Roles of cardiac output and peripheral resistance in mediating blood pressure response to stress in rats.

The change in arterial blood pressure (BP) in response to presentation of an acute behavioral stress (i.e., classical conditioning) in rat includes an initial rapid rise (C1) followed by a delayed, but more sustained, pressor response (C2). The purpose of this experiment is to determine the patterns of change in cardiac output (CO) and total peripheral vascular resistance (TPR) that are associated with the behaviorally induced pressor response. A blood flow probe was implanted around the ascending aorta, and a catheter was implanted in a femoral artery in 10 male Sprague-Dawley rats. The rats were trained by a 15-s tone (CS+) followed by a 0.5-s tail shock; another tone (CS-), never followed by shock, served as a behavioral control. BP responded to the stressful stimulus (CS+) by a rapid C1 increase (8 +/- 1 mmHg; mean +/- SE) followed by the delayed C2 response (2 +/- 0.3 mmHg); the unconditioned response to shock was a 9 +/- 2 mmHg increase in BP. The C1 BP increase produced a significant increase in TPR (10 +/- 1 dyn.s/cm5); CO was not significantly changed. TPR decreased during C2 (-4 +/- 2 dyn.s/cm5), whereas CO was significantly increased (2 +/- 1 ml/min). These data contribute to our understanding of how the autonomic nervous system organizes the cardiovascular response to a suddenly perceived behavioral stress.