The role of potassium channels and calcium in the relaxation mechanism of magnesium sulfate on the isolated rat uterus

Abstract

MgSO 4 is used as a tocolytic agent. It is considered to be a calcium channel antagonist, but a different mechanism of its action might be involved. The aim of this study was to examine the contribution of calcium concentrations and potassium channels in the mechanism of MgSO 4 -mediated uterine relaxation. Isolated uteri from female Wister rats were treated with increasing MgSO 4 concentrations (0.1-30 mM). MgSO 4 induced dose-dependent inhibition of spontaneous activity. Addition of Ca 2+ (6 mM and 12 mM) stimulated uterine contractile activity and attenuated the inhibitory activity of MgSO 4 . In order to analyze the role of different subtypes of potassium channels, Ca 2+ -stimulated uteri were pretreated with glibenclamide (Glib), a selective ATP-sensitive potassium channel inhibitor (KATP), tetraethylammonium (TEA), a non-specific inhibitor of large conductance calcium-activated potassium channels (BKCa), and 4-aminopyridine (4-AP), a voltage-sensitive potassium channel inhibitor (Kv), at concentrations that had no effect per se. Pretreatment with 4-AP had no effect on MgSO 4 -mediated relaxation of Ca 2+ -stimulated uteri. The relaxing effect of MgSO 4 was potentiated by pretreatment with glibenclamide. Pretreatment with TEA attenuated the MgSO 4 -mediated decrease in frequency. Our results suggest that MgSO 4 acts as a general calcium antagonist that influences Ca 2+ -mediated potassium channels. Furthermore, it seems that MgSO 4 uterine relaxation activity is partially mediated by selective ATP-sensitive potassium channels, suggesting an ATP-dependent role.