Abstract
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 μg/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.