Effects of alpha-adrenoceptor-stimulating drugs on baroreceptor reflexes in conscious cats

The action of some alpha-adrenoceptor stimulating drugs with a central effect (clonidine, alpha-methyldopa, reserpine) on baroreceptor reflexes was studied in conscious cats (both in the resting condition and when influenced by emotional tension or electrical stimulation of the hypothalamus). The sedative effect of these drugs was observed simultaneously with bradycardia and the increase of baroreceptor reflexes. Clonidine and reserpine (in 6-24 h after injection) lowered blood pressure while alpha-methyldopa (40 mg/kg) increased it. Confrontation with a dog or electrical hypothalamic stimulation produced hypertensive reactions and diminished the baroreflexes. All drugs reduced the emotional and hypertensive reactions caused by natural stress situations and restored baroreceptor reflexes. On the other hand, neither clonidine nor alpha-methyldopa changed the decrease of baroreceptor reflexes caused by electrical hypothalamic stimulation. It is supposed that central alpha-adrenoceptor stimulating drugs do not influence processes of hypothalamic modulation of baroreceptor reflexes. The increase in baroreflex activity after clonidine, alpha-methyldopa and reserpine appears to be due to a direct effect of the drugs on the central neurones mediating baroreceptor reflexes and to the tranquillizing action of these drugs.