Hypotensive action of an aqueous extract of Pimenta dioica (Myrtaceae) in rats

The intra-venous (i.v.) hypotensive action of the final aqueous fraction of Pimenta dioica was studied in Spontaneously Hypertensive Rats (SHR). The r ats were anaesthetized (sodium pentobarbital 50 mg/kg), the trachea, right carotid artery and jugular vein were cannulated for adequate ventilation, d irect blood pressure measurement and intra-venous administration of extract s, solutions and drugs. The arterial line was connected to a pressure trans ducer (Viggo-Spectramed model P23 XL) and a polygraph (Grass model 7H) and monitored continuously during the first five minutes after plant extract ad ministration and then at 5 and 15 minute intervals for one hour. Responses were taken as the maximum pressure changes observed during this period. Inc reasing doses of the final aqueous fraction were given i.v. to groups of si x SHR each. It produced a dose dependent decrease in blood pressure and the ED50 was 45 mg/kg. To discard that the hypotensive effect of the extracts was due to its ionic composition, a solution containing KCI, NaCl, CaCl2 an d MgCl2 equivalent to the ion contents present in a dose of 50 mg/kg of tot al aqueous extract was injected to Sprague-Dawley rats (SDN) using the same method as described above. It did not produce significant changes in blood pressure. pharmacological antagonistic studies were done injecting either autonomic ganglion, alpha adrenoceptor, beta adrenoceptor and cholinergic r eceptor blockers prior to extract administration in SHR rats. Atropine, pro pranolol and phentolamine did not affect the hypotensive effect of the fina l aqueous fraction. With hexamethonium (autonomic ganglion blocker) the hyp otensive response was diminished in a significant way (p<0.05). The hypoten sive action of the final aqueous extract was not mediated through cholinerg ic, alpha or beta adrenergic receptors. The extract may posses vasorelaxing activity which could not be evident after autonomic ganglion blockade due to extreme vasodilation present prior to extract administration. Future stu dies should address the question of a possible direct vasodilating effect o f the extracts.