Abstract
Objective: Genetically hypertensive BPH/2J mice are recognized as a neurogenic model of hypertension, primarily based on sympathetic overactivity and greater neuronal activity in cardiovascular regulatory brain regions. Greater activity of the central renin angiotensin system (RAS) and reactive oxygen species (ROS) reportedly contribute to other models of hypertension. Importantly the peripheral RAS contributes to the hypertension in BPH/2J mice, predominantly during the dark period of the 24 h light cycle. The aim of the present study was to determine whether central AT1 receptor stimulation and the associated ROS signaling contribute to hypertension in BPH/2J mice in a circadian dependent manner.Methods: Blood pressure (BP) was measured in BPH/2J and normotensive BPN/3J mice (n = 7–8) via pre-implanted telemetry devices. Acute intracerebroventricular (ICV) microinjections of AT1 receptor antagonist, candesartan, and the superoxide dismutase (SOD) mimetic, tempol, were administered during the dark and light period of the 24 h light cycle via a pre-implanted ICV guide cannula. In separate mice, the BP effect of ICV infusion of the AT1 receptor antagonist losartan for 7 days was compared with subcutaneous infusion to determine the contribution of the central RAS to hypertension in BPH/2J mice.Results: Candesartan administered ICV during the dark period induced depressor responses which were 40% smaller in BPH/2J than BPN/3J mice (Pstrain < 0.05), suggesting AT1 receptor stimulation may contribute less to BP maintenance in BPH/2J mice. During the light period candesartan had minimal effect on BP in either strain. ICV tempol had comparable effects on BP between strains during the light and dark period (Pstrain > 0.08), suggesting ROS signaling is also not contributing to the hypertension in BPH/2J mice. Chronic ICV administration of losartan (22 nmol/h) had minimal effect on BPN/3J mice. By contrast in BPH/2J mice, both ICV and subcutaneously administered losartan induced similar hypotensive responses (−12.1 ± 1.8 vs. −14.7 ± 1.8 mmHg, Proute = 0.31).Conclusion: While central effects of peripheral losartan cannot be excluded, we suggest the hypotensive effect of chronic ICV losartan was likely peripherally mediated. Thus, based on both acute and chronic AT1 receptor inhibition and acute ROS inhibition, our findings suggest that greater activation of central AT1 receptors or ROS are unlikely to be mediating the hypertension in BPH/2J mice.
Highlights
BPH/2J mice are a genetic model of hypertension selectively bred from the same base population as their normotensive BPN/3J control strain (Schlager, 1974)
During the light period, mean arterial pressure (MAP) in BPH/2J mice was 18% greater than in BPN/3J mice (Pstrain = 0.01) and heart rate (HR) tended to be higher in BPH/2J mice (Pstrain = 0.05) but activity was comparable between strains (Pstrain = 0.9)
Our acute pharmacological studies demonstrate that central AT1 receptor activity is crucial for normal regulation of blood pressure (BP) during the dark period but less so during the light period, whilst central reactive oxygen species (ROS) seem to be more important in controlling the cardiovascular response to stress
Summary
BPH/2J mice are a genetic model of hypertension selectively bred from the same base population as their normotensive BPN/3J control strain (Schlager, 1974). Studies report that whole brain catecholamine levels were low in BPH/2J mice and more discrete evaluation revealed that catecholamine levels were low in the hypothalamus, midbrain, medulla, and thoracic spinal cord of BPH/2J mice (Schlager and Freeman, 1983; Schlager et al, 1983; Denoroy et al, 1985) Whether these differences in catecholamine level were due to lower synthesis or increased turnover is unknown but it does highlight a difference in the CNS that is specific to the hypertensive mice. We have shown that neuronal overactivity in the medial amygdala is a major contributor to the sympathetically mediated hypertension in BPH/2J mice (Jackson et al, 2014) Taken together these studies suggest that the CNS is likely involved in the sympathetically mediated hypertension in BPH/2J mice, but the mechanism remains to be determined
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