Human study, using oral magnesium oxide (1g daily) for 4 weeks. The 1g magnesium oxide provides 600mg elemental magnesium, so the dose is not that high. With a more bioavailable form like magnesium bicarbonate or glycinate a person will probably get the same effects from 300mg-400mg elemental magnesium.
http://hyper.ahajournals.org/content/13/3/227.long
"...Therefore, we concluded that appropriate oral magnesium intake might be effective as a nonpharmacological treatment for essential hypertension."
http://hyper.ahajournals.org/content/13/3/227.long
"...Therefore, we concluded that appropriate oral magnesium intake might be effective as a nonpharmacological treatment for essential hypertension."