Smooth muscle contraction has a relatively high requirement for free magnesium (Mg²⁺). In this study we examined the effect of Mg²⁺ concentration ([Mg²⁺]) on Ca²⁺-dependent stress development and stress maintenance, myosin ATPase activity, and myosin light chain (MLC) phosphorylation levels in Triton X-100 detergent-skinned fibers of the swine carotid media. Increasing [Mg²⁺] in a stepwise fashion from 0.1 to 6 mM 1) decreased the magnitude and Ca²⁺ sensitivity of stress development but augmented the amount of stress maintained without proportional MLC phosphorylation, 2) produced a greater decrease in the Ca²⁺ sensitivity of MLC phosphorylation than that of stress development, and 3) decreased myosin ATPase activity. These findings demonstrate that Mg²⁺ differentially modulates the MLC phosphorylation-dependent development of stress and the MLC phosphorylation-independent maintenance of stress. We suggest that increases in [Mg²⁺] enhance stress maintenance by increasing [MgADP], thus increasing the number of cross bridges in a force-generating state, and by a direct effect on the pathway responsible for Ca²⁺-dependent, MLC phosphorylation-independent contractions.

vascular smooth muscle; latch state; calcium; magnesium; permeabilized fibers; myosin light chain phosphorylation; myosin adenosinetriphosphatase activity