The effect of propranolol and other antihypertensive drugs on preganglionic sympathetic activity in the cat.

Preganglionic sympathetic nerve activity was recorded from the white rami of T3/T4 along with blood pressure, heart rate and femoral arterial conductance in vagotomised, atropinised cats under alpha-chloralose/pentobarbitone anaesthesia, and paralysed by gallamine. Injection of propranolol or trimetaphan caused falls in mean blood pressure and concomitant rises in preganglionic sympathetic nerve activity. Propranolol was then infused over 3 hr with a stepwise increase in dose rate. This infusion caused a secondary fall in blood pressure associated with a delayed increase in preganglionic sympathetic nerve activity. Infusion of clonidine and sodium nitroprusside over 3 hr with a stepwise increase in dose rate caused falls in blood pressure, but clonidine caused a decrease while sodium nitroprusside caused an increase in preganglionic sympathetic nerve activity. It is therefore suggested that propranolol interferes at more than one site, along the reflex arc involved in compensation for the propranolol-induced fall in cardiac output.