Magnesium relaxes arterial smooth muscle by decreasing intracellular Ca2+ without changing intracellular Mg2+.

Elevations in extracellular Mg2+] (Mg2+]o) relax vascular smooth muscle. We tested the hypothesis that elevated Mg2+]o induces relaxation through reductions in myoplasmic Ca2+] and myosin light chain phosphorylation without changing intracellular Mg2+] (Mg2+]i). Histamine stimulation of endothelium-free swine carotid medial tissues was associated with increases in both Fura 2- and aequorin-estimated myoplasmic Ca2+], myosin phosphorylation, and force. Elevated Mg2+]o decreased myoplasmic Ca2+] and force to near resting values. However, elevated Mg2+]o only transiently decreased myosin phosphorylation values: sustained Mg2+]o-induced decreases in myoplasmic Ca2+] and force were associated with inappropriately high myosin phosphorylation values. The elevated myosin phosphorylation during Mg2+]o-induced relaxation was entirely on serine 19, the Ca2+/calmodulin-dependent myosin light chain kinase substrate. Myoplasmic Mg2+] (estimated with Mag-Fura 2) did not significantly increase with elevated Mg2+]o. These results are consistent with the hypothesis that increased Mg2+]o induces relaxation by decreasing myoplasmic Ca2+] without changing Mg2+]i. These data also demonstrate dissociation of myosin phosphorylation from myoplasmic Ca2+] and force during Mg(2+)-induced relaxation. This finding suggests the presence of a phosphorylation-independent (yet potentially Ca(2+)-dependent) mechanism for regulation of force in vascular smooth muscle.