The effect of prazosin, indoramin and phentolamine on sympathetic nerve activity.

The failure of alpha 1-adrenoceptor antagonists, prazosin and indoramin, to cause a reflex tachycardia was investigated in the anaesthetised cat. Recordings were made of preganglionic sympathetic nerve activity from the third or fourth white ramus communicans, femoral arterial conductance, heart rate and blood pressure. A 1 h infusion of prazosin or indoramin caused falls in preganglionic sympathetic nerve activity, heart rate and blood pressure. Infusion of phentolamine differed, in that there was an increase in preganglionic sympathetic nerve activity and heart rate along with a fall in blood pressure. Indoramin differed from prazosin and phentolamine in that it only caused a small increase in femoral arterial conductance. It is suggested that the ability of prazosin and indoramin to cause a reduction in central thoracic sympathetic outflow may explain the failure of these drugs to cause a reflex tachycardia in man and experimental animals.