Cardiac norepinephrine stores and chronotropic reflex response after thoracotomy

Although alterations in cardiac reflex activity, as well as in cardiac norepinephrine stores, have been shown following thoracic operations, the physiological relationship between these alterations has not been investigated. Cardiac norepinephrine stores were examined in 17 normal dogs, and in 16 dogs 1-6 wk after a right thoracotomy. The heart rate response to induced arterial hypotension (mean decrease 17-20 mmHg) prior to and following surgery was also examined in 10 dogs. A significant depression in right atrial norepinephrine stores from normal levels of 1.95 +/- 0.13 mug/g to 1.30 +/- 0.25 mug/g (33%) (P less than 0.05) was observed during the first wk following surgery. During the same period the heart rate response to a fall of 17-20 mmHg systolic arterial pressure was clearly reduced from 52.8 +/- 6.9 beats/min to 17.0 +/- 4.8 beats/min (67.8%; P less than 0.05). Right atrial norepinephrine stores at the time of the initial operation and when reflex responses were most markedly depressed demonstrated that the decrease in norepinephrine stores was paralleled by attenuation of the chronotropic response to hypotension.