Characterization of magnesium-induced contractions in detergent-skinned swine carotid media

Ca(2+)-calmodulin-dependent phosphorylation of the 20-kDa smooth muscle myosin light chain (MLC) results in high shortening velocities and rapid stress development. The stress maintained after a reduction in Ca2+ is associated with a decrease in MLC phosphorylation and velocity of shortening. This Ca(2+)-dependent stress without proportional MLC phosphorylation has been termed "latch" and has been postulated to reflect a population of dephosphorylated noncycling cross bridges or "latch bridges." Mg2+ is necessary for contraction of smooth muscle, and in high concentrations, Mg2+ elicits contractions that are MLC phosphorylation independent. The purpose of this study was to test the hypothesis that high concentrations of Mg2+ directly induce latch-bridge formation. This was accomplished by comparing the characteristics of Mg(2+)-induced contractions of Triton X-100-skinned swine carotid media with the known characteristics of the Ca(2+)-dependent latch state. In the absence of Ca2+, free Mg2+ (3-20 mM) caused an increase in the velocity of shortening and a concentration-dependent increase in stress, with no detectable increase in MLC phosphorylation. Mg(2+)-induced contractions could be supported by CTP, which is a substrate for the actin-activated myosin adenosinetriphosphatase but not the MLC kinase. Stress development in response to Mg2+ was abolished at long tissue lengths, which also inhibit the expression of latch bridges. The calmodulin antagonist, trifluoperazine (TFP), inhibited the MLC phosphorylation-independent contractions elicited by Mg2+. TFP also inhibited the latch state. The results of this study support the existence of a regulatory system in vascular smooth muscle that is independent of the MLC phosphorylation system and can be directly activated by pharmacological levels of Mg2+.