Effects of abrupt cessation of treatment with clonidine and guanfacine on blood pressure and heart rate in spontaneously hypertensive rats.

Present experiments were undertaken to compare the effects of clonidine and guanfacine withdrawal on blood pressure in spontaneously hypertensive rats. Five week oral administration of clonidine at a dose of approximately 300 microgram/kg/day produced a significant hypotension. After acute interruption of clonidine administration, a marked blood pressure rise was observed. Peak blood pressure rise was reached within 24 hours after clonidine withdrawal. A significant increase in plasma norepinephrine concentration was observed when clonidine treatment was discontinued. On the other hand, long-term administration of guanfacine at a dose of approximately 3 mg/kg/day produced a significant hypotension. After a five week administration of guanfacine, abrupt cessation did not produce a rapid blood pressure rise. Catecholamine concentration in urine and plasma was not influenced either by guanfacine administration or its withdrawal.