title: Magnesium relaxes arterial smooth muscle by decreasing intracellular Ca2+ without changing intracellular Mg2+.
abstract: 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.
authors.affiliation: Department of Internal Medicine, University of Virginia Health Sciences Center, Charlottesville 22908.
authors.forename: E K
authors.initials: EK
authors.lastname: D'Angelo
authors.forename: H A
authors.initials: HA
authors.lastname: Singer
authors.forename: C M
authors.initials: CM
authors.lastname: Rembold
mesh_terms: D000331:Aequorin; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D016257:Fura-2; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology*; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D010766:Phosphorylation; D013552:Swine
chemical_list: D000331:Aequorin; D008274:Magnesium; D002118:Calcium; D016257:Fura-2