Intracellular action of magnesium in airways smooth muscle relaxation in rabbit
This study was designed to examine whether magnesium has an intracellular action on airways smooth muscle contraction. Tracheal muscle strips were obtained from six male rabbits and perfused in a bathing chamber. The muscle was incubated in Krebs solution for 15 min. Then, the bath solution was changed to a Ca++ free (calcium free) solution for 10 min. 10(-4) M Ach (acetyl choline) applied in the Ca++ free solution evoked a muscle contraction, at about 60% of initial contraction induced by 10 Ach in Krebs solution. After control contraction, magnesium sulfate (range 10(-4) - 1 M) was applied in Ca++ free solution. At each dose of magnesium pretreatment the response of the muscle to 10(-4) M Ach in Ca++ free solution was measured. Magnesium caused a decrease in Ach-induced contraction in a dose-dependent manner. 10(-1) M MgSO4 depressed to 13.28% of the contraction of Ach in Calcium free solution. The time to peak contraction and to washout, were also decreased. The concentration for 50% inhibition (IC50) of the maximal contraction was estimated to be 10(-3) M MgSO4. Re-admission of Krebs solution to muscle strip, resulted in contraction as a consequence of massive influx of Ca++ that occurs during the phase of Ca++ repletion. These data suggest that magnesium has an intracellular action and inhibits smooth muscle contraction in a Ca++ free solution probably by inhibiting Ca++ release from the sarcoplasmic reticulum.