Reflex hemodynamic responses initiated from the thoracic aorta

In anesthetized vagotomized cats with both carotid arteries occluded, stretch of the walls of the thoracic aorta performed without obstructing aortic blood flow induced reflex increases in arterial blood pressure (systolic: 136 ± 4 [SE] to 170 ± 7 mm Hg), heart rate (230 ± 10 to 236 ± 11 beats/min), and maximum rate of rise of left ventricular pressure (dP/dt max) (2,337 ± 256 to 3,155 ± 302 mm Hg/sec). In cats with spinal transection, similar increases were observed. These responses were abolished by infiltrating the walls of the thoracic aorta with xylocaine. In adrenalectomized cats with intact central nervous systems, reflex responses were reduced but were still statistically significant. Phenoxybenzamine abolished the pressor response but not the increases in heart rate and dP/dt max. Propranolol drastically reduced the increases in heart rate and dP/dt max but not the pressor response. It is concluded that stretch of the thoracic aorta induced an increase in sympathetic activity affecting the heart, the peripheral vessels, and, probably, the adrenal glands through a spinal reflex.