Hypotensive effect of clonidine and regional noradrenaline concentration in the brain of normotensive and hypertensive rats after acute and long-term treatment.

The following experiments were performed: a) the effects of increasing doses (0.025-0.2 mg/kg p.o.) of clonidine on the blood pressure and heart rate in SHR and normotensive rats (acute treatment), b) the influence of an effective dose (0.05 mg/kg) of clonidine on the same parameters during 3-week administration, c) the effect of clonidine (0.05 mg/kg) on tissue noradrenaline (NA) concentration in the medulla, striatum and hypothalamus after acute and long-term treatment of SHR and normotensive rats. The experiments were carried out on unanaesthetized animals. Blood pressure and heart rate were measured by on indirect method. Tissue NA was assayed fluorimetrically. Clonidine in a single dose caused a significant fall of blood pressure and heart rate. These effects were much more pronounced in SHR. The effective dose (0.05 mg/kg) in acute experiments was ineffective in long-term (3 weeks) treated rats. This dose of clonidine had no detectable effect on regional NA concentration in rat brain with the exception of the hypothalamus where the concentration of NA was markedly increased (SHR in acute experiments).