Role of endothelium and nitric oxide synthesis on garlic extract induced vasorelaxation

Garlic is thought to have a variety of therapeutic applications including lowering of blood pressure and inhibition of platelet aggregation. In the present study, we investigated the vasorelaxant effect of both water and ethanol extracts of garlic on isolated rat thoracic aorta (TA) and intralobar pulmonary arteries (PA) in the presence and absence of endothelium (EC). In both EC-intact TA and PA, precontracted with 0.1-1 μM phenylephrine, addition of 1-300 μg/ml water extract of garlic (GW) resulted in a slow-developing, dose-dependent relaxation reaching a maximum >90% and an IC50 of approximately 30 μg/ml. Ethanol extracts of garlic failed to elicit any vascular response. Prior exposure to a maximum 300 μg/ml GW did not result in any tachyphylactic effect, while 20 mins exposure to 1000 μg/ml GW resulted in a complete tachyphylaxis. Pretreatment of EC-intact TA and PA with 5 μM indomethacin, specific inhibitor of cyclooxygenase and vasodilatory prostanoids production, decreased the Emax to -65± 5% and -79± 7%, respectively. Combined indomethacin and 0.3 mM L-NAME (specific inhibitor of NO synthesis) treatment further reduced the relaxant responses to -42±4% and -66±9% in TA and PA, respectively. Mechanical disruption of EC, on the other hand, completely abolished the GW relaxation, with no significant change in the vascular smooth cell response. These results demonstrate that water extract, but not the organic extract, of garlic is capable of eliciting an EC-dependent relaxation which is dependent in part on the production of both vasodilatory prostanoids and nitric oxide.