Cardiovascular effects of the essential oil of Ocimum gratissimum leaves in rats: role of the autonomic nervous system.

1. The cardiovascular effects of intravenous (i.v.) administration of the essential oil of Ocimum gratissimum (EOOG) were investigated in rats. In addition, the present study examined: (i) whether the autonomic nervous system is involved in the mediation of EOOG-induced changes in mean aortic pressure (MAP) and heart rate (HR); and (ii) whether these changes could be attributed, at least in part, to the actions of eugenol, the major constituent of EOOG. 2. In both pentobarbitone-anaesthetized and conscious rats, i.v. bolus injections of EOOG (1-20 mg/kg) elicited immediate and dose-dependent decreases in MAP and HR. These responses to EOOG were of the same order of magnitude irrespective of whether the animal was under general anaesthesia. 3. Pretreatment of anaesthetized rats with bilateral vagotomy did not significantly modify the EOOG-induced dose-dependent hypotension, whereas it significantly reduced the bradycardia at the highest dose used. 4. In conscious rats, i.v. injections of bolus doses (1-10 mg/kg) of eugenol also elicited immediate and dose-dependent decreases in MAP and HR. Intravenous pretreatment of conscious rats with either methylatropine (1 mg/kg) or hexamethonium (30 mg/kg) significantly reduced the EOOG-induced dose-dependent bradycardia without affecting the hypotension. 5. These data show, for the first time, that i.v. administration of EOOG to either anaesthetized or conscious rats induces an immediate and significant hypotension and bradycardia, which appear to be due, at least in part, to the actions of the major constituent of EOOG, eugenol. These cardiovascular effects appear to be mediated by different pathways because only EOOG-induced hypotension appears to be independent of the presence of an operational autonomic nervous system. This may suggest that the hypotensive activity of EOOG results from its vasodilatory effects directly upon vascular smooth muscle.