Continuous infusion and withdrawal of methyldopa in the spontaneously hypertensive rat.

In conscious spontaneously hypertensive rats, prepared with permanently indwelling abdominal aortic catheters, the effect on heart rate and blood pressure of a 12-day continuous intragastric infusion of methyldopa (200 mg/kg/day) and its withdrawal were studied. Throughout the infusion period, the blood pressure was significantly reduced, whereas slight but consistent tachycardia was observed. After sudden discontinuation of treatment, heart rate and blood pressure returned to the control level. Overshoots in these parameters were not observed. The appearance of blood pressure upswings after methyldopa withdrawal was limited to two to three per hour. These results contrast with the findings obtained earlier by us with continuous infusion and withdrawal of clonidine in the rat. Clonidine induces hypotension and bradycardia during infusion in the SH rat, whereas a marked overshoot in heart rate and a high frequency of blood pressure upswings appear after cessation of treatment.