transporter protein possesses a binding site for Na+ and a binding site for Cl-, which is also the binding site for MTZ. Na+ binds to its site and increases the affinity for Cl-/MTZ. The binding of Cl- to the transporter enables the import of Na+ and Cl- across the tubule membrane. MTZ, however, when present competes with Cl- for the binding site on the transporter and prevents the transport of Na+ and Cl-.
authors.affiliation: Department of Medicine, University of California, San Diego, La Jolla 92093.
authors.forename: J M
authors.initials: JM
authors.lastname: Tran
authors.forename: M A
authors.initials: MA
authors.lastname: Farrell
authors.forename: D D
authors.initials: DD
authors.lastname: Fanestil
mesh_terms: D000818:Animals; D002712:Chlorides / Q000494:pharmacology; D004232:Diuretics / Q000378:metabolism*; D007477:Ions; D007668:Kidney / Q000378:metabolism*; D008094:Lithium / Q000494:pharmacology; D008297:Male; D008566:Membranes / Q000378:metabolism; D008788:Metolazone / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000494:pharmacology
chemical_list: D002712:Chlorides; D004232:Diuretics; D007477:Ions; D008094:Lithium; D012964:Sodium; D008788:Metolazone