Cardiovascular effects of low and high doses of clonidine in rats.

Clonidine was a partial agonist of adrenoceptors and the effects of a range of clonidine doses on the mean arterial pressure and heart rate of rats were evaluated. It was found that low doses of clonidine, 30-300 micrograms/kg, induced a significant hypotension whereas high doses of clonidine, 3-18 mg/kg, were unable to decrease the blood pressure significantly. The reversal by high doses of clonidine of the hypotension induced by low dose of clonidine was possibly due to the blood vessel contraction induced by a high dose of clonidine. On the other hand, both low and high doses of clonidine induced bradycardia in the in vivo as well as in vitro studies.