Mechanism of trypsin-induced contraction in the rat myometrium: the possible involvement of a novel member of protease-activated receptor

1 The mechanism of trypsin-induced contraction in the rat myometrium was in vestigated using front-surface fluorimetry on fura-PE3-loaded strips. The e xpression of protease-activated receptors (PARs) in the rat myometrium was determined by reverse transcription polymerase chain reaction (RT PCR). 2 In non-pregnant rats, 10 muM trypsin developed a force of up to 30.5 +/- 5.1% of that obtained during the 40 m-m K+-depolarization-induced contracti on. In pregnant rats. the maximal level of the cytosolic Ca2+ concentration and tension obtained with 3 muM trypsin was 143.2+/-6.0% and 63.2+/-7.9%. respectively. The depletion of the extracellular Ca2+ abolished the trypsin -induced contraction, 3 Trypsin-induced contraction was abolished by the pre-treatment of a serin e protease inhibitor. PAR1-activating peptide (PAR1-AP) caused a potent con traction of the myometrium. while neither PAR2-AP nor PAR4-AP induced any c ontraction. 4 RT PCR analysis detected the expression of PAR1 mRNA. However. neither PA R2 nor PAR4 mRNA was detected in the rat myometrium. 5 Once the strips were stimulated with thrombin, the subsequent application of thrombin failed to induce any contraction. while trypsin induced a cont raction similar to that observed without the prestimulation with thrombin. Once the strip was stimulated with trypsin, neither trypsin nor thrombin in duced any contraction. The response to PAR1-AP remained after the pre-stimu lation with thrombin and trypsin. 6 In Conclusion, PAR1 was the only known receptor for trypsin expressed in the rat myometrium, but it was suggested to be cleaved an inactivated by tr ypsin.-Trypsin was thus suggested to contract the rat myometrium via a nove l type of PAR, which might be upregulated during pregnancy.