Plasma clonidine in relation to blood pressure, catecholamines, and renin activity during long-term treatment of hypertension.

Eight patients with essential hypertension were treated with 50 microgram oral clonidine four times daily for 4 wk followed by 75 to 150 microgram four times daily for another 4 to 16 wk. Before treatment and at the end of each treatment period the patients were hospitalized and their plasma clonidine concentrations (P-CLON) were determined repeatedly during one dosage interval. The relationship of P-CLON to blood pressure, heart rate, plasma norepinephrine (PNE), and plasma renin activity (PRA), as well as to conventional kinetic parameters, was calculated. With the two dosages used the half-life (t 1/2 s) of clonidine absorption were 1.05 and 0.83 hr, the biological t 1/2 s 8.7 and 7.9 hrs, and the total oral clearances 4.7 and 5.0 ml X min-1 X kg-1. The log of P-CLON correlated with the maximum percentage decrease of mean arterial blood pressure (MAP) (r = 0.91, P less than 0.001) in five patients, whereas in three patients there was no further decrease of MAP after increase of plasma clonidine level. The logarithm of P-CLON correlated with percentage reduction of PNE (r = 0.67, P less than 0.01). In six patients in whom clonidine induced a reduction of PRA, there was a relationship between log P-CLON and percentage PRA reduction (r = 0.91, P less than 0.001) and also between the percentage PRA and PNE reduction (r = 0.74, P less than 0.01).