Chloride Channel Blockers 5-nitro-2-(3-phenylpropylamino) Benzoic Acid and Anthracene-9-Carboxylic Acid Inhibit Contractions of Pregnant Rat Myometrium in Vitro

Abstract

Objective

We compared in vitro relaxant effect of chloride channel modulators, such as 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and anthracene-9-carboxylate (9-AC), and β₂-adrenergic agonists, such as ritodrine, in pregnant rat myometrium.

Methods

Isolated myometrial strips were obtained from eight pregnant rats, and the strips were mounted in organ baths for recording isometric tension. The effects of 10⁻⁸−10⁻⁴ M ritodrine, 10⁻⁶−3 × 10⁻⁴ M NPPB, and 10⁻⁶−10⁻³ M 9-AC on spontaneous contractions were recorded.

Results

Ritodrine (10⁻⁸−10⁻⁵ M) completely inhibited the amplitude and frequency of spontaneous contrations in myometrial strips isolated from pregnant rats in a concentration-dependent manner, but the relaxant effect of ritodrine at 10⁻⁴ M concentration resulted in tachyphylaxis. The chloride channel blocker NPPB (10⁻⁶−3 × 10⁻⁴ M) and the chloride transport inhibitor 9-AC (10⁻⁵−10⁻³ M) decreased the amplitude of spontaneous myometrial contractions in a concentration-dependent manner; the maximum inhibition produced by the highest tested concentration of each drug was 43.8% and 42.1% of the original degree of spontaneous contractions, respectively. The frequency of myometrial contractions was significantly inhibited by NPPB and 9-AC beginning with the concentration of 10⁻⁴ M.

Conclusion

NPPB and 9-AC appear to be effective relaxants of pregnant rat myometrium. These effects of NPPB and 9-AC might be therapeutically advantageous in clinical management of preterm labor.