YOHIMBINE PHARMACOKINETICS AND INTERACTION WITH THE SYMPATHETIC NERVOUS-SYSTEM IN NORMAL VOLUNTEERS

The pharmacokinetics of yohimbine and its effects on sympathoadrenal function were studied in 13 young, healthy, male volunteers after an IV bolus dose of 0.25 or 0.5 mg . kg-1. Pharmacokinetic analysis showed that distribution was rapid, with a half life between 0.4 and 18 min, and the elimination half life ranged between 0.25 and 2.5 h. The volume of distribution (V(SS)) was 74 l, (range 26 to 127 l). Only 0.5 to 1 % of unchanged yohimbine was found in the urine, indicating that the major part of the drug was eliminated by hepatic clearance. Total plasma clearance was 117 l . h-1, which exceeds the hepatic plasma flow. This means that yohimbine is a high extraction drug with considerable extra-hepatic metabolism. Fractional urine sampling revealed that 0.5-1 % of unchanged yohimbine was excreted in urine in a biphasic manner. The data also suggested the existence of a slower elimination phase, with a half life of 13 h. The venous plasma concentration of noradrenaline (NA) increased 3-fold within 15 min after the yohimbine injection while plasma adrenaline (A) and neuropeptide Y-like immunoreactivity (NPY-LI) remained unchanged. The plasma concentration-effect relationship of the changes in circulating NA followed counterclockwise hysteresis. The results show that the hyperadrenergic state elicited by therapeutic doses of the alpha2-adrenergic autoreceptor antagonist, yohimbine, is due to an interaction with NA but not to release of A or NPY in man.