MECHANISMS OF GALANIN-INDUCED CONTRACTION IN THE RAT MYOMETRIUM

1 A neuropeptide, galanin, regulates the reproductive process and dire ctly induces myometrial contraction. The aim of this study was to dete rmine the mechanism of galanin-induced myometrial contraction. For thi s purpose, we simultaneously measured intracellular Ca2+ concentration ([Ca2+](i)) and tension using fura-PE3-fluorometry and the rat longit udinal myometrium. The effect of galanin on the Ca2+ sensitivity of th e contractile apparatus was examined in beta-escin permeabilized strip s. The expression of galanin and the galanin receptors mRNAs in the ra t myometrium were determined by reverse transcription polymerase chain reaction (RT-PCR). 2 Galanin (10-300 nM) induced phasic contraction w ith or without oscillation in the pregnant rat myometrium in a concent ration-dependent manner. The maximal response was obtained at 100 nM. There was no significant difference either in the maximal responses or EC50 values for galanin-induced myometrial contractions among myometr iums from non-pregnant and pregnant (day 4, day 11, day 20, day 22) ra ts. 3 In the day 20 and 22 pregnant myometriums, assigning the levels of [Ca2+](i) and tension at 40 mM K+- depolarization to be 100%, galan in increased the [Ca2+](i) and tension to 126.9+/-2.9% and 116.3+/-2.7 %, respectively. Diltiazem (10 mu M) inhibited the galanin-induced ele vation of [Ca2+](i) and tension to 71.9+/-2.4% and 16.2+/-0.7%, respec tively. Ni2+, by itself, decreased the basal [Ca2+](i) to -50.2+/-3.9% without affecting resting tension. After Ni2+ treatment, galanin-indu ced increases in [Ca2+](i) and tension were -19.6+/-3.4% and 0.9+/-0.1 %, respectively. In myometrium treated with diltiazem, no oscillation in [Ca2+](i) and tension was observed. In Ca2+-free solution with 0.1 mM EGTA, galanin increased [Ca2+](i) from -40.2+/-2.7% to -18.0+/-2.6% and induced transient contraction (3.6+/-0.8%). 4 In beta-escin perme abilized myometrium, galanin enhanced the contraction induced by 0.3 m u M Ca2+ in the presence of GTP. In the presence of GDP beta S (1 mM) instead of GTP, galanin failed to increase the Ca2+ sensitivity of the contractile apparatus. 5 RT-PCR revealed that galanin mRNA was hardly expressed in the non-pregnant rat myometrium and increased to reach a maximal level at mid pregnancy (day 11), but decreased to the same le vel as in the non-pregnant myometrium at term (day 22). Type 2 galanin receptor (GALR2) mRNA was found to be expressed in the rat myometrium whereas type 1 galanin receptor (GALR1) mRNA expression was not detec ted. 6 In conclusion, galanin induces contraction of the rat myometriu m by increasing [Ca2+](i) as well as by increasing Ca2+ sensitivity of the contractile apparatus. Galanin-induced increases in [Ca2+](i) are caused by both intracellular Ca2+ release and Ca2+ influx from extrac ellular space. The responsiveness of the rat myometrium to galanin doe s not change during pregnancy. The galanin mRNA is expressed in the ra t myometrium and it is upregulated during mid-pregnancy. Rat myometriu m expresses GALR2 but not GALR1 mRNA. By changing mRNA expression in t he myometrium during pregnancy, galanin may act as a paracrine or auto crine mediator in the regulation of myometrial contractility.