Angiotensin II Type I Receptor Modulates Intracellular Free Mg2+ in Renally Derived Cells via Na+-dependent Ca2+-independent Mechanisms

Abstract

Treatment of Madin-Darby canine kidney (MDCK) cells with the peptide hormone angiotensin II (Ang II) results in an increase in the concentrations of cytosolic free calcium ([Ca2+]i) and sodium ([Na+]i) with a concomitant decrease in cytosolic free Mg2+ concentration ([Mg2+]i). In the present study we demonstrate that this hormone-induced decrease in [Mg2+]i is independent of [Ca2+]i but dependent on extracellular Na+. [Mg2+]i, [Ca2+]i, and [Na+]i were measured in Ang II-stimulated MDCK cells by fluorescence digital imaging using the selective fluoroprobes mag-fura-2AM, fura-2AM, and sodium-binding benzofuran isophthalate (acetoxymethyl ester), respectively. Ang II decreased [Mg2+]i and increased [Na+]i in a dose-dependent manner. These effects were inhibited by irbesartan (selective AT1 receptor blocker) but not by PD123319 (selective AT2 receptor blocker). Imipramine and quinidine (putative inhibitors of the Na+/Mg2+ exchanger) and removal of extracellular Na+ abrogated Ang II-mediated [Mg2+]i effects. In cells pretreated with thapsigargin (reticular Ca2+-ATPase inhibitor), Ang II-stimulated [Ca2+]i transients were attenuated (p < 0.01), whereas agonist-induced [Mg2+]i responses were unchanged. Clamping the [Ca2+]i near 50 nmol/liter with 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid tetrakis(acetoxymethyl ester) inhibited Ang Il-induced [Ca2+]i increases but failed to alter Ang Il-induced [Mg2+]i responses. Benzamil, a selective blocker of the Na+/Ca2+ exchanger, inhibited [Na+]i but not [Mg2+]i responses. Our data demonstrate that in MDCK cells, AT1 receptors modulate [Mg2+]i via a Na+-dependent Mg2+ transporter that is not directly related to [Ca2+]i. These data support the notion that rapid modulation of [Mg2+]i is not simply a result of Mg2+ redistribution from intracellular buffering sites by Ca2+ and provide evidence for the existence of a Na+-dependent, hormonally regulated transporter for Mg2+ in renally derived cells.