Beta-endorphin contributes to the antihypertensive effect of clonidine in a subset of patients with essential hypertension.

Naloxone [0.4 mg iv.] increased blood pressure and heart rate of 13 clonidine-treated [0.3 mg per os for 3 days] patients with essential hypertension [reacting group] while it has no such effect in 11 clonidine-treated patients [non-reacting gr.] Clonidine increased plasma beta-endorphin concentration of the reacting patients by 17.53 +/- 1.68 pM/1 and in the non-reacting ones by 5.91 +/- 0.88 pM/1. Significant linear correlation was found between the clonidine-induced increase in plasma beta-endorphin level and the naloxone-induced change in mean blood pressure [r = 0.9572, n:24, p less than 0.001]. In another group of 8 patients clonidine [0.15 mg iv.] decreased mean blood pressure but naloxone, 30 min after the clonidine injection, did not reverse the clonidine hypotension. We suggest that beta-endorphin, released by chr. clonidine therapy, contributes to the anti-hypertensive effect only in the reacting group.