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