Guanfacine and clonidine: antihypertensive and withdrawal characteristics after continuous infusion and its interruption in the spontaneously hypertensive and normotensive rat.

In conscious unrestrained spontaneously hypertensive and normotensive rats, prepared with permanently indwelling abdominal aortic catheters, the effects of blood pressure and heart rate of a 12-day continuous subcutaneous infusion of guanfacine (10 mg/kg/day) and clonidine (500 micrograms/kg/day) and sudden interruption of these treatments were studied. Both drugs significantly and consistently reduced the mean arterial pressure and heart rate throughout the infusion period in the SH rats, but not in the normotensive animals. The magnitude of the effects of both drugs in the SH rat were similar. Following withdrawal of treatment with guanfacine, a discontinuation syndrome was evoked, much less severe than that observed after suspension of the infusion with clonidine. The withdrawal syndromes were characterized by an overshoot of heart rate and a period of blood pressure lability. In spite of the ineffectiveness of guanfacine and clonidine to reduce blood pressure and heart rate consistently in the normotensive rat, similar withdrawal patterns as those found in the SH rat were observed. These findings are in general agreement with the results previously found in clinical studies in hypertensive patients. The spontaneously hypertensive rat may prove a suitable animal model for pre-clinical studies of discontinuation symptoms after cessation of treatment with antihypertensive drugs.