Evidence for a peripheral component in the sympatholytic actions of clonidine and guanfacine in man

The time courses of the changes in plasma growth hormone and noradrenaline concentrations in response to 15 min infusions of clonidine 0.2 mgs and guanfacine 2 mgs, were studied in six normal volunteers, in a double-blind, randomised, cross-over study. Plasma noradrenaline fell within 15 min of the commencement of drug administration, by 36 +/- 14% after clonidine (p less than 0.05) and by 32 +/- 11% (p less than 0.05) after guanfacine. Plasma growth hormone was not significantly elevated until the 30th minute to 12.0 +/- 4.7 lU/ml (p less than 0.05) after clonidine and 14.7 +/- 11.5 lU/ml (p less than 0.05) after guanfacine, having been undetectable prior to both drugs. The reduction in plasma noradrenaline by these alpha 2-adrenergic agonists, prior to activation of central adrenoceptors as detected by changes in plasma growth hormone, is evidence for a peripheral component in their sympatholytic effect.