Cardiac reflex activity in chemically sympathectomized dogs in response to hypotension or hypertension

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Abstract
The cardiac responses to hypotension and hypertension induced by intravenous injections of nitroglycerin and phenylephrine were studied in unanesthetized dogs before and after chemical sympathectomy with 6-hydroxydopamine (6-OHDA) (50 mg/kg) and β-receptor blockade with sotalol (8 mg/kg). Three days after sympathectomy, the fall in blood pressure induced by nitroglycerin administration was increased significantly whereas the reflex tachycardia was markedly decreased, although it was not completely abolished. A significant increase in heart rate (27 ± 5 beats/min) persisted after 6-OHDA pretreatment. A similar cardiac response to nitroglycerin was observed after β-receptor blockade in normal and sympathectomized dogs. The chronotropic response observed after sympathetic blockade was accompanied by a disappearance of the normally observed sinus arrhythmia, and its magnitude seemed to be related to the initial heart rate. In contrast, the reflex slowing of the heart after phenylephrine-induced hypertension remained unchanged after pretreatment with 6-OHDA or sotalol. However, the blood pressure threshold necessary to trigger a reflex bradycardia was lower after chemical sympathectomy. The present study suggests that reflex tachycardia in response to hypotension results from reciprocal changes in sympathetic and parasympathetic activity of the heart while the bradycardia in response to hypertension is predominantly of vagal origin.

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