In vitro inhibitory effect of methanol leaf extract of Stachytarpheta jamaicensis (Verbenaceae) on non-pregnant rat uterus

Purpose: To investigate the effect of the methanol extract of the plant, Stachytarpheta jamaicensis (Verbenaceae) (SJ) on uterine smooth muscles of non-pregnant female rats, with the aim of examining the oxytocic or otherwise effect of the extract.
Methods: In the first phase of experiments, the effects of SJ (0.41 and 4.01 mg/ml) on oxytocin (OT) induced uterine contractions were determined and repeated after addition of salbutamol (SBL) (41.7 nM). In the second phase, verapamil (VER) (2.03 μM), SBL (41.7 nM), and SJ (0.41 and 4.01 mg/ml) were applied to the tissues after pre-contraction with K+ (80 mM). In the third phase, the effects of VER, SBL, and SJ on CaCl₂-induced contractions (0.03 – 10.83 mM) were examined. In the fourth and final phases, the second phase experiments were repeated in a calcium-free medium and in the absence and presence of propranolol (1.54 mM) respectively.
Results: SJ exhibited significant inhibitory effects on OT and CaCl₂ induced uterine contractions (p < 0.05). The EC₅₀ for OT increased from 1.92 ± 0.12 to 7.16 ± 0.16 nM and that for CaCl₂ increased from 0.19 ± 0.09 to 0.76 ± 0.11 mM in the presence of the extract. SJ also significantly inhibited KCl- induced contraction by 39.15 ± 2.13 and 53.23 ± 1.58 % for 0.41 and 4.01 mg/ml of the extract, respectively (p < 0.01); this inhibition was unaffected by propranolol. On the other hand, SBL and VER showed inhibition of 77.25 ± 1.85 and 79.44 ± 2.27 %, respectively.
Conclusion: SJ exerts uterine inhibitory effects in rats which appear unrelated to β₂-adrenergic receptor stimulation but possibly through inhibition of calcium entry into the cytoplasm.

Keywords
Oxytocin; Calcium-free; Uterine contraction; β₂-Adrenergic receptor stimulation; verapamil; salbutamol; Propranolol; S. jamaicensis