Mechanisms of galanin-induced contraction in the rat myometrium

Abstract

1. A neuropeptide, galanin, regulates the reproductive process and directly induces myometrial contraction. The aim of this study was to determine the mechanism of galanin-induced myometrial contraction. For this purpose, we simultaneously measured intracellular Ca²⁺ concentration ([Ca²⁺]_i) and tension using fura-PE3-fluorometry and the rat longitudinal myometrium. The effect of galanin on the Ca²⁺ sensitivity of the contractile apparatus was examined in β-escin permeabilized strips. The expression of galanin and the galanin receptors mRNAs in the rat myometrium were determined by reverse transcription polymerase chain reaction (RT-PCR).

2. Galanin (10–300 nM) induced phasic contraction with or without oscillation in the pregnant rat myometrium in a concentration-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 myometriums from non-pregnant and pregnant (day 4, day 11, day 20, day 22) rats.

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%, galanin increased the [Ca²⁺]_i and tension to 126.9±2.9% and 116.3±2.7%, respectively. Diltiazem (10 μM) inhibited the galanin-induced elevation of [Ca²⁺]_i and tension to 71.9±2.4% and 16.2±0.7%, respectively. Ni²⁺, by itself, decreased the basal [Ca²⁺]_i to −50.2±3.9% without affecting resting tension. After Ni²⁺ treatment, galanin-induced 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 β-escin permeabilized myometrium, galanin enhanced the contraction induced by 0.3 μM Ca²⁺ in the presence of GTP. In the presence of GDPβS (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 level 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 detected.

6. In conclusion, galanin induces contraction of the rat myometrium 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 extracellular space. The responsiveness of the rat myometrium to galanin does not change during pregnancy. The galanin mRNA is expressed in the rat myometrium and it is upregulated during mid-pregnancy. Rat myometrium expresses GALR2 but not GALR1 mRNA. By changing mRNA expression in the myometrium during pregnancy, galanin may act as a paracrine or autocrine mediator in the regulation of myometrial contractility.

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