Autonomic nervous system control of heart rate during baroreceptor activation in conscious and anesthetized rats

Heart rate and blood pressure were recorded in conscious, freely behaving rats through a catheter in the tail artery during administration of nitroprusside or phenylephrine through a catheter in the jugular vein. The sympathetic and parasympathetic components were distinguished by treating the rats with atenolol or methyl-atropine. Reflex bradycardia induced by all doses of phenylephrine was almost totally blocked blocked following methyl-atropine treatment. Reflex tachycardia induced by small to moderate doses of nitroprusside was attenuated to an equal extent following atropine or atenolol treatment. A similar experimental schedule was followed with a separate group of rats to determine the effects of pentobarbital and urethane anesthesia on the baroreceptor reflex. Both pentobarbital and urethane equally attenuated the tachycardia response to a decrease in blood pressure. However, pentobarbital anesthesia resulted in a greater attenuation of the bradycardia response to an increase in blood pressure than did urethane anesthesia. These data support the conclusion that the parasympathetic nervous system is primarily responsible for baroreceptor reflex-induced bradycardia in conscious rats. The sympathetic and parasympathetic systems contribute equally to control baroreceptor reflex-induced tachycardia except in extreme acute hypotension when the tachycardia is predominantly due to the activation of sympathetic nerves. The findings of the second experiment indicate that pentobarbital and urethane affect sympathetic systems differently.