Anesthesia Blocks the Cardiovascular Response to Baroreceptor Unloading in Rats

Baroreflex responsiveness and orthostatic stability in humans can be assessed by a variety of approaches including exposure to graded levels of lower body negative pressure (LBNP). However, such approaches have limited applicability in animal studies owing to the need to anesthetize or sedate the animal. We recently reported a novel approach for the assessment of baroreceptor responsiveness in the awake rat using LBNP, and presented preliminary findings that 3% isoflurane anesthesia completely blocked the normally robust baroreflex. In the present study, we sought to extend these findings by studying the effects of several common anesthetics on LBNP responsiveness. Blood pressure (BP) and heart rate (HR) responses to progressive levels of LBNP were first made in awake rats (male and female), followed by measurements under various anesthetics regimens: 1) pentobarbital; 2) ketamine plus xylazine; 3) isoflurane at 3%, 2%, and 1.5%; 4) urethane delivered as an i.p. bolus, slow i.p. infusion, and slow i.v. infusion. As previously reported, BP in awake rats was well maintained up to -15 mmHg LBNP, accompanied by a robust baroreflex tachycardia. Despite varying effects on steady-state BP and HR, all of the anesthetics tested severely or completely blocked the ability to maintain BP during LBNP and completely blocked reflex tachycardia. BP, but not reflex tachycardia, during LBNP was partially preserved only in those rats treated with i.v. urethane. These data confirm that the functional baroreflexes that normally maintain BP during orthostatic challenge are blocked by commonly used anesthetics.