Neural and hormonal control of blood pressure and vascular conductance during hemorrhage in hypertensive rabbits

• Main Author: Weichert, Gabriele Elizabeth
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/7498

The cardiovascular response to hemorrhage has been well characterized in conscious normotensive rabbits. When the autonomic nervous system is intact, baroreflex mechanisms cause peripheral vasoconstriction and tachycardia, thereby maintaining blood pressure. When the autonomic nervous system is blocked, the vasoconstrictor hormones vasopressin (AVP) and angiotensin II (ANG Il) are released early during hemorrhage, and have significant effects on peripheral vascular tone and maintenance of blood pressure. In hypertension, the reflex control of blood pressure in response to hemorrhage is not well understood. It was the goal of the experiments performed in this thesis to characterize the control of blood pressure and vascular conductance during hemorrhage in conscious renal wrap hypertensive rabbits. In particular, direct blood flow measurements allowed the control of hindlimb, mesenteric and renal vascular conductance to be characterized. In addition, a morphological analysis of hindlimb, mesenteric and renal blood vessels from renal wrap hypertensive rabbits was performed in order to further understand functional alterations to the vasculature in this hypertensive model. During hemorrhage, renal wrap hypertensive rabbits had an impaired ability to control blood pressure. When the autonomic nervous system was intact, this was associated with an impaired ability to control heart rate and hindlimb conductance. When the autonomic nervous system was pharmacologically blocked, the impaired ability to control blood pressure was associated with an impairement in hormonally mediated (AVP and ANG II) control of hindlimb vascular conductance. There was no evidence to suggest that neural or hormonal control of renal and mesenteric vascular conductance was impaired in the hypertensive rabbits during hemorrhage. Morphological analysis of hypertensive blood vessels showed that the hindlimb, renal and mesenteric blood vessels had an increased wall-to-lumen ratio indicative of non-specific vascular hyper-responsiveness. The impaired ability of the hypertensive rabbits to control blood pressure during hemorrhage was associated with an impaired ability to control the hindlimb, but not the mesenteric and renal vascular beds. These findings suggest that, in renal wrap hypertensive rabbits, there is either a differential control of sympathetic nerve activity or that there is a differential degree of vascular reactivity to these individual vascular beds.