Magnesium sulphate suppresses ATP receptors in myometrial cells to inhibit uterine contractility for tocolysis

Abstract

TO INHIBIT UTERINE CONTRACTILITY FOR TOCOLYSIS HIROSHI MIYOSHI, SATOSHI URABE, RIKAKO SHIMAZUTSU, OSAMU SAMURA, HISAYA FUJIWARA, YOSHIKI KUDO, Hiroshima University, OB/GYN, Hiroshima, Japan OBJECTIVE: Magnesium sulphate is commonly used in clinical treatment for tocolysis. Magnesium ions (Mg) suppress the strength and frequency of contractions but the mechanism is less understood. We have reported that non-selective cation channels (NSCCs) were suggested to regulate pacemaker potentials to generate rhythmical contractions. One candidate for myometrial NSCC is a ATP receptor. The purpose of this study is to examine the effect of Mg on ATP receptor currents and identify myometrial ATP receptors. STUDY DESIGN: ATP receptor currents and membrane potentials were recorded from rat myometrial cells isolated at days 18-22 of pregnancy using the patch clamp method and microelectrodes. The expression of ATP receptors was measured in pregnant rat myometrium by real time PCR (ABI 7700) with TaqMan probes (ABI). RESULTS: Extracellular ATP induced NSCC currents in these preparations. These currents were induced by ADP, GTP, UTP, 2-methylthio ATP and abmethylene ATP and were blocked by suramin and PPADS (blockers of P2X receptor). ATP depolarized cell membrane from a resting potential of 50 mV to 20 mV, indicating these currents may regulate uterine contractility. External application of Mg inhibited these currents, and also suppressed the ATP induced depolarization. The expression of mRNA was checked for P2X receptors. P2X1, 4 and 7 were measurable and the expression of P2X4 in the late pregnancy was increase by four times compared with non-pregnant myometrium. The properties of ATP receptor currents were similar to those of P2X4 or P2X7 currents. These findings indicate P2X4 or P2X7 are the main functional component of ATP receptor in myometrium. CONCLUSION: ATP receptor currents are observed in myometrial cells and are suggested to regulate the frequency of contractions. Blockade of ATP receptors by Mg may be one of the mechanisms for inhibiting uterine contraction in tocolysis.