Mice with selective angiotensin type 1A receptor deletion from catecholaminergic neurons show altered angiotensin II-dependent hypertension.

Background: Angiotensin II (AngII) acts via several tissues, including the brain to exert its well established roles in the regulation of blood pressure (BP) and fluid and electrolyte regulation. Manipulation of brain AngII can also induce metabolic changes including lean phenotype and increased metabolic rate possibly via modulation of the sympathetic nervous system. These actions are mediated via activation of the AngII type 1A receptor (AT1AR). In addition, chronic low dose infusion of Ang II induces hypertension via activation of the AT1AR. Whilst the involvement of AT1AR in specific cells of the kidney has been demonstrated in AngII-dependent hypertension it is not clear whether other tissues are involved. The present study aimed to investigate whether targeted deletion of AT1ARs from the catecholaminergic cells affects the baseline metabolic parameters and blood pressure (BP) and the response to sustained infusion of AngII. Methods: All experiments were approved by the University of Melbourne Animal Experimentation Ethics Committee in accordance with guidelines of the Australian National Health and Medical Research Council. Mice (C57Bl/6 background) with cell-selective deletion of the AT1AR from catecholaminergic cells were generated by crossing AT1AR floxed (AT1ARfl/fl) mice with mice expressing crerecombinase (Cre) under the control of the tyrosine hydroxylase (TH) promoter. Two different genotypes AT1ARfl/fl;THCre+/- (knockouts) and AT1ARfl/fl;TH-Cre-/- (controls) were used in this study. In vitro autoradiography was performed to confirm the loss of AT1R binding from catecholaminergic cells. All surgical procedures were performed under surgical anesthesia (loss of pedal withdrawal reflex) using either isoflurane delivered by a gas mask (^~2% in oxygen) or ketamine/xylacine i.p. (75 mg/kg:10 mg/kg). All animals received a long-acting, non-steroidal anti-inflammatory agent at the start of the surgical procedure (caroprofen 5 mg/kg). Telemetry devices were used to record baseline BP and heart rate (HR). For the metabolic studies, mice were housed individually in metabolic cages with free access to food and water. After 1 day acclimatization 24 hour urine output and food and water intake were recorded. AngII-dependent hypertension was induced by subcutaneous implantation of mini-osmotic pumps designed to infuse Ang II (500 ng/kg/min) for 4 weeks. Results: AT1R binding was completely absent in the adrenal medulla and thoracic sympathetic ganglia of the AT1AR fl/fl;TH-Cre+/- mice. Basal systolic BP and HR were not different between the groups (mean±SEM; 109± 1mmHg vs. 108 ± 2 mmHg and 553± 10 bpm vs. 573 ± 10 bpm). Body weight, water intake, and 24 h urine volumes were also not different between groups. Infusion of Ang II in AT1ARfl/fl;TH-Cre-/- induced a gradual pressor response that was increased above baseline by day 5 of infusion. The maximal change in BP occurred at day 13. After day 17 BP gradually decreased toward baseline. The trajectory of BP increase was less in the AT1A fl/fl;TH Cre+/- mice throughout the infusion with a significant decrease in the maximal response at day 13 of infusion (150± 3 mmHg vs. 133 ± 2 mmHg; P<0.05). The reduced pressor response was associated with reduced fluid and electrolyte retention, reduced activation of the sympathetic nervous system and loss of superoxide formation in key brain cardiovascular nuclei. Conclusions: AT1AR knockout from catecholaminergic cells does not elicit changes in metabolic state, baseline systolic BP or HR but expression of the receptor by these cells is essential for the full development of AngII hypertension. [ABSTRACT FROM AUTHOR]