Ca2+ coupling in vascular smooth muscle: Mg2+ and buffer effects on contractility and membrane Ca2+ movements.

An examination of the literature, over the past two decades, reveals that (1) in studies of different types of vascular smooth muscles, Mg2+ is often either left out of physiological salt solutions or reduced in concentration compared with that in blood; and (2) when excitation--contraction coupling processes have been examined in isolated vascular tissues and cells, a number of artificial (synthetic) amine and organic zwitterion buffers have often been substituted for the naturally occurring bicarbonate and phosphate anions found in the blood and in cells. The influence of extracellular magnesium ions ([Mg2+]0) on tone, contractility, reactivity, and divalent cation movements in vascular smooth muscles, and how they may relate to certain vascular disease states, is reviewed. Data are presented and reviewed which indicate that many of the most commonly used artificial buffers (e.g., Tris, HEPES, MOPS, Bicine, PIPES, imidazole) can exert adverse effects on contractility and reactivity of certain arterial and venous smooth muscles. The data reviewed herein suggest that [Mg2+]0 and membrane Mg are important in the regulation of vascular tone, vascular reactivity, and in control of Ca uptake, content, and distribution in smooth muscle cells. [HCO3-]0 and (or) PO4(2-) anions may be important for normal maintenance of excitability and reactivity and in the control of Ca uptake, content, and distribution in smooth muscle cells.