Magnesium and membrane function in vascular smooth muscle.

Since a tight electromechanical coupling exists in vascular smooth muscle, even small shifts of the membrane potential are sufficient to change the vascular lumen. The extracellular H+ and K+ concentrations are important effectors for the adjustment of the membrane potential. The ion concentrations in the immediate neighbourhood of the cell membrane can be influenced by the microdynamic binding properties of the basement membrane and the other vascular connective tissue. These structures are polyanionic macromolecules to which mono- and divalent cations are extensively bound, and which are separated from the vascular smooth muscle cell membranes by tiny cleft spaces. The ion binding properties of vascular connective tissue were therefore studied in dependence on proton and cation concentration. The pH-dependent binding of monovalent cations to vascular connective tissue is dependent on the concentration and affinity constant of the ion species in question. The mode of interaction is competition. For instance, the actual K binding characteristic means that an increase of [K+]o close to the cell membrane cold ensue from a diminished K+ binding ability under alkalosis. Depolarization and contraction of vascular smooth muscle cells result. Divalent cation binding to vascular connective tissue it additionally dependent on conformational changes. Already physiological concentrations of Mg++ ions can induce a specific change in configuration, which enables K+ ions to bind cooperatively. This means that with extracellular Mg++ deficiency not only less Mg++ ions are bound to vascular connective tissue but also less K+ ions. [K+]o would increase near the cell membrane, depolarization and vasoconstriction would occur.