Influence of clonidine on cardiac vagal component of baroreceptor reflex in normotensive and spontaneously hypertensive rats.

In normotensive (NR) and spontaneously hypertensive rats (SHR) the cardiac vagal component of baroreceptor reflex was checked before and after intravenous infusion of clonidine (25-30 microgram/kg). 1) The decrease of heart rate due to the arterial baroreceptor reflex was the same in NR and in SHR. 2) Intravenous injection of clonidine produced much greater decrease of the heart rate in SHR than in NR. 3) After administration of clonidine the stimulation of arterial baroreceptors decreased the heart rate to the same low level in NR and SHR. In these conditions the maximal neurogenic slowing of the heart rate was probably achieved. 4) Atropine abolished almost completly heart rate change following baroreceptor stimulation or/and clonidine administration.