Barium is a divalent earth metal; it is stable and highly toxic upon acute and chronic exposure on man. It is present in
many products and involved in a number of industrial processes. Its molecular action is at least the interaction with
plasma membrane channels. The two divalent physiological ions, calcium and magnesium have specific sites of action and
the question is to what extend these three divalent cations interact in smooth muscle.
Muscle strips from rat aorta and gastric antrum and fundus were investigated with conventional organ bath technique.
Single cell electrophysiology was performed using patch-clamp technique.
In single cells, where only calcium channels were investigated (potassium channels were blocked by caesium ions),
magnesium ions reduced the inward current in the millimolar range. Barium ions administered up to 10 mM increased the
inward current markedly but reduced the dynamic of the current.
In those cells, where potassium channels were untreated by caesium ions, barium reduced the outward current in the
range between 100 μM and 3 mM. Especially calcium dependent potassium currents and inward rectifier potassium
currents were strongly inhibited.
In organ bath studies, the administration of barium to smooth muscle showed a biphasic pattern. Low concentrations of
barium (up to 1 mM) increased the contractile force and contraction amplitudes. Higher concentrations (1 -10 mM)
reduced and aborted the contractions. Increased concentrations of magnesium administered up to 3 mM in the presence
of low barium normalized the amplitude of the contractions.
It is concluded, that barium can depolarize membrane potential by block of potassium channels. The increased open
probability of calcium channels can be normalized by a magnesium antagonism. The increase of extracellular magnesium
might be useful in the treatment of acute barium administration.