Effects of magnesium ions on detrusor contraction in rat

Contraction of smooth muscle including detrusor is initiated by an increase in the concentration of intracellular free calcium ions. Magnesium affects the translocation of calcium ions through the smooth muscle cell membrane. The effect of magnesium ions on detrusor contraction was investigated in vitro using the rat detrusor muscle. Detrusor contractility caused by intramural nerve stimulation increased in a magnesium-free solution but decreased in a high-magnesium solution (MgCl2 5 mM). Cumulative addition of MgCl2 suppressed the detrusor response induced by field stimulation dose-dependently, and 30 mM MgCl2 completely eliminated it. Mg ion also suppressed atropine-resistant detrusor contraction in a dose-dependent manner. After blocking nerve-mediated detrusor contractility with tetrodotoxin, Mg ions inhibited spontaneous detrusor contractions and reduced tonus of the muscle strips. Mg ions suppressed the detrusor contraction induced by carbachol and potassium. The effect of Mg ions on detrusor contraction was enhanced by verapamil but inhibited by Bay-K8644. These results suggest that Mg ions suppress detrusor muscle contractility through an inhibitory mechanism acting on calcium channels in the smooth muscle cell membrane.