Factors modifying contraction-relaxation cycle in vascular smooth muscleshttp://ajpheart.physiology.org/content/243/5/H641.short
"Contraction-relaxation cycles in vascular smooth muscles are largely dependent on the regulation of free Ca2+ in the myoplasm, as is the case in skeletal and cardiac muscles."
Calcium Movements, Distribution, and Functions in Smooth Musclehttp://www.vm.a.u-tokyo.ac.jp/yakuri/pa ... %20rev.pdf
"MAGNESIUM ION. In rat aortic cells (Zhang et al., 1992), removal of external Mg2+ increased [Ca2+]i and changed cell shape. In rabbit aorta and ear artery, rat aorta and guinea pig aorta, removal of external Mg2+ gradually augmented the caffeine-induced contraction without changing the contraction induced by norepinephrine or high K+, possibly by decreasing cytosolic Mg2+ level and activating CICR (Karaki et al., 1987). In rabbit urinary bladder detrusor muscle (Yu et al., 1995), addition of Mg2+ inhibited carbachol-induced contraction accompanied by a decrease in [Ca2+]i. In porcine trachea (Kumasaka et al., 1996), addition of Mg2+ inhibited contractions and increased [Ca2+]i elicited with high K+ or carbachol. In rat carotid artery (Karaki, 1989b), addition of Mg2+ relaxed contraction induced by high K1 accompanied by a decrease in [Ca2+]i. In swine carotid artery (D’Angelo et al., 1992), addition of Mg2+ decreased histamine-stimulated [Ca2+]i and force to resting values. However, Mg2+ only transiently decreased MLC phosphorylation, suggesting that Mg2+ induces relaxation by decreasing [Ca2+]i and, also, by dissociating MLC phosphorylation from [Ca2+]i and force. This finding also suggests the presence of an MLC phosphorylation-independent (yet potentially Ca2+-dependent) mechanism for regulation of force in vascular smooth muscle."
"Vascular smooth muscle refers to the particular type of smooth muscle found within, and composing the majority of the wall of blood vessels.
Arteries have a great deal more smooth muscle within their walls than veins..."
so magnesium either has to work harder to be effective in arterial smooth muscle to be effective, therefore its job would be easier in venous smooth muscle, or it has less to work with in venous smooth muscle, and would be less of a factor in relaxation.
however, given the fact that magnesium appears to regulate calcium/contraction-relaxation in all varieties of smooth muscle, it may be reasonable to assume benefit in veins also.