Angiotensin II type I receptor modulates intracellular free Mg2+ in renally derived cells via Na+-dependent Ca2+-independent mechanisms

Treatment of Madin-Darby canine kidney (MDCK) cells with the peptide hormon e angiotensin II (Ang II) results in an increase in the concentrations of c ytosolic free calcium ([Ca2+](i)) and sodium ([Na+](i)) with a concomitant decrease in cytosolic free Mg2+ concentration ([Mg2+](i)). In the present s tudy we demonstrate that this hormone induced decrease in [Mg2+](i) is inde pendent of [Ca2+](i) but dependent on extracellular Na+. [Mg2+](i), [Ca2+]( i), and [Na+](i) were measured in Ang II-stimulated MDCK cells by fluoresce nce digital imaging using the selective fluoroprobes mag-fura-2AM, fura-2AM , and sodium-binding benzofuran isophthalate (acetoxy methyl ester), respec tively. Ang II decreased [Mg2+](i) and increased [Na+](i) in a dose-depende nt manner. These effects were inhibited by irbesartan (selective AT(1) rece ptor blocker) but not by PD123319 (selective AT(2) receptor blocker). Imipr amine and quinidine (putative inhibitors of the Na+/Mg2+ exchanger) and rem oval of extracellular Na+ abrogated Ang II-mediated [Mg2+](i) effects, In c ells pretreated with thapsigargin (reticular Ca2+-ATPase inhibitor), Ang II -stimulated [Ca2+](i) transients were attenuated (p < 0.01), whereas agonis t-induced [Mg2+](i) responses were unchanged. Clamping the [Ca2+](i) near 5 0 nmol/liter with 1,2-bis(2-aminophenoxy)ethane-N,N,N' N'-tetraacetic acid tetrakis(acetoxymethyl ester) inhibited Ang II-induced [Ca2+](i) increases but failed to alter Ang II-induced [Mg2+](i) responses. Benzamil, a selecti ve blocker of the Na+/Ca2+ exchanger, inhibited [Na+](i) but not [Mg2+](i) responses. Our data demonstrate that in MDCK cells, AT(1) receptors modulat e [Mg2+](i) via a Na+-dependent Mg2+ transporter that is not directly relat ed to [Ca2+](i). These data support the notion that rapid modulation of [Mg 2+](i) is not simply a result of Mg2+ redistribution from intracellular buf fering sites by Ca2+ and provide evidence for the existence of a Na+-depend ent, hormonally regulated transporter for Mg2+ in renally derived cells.