Stimulation of phrenic nerve activity by salicylate

To assess the possibility that salicylate stimulates VE by direct excitation of phrenic motoneurons, we compared two groups of anesthetized vagotomized dogs with respect to increases in phrenic nerve activity elicited by a large dose of sodium salicylate (225 mg/kg). The sole difference between the two groups of animals was the condition of the spinal cord (SC); SC remained intact in one group of animals (i.e., intact animals) whereas the other group of animals (i.e., T1 spinal-transected animals) underwent complete transection of the SC at the first thoracic level. Both groups of animals were ventilated by a respirator and arterial PCO2 was maintained constant throughout all experiments. Following salicylate infusion, intact animals exhibited two- to threefold increases in the frequency of phrenic nerve bursts and three- to fivefold increases in moving average minute phrenic activity (i.e., the summation of peak integrated burst activity per minute). In contrast, salicylate infusion into T1 spinal-transected animals elicited no statistically significant increase in the frequency of phrenic nerve bursts while increases in minute phrenic activity were limited to 32 +/- 8%. Since T1 spinal transection markedly diminishes increases in phrenic nerve activity elicited by salicylate, we conclude that salicylate stimulates VE by a reflex mechanism whose afferent pathways originate in metameres below T1.