Effects of plants popularly used against hypertension on norepinephrine-stimulated guinea pig atria

High blood pressure is often caused by the release of excessive amounts of epinephrine and norepinephrine by the sympathetic nervous system and the adrenal medulla. This condition is associated with an increased vascular resistance and/or cardiac output, the product of cardiac contractility and beating frequency. Thus, an elevated blood pressure can be lowered, among others, by decreasing the contractility and/or beating frequency of the heart. In this study, we used an isolated organ model to investigate whether such a mechanism may play a role in the presumed blood pressure-lowering effects of ten popularly used plant extracts. The plants used in this study were Annona muricata, Artocarpus altilis, Averrhoa bilimbi, Cocos nucifera, Commelina virginica, Gossypium barbadense, Mangifera indica, Momordica charantia, Phyllanthus amarus, and Solanum melongena. Aqueous extracts were prepared, and assessed at 0.001 to 1 mg/mL for their ability to reduce the increased relative contractility and/or beating frequency of isolated guinea pig atria caused by 5 x 10-6 M norepinephrine. Incubations were in Ringer-Locke buffer, in 100% O2 and at 30 °C, and were for 3 min with 3-min intervals. Results (means ± SDs; n ≥ 3) were in g/sec and beats/min, and expressed with respect to readings found with Ringer-Locke buffer alone. The extracts from A. bilimbi (leaves), P. amarus (whole plants), and S. melongena (unripe fruits) decreased the contractility of the norepinephrine-stimulated atria by 50 to 100%, but did not affect their beating frequency. In addition, the S. melongena extract decreased the contractility of spontaneously beating atria by 90% without affecting the frequency. The remaining samples did not exert any effect under the above-mentioned conditions. The negative-inotropic effects of the A. bilimbi, P. amarus, and S. melongena extracts in the isolated guinea pig atria suggest that reduction of the cardiac output may play a role in their presumed blood pressure-lowering effects. These actions might be associated with alterations in intracellular calcium metabolism and/or phenomena involving the muscarinic receptor, but this must be determined in future studies.