Effect of captopril on sympathetic preganglionic efferent activity in cats

In 13 decerebrate cats, we studied the effects of captopril (10 mg/kg iv bolus) on the background discharge of thoracic preganglionic sympathetic fibers. After drug administration there was an initial reduction in systolic arterial pressure (SAP), which was followed by a later inhibition of sympathetic discharge (from 2.7 +/- 0.5 to 0.79 +/- 0.1 imp/0.1 s; P < 0.01). Captopril significantly reduced the excitatory response of sympathetic fibers to premature ventricular contraction (70 +/- 17 vs. 257 +/- 30%), inferior vena cava obstruction (176 +/- 56 vs. 315 +/- 85%) and asphyxia (143 +/- 20 vs. 245 +/- 51%). Vice versa the sympathetic response to aortic occlusion was unaffected (-58 +/- 8 vs. -62 +/- 6%). A similar reduction in sympathetic discharge was observed after captopril administration in anesthetized cats (n = 3). On the contrary, no changes in background neural discharge were noticed in decerebrate-spinalized cats (n = 5), despite comparable hemodynamic effects. These data indicate that captopril reduces sympathetic efferent activity and its responsiveness to excitatory stimuli. The lack of neural effects in decerebrate-spinalized cats is consistent with a brain stem site of action of captopril.