Lack of an effect of age on the response to clonidine

The plasma concentration and appearance rate of norepinephrine are increased in the elderly. A hypothesis to explain this observation is that the elderly have a diminished response of the alpha 2-adrenoreceptor in the brainstem that modulates peripheral sympathetic tone. To evaluate the effect of age on alpha 2-adrenoreceptor function, we studied 12 healthy elderly subjects and 12 healthy young volunteers and compared the decrease in plasma norepinephrine and blood pressure in response to increasing doses of orally administered clonidine. We found that, for the same plasma clonidine concentration, the blood pressure and plasma norepinephrine concentration fell equivalently in both groups. These data imply that the increased plasma norepinephrine and the elevated blood pressure in the healthy elderly population do not appear to be secondary to a decrease in alpha 2-adrenergic response to an agonist in the central nervous system.