Differential Regulation of Circulating Mg 2+ in the Rat by β 1 - and β 2 -Adrenergic Receptor Stimulation

Abstract

Abstract Extracellular Mg 2+ homeostasis was studied in vivo in the anesthetized rat. Animals were infused with isoproterenol (ISO) for 10 minutes, and serum Mg 2+ was measured after the infusion and then 10 and 20 minutes later. A dose-dependent increase in circulating Mg 2+ was observed in animals infused with ISO at a rate of 0.1 μg · kg −1 · min −1 or higher. The time course of the effect demonstrated that circulating Mg 2+ continued to increase 20 minutes after the end of the ISO infusion. A predicted maximal increase in serum Mg 2+ concentration of 19.3% was derived with a predicted EC 50 of 0.08 μg · kg −1 · min −1 . The maximal percent increase corresponded to a net increase of 6.7 μmol/300 g body wt. Because infusion of ISO resulted in changes in hemodynamic parameters, most notably a drop in blood pressure, a group of animals was infused with nitroprusside to mimic the hypotensive response via a nonadrenergic mechanism. Under these conditions, there was a transient increase in circulating Mg 2+ that was largely inhibited by propranolol, indicating that hypotension per se was not responsible for the mobilization of Mg 2+ . Infusion of salbutamol, but not prenalterol, also induced an increase in circulating Mg 2+ . Pretreatment with butoxamine, ICI-118551, or propranolol prevented the ISO-induced increase in serum Mg 2+ . Pretreatment with atenolol minimally affected the ISO-induced changes in circulating Mg 2+ . Pretreatment with CGP-20271A actually enhanced the ISO-induced increase in circulating Mg 2+ . This evidence demonstrates the existence of a pool of Mg 2+ that is mobilized into the circulation in response to selective β 2 -adrenergic stimulation.