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1 Department of Pharmacology and Toxicology, Wright State University School of Medicine, Dayton, OH, USA
2 Department of Pharmacology and Toxicology, Wright State University School of Medicine, Dayton, OH, USA; University of Sao Paulo School of Medicine, Ribeirao Preto-SP, SP, Brazil
3 Department of Pharmacology and Toxicology, Wright State University School of Medicine, Dayton, OH, USA; Department of Pharmacology and Toxicology, Wright State University School of Medicine, Dayton, OH, USA
4 University of Sao Paulo School of Medicine, Ribeirao Preto-SP, SP, Brazil
* To whom correspondence should be addressed. E-mail: mariana.morris{at}wright.edu.
Studies examined the role of angiotensin (Ang) AT1a receptors in cardiovascular autonomic control by measuring arterial pressure (AP) and heart rate (HR) variability and the effect of autonomic blockade in mice lacking AT1a receptors (AT1a-/-). Using radiotelemetry in conscious AT1a +/+ and AT1a -/- mice, we determined: 1) AP and pulse interval (PI) variability in time and frequency (spectral analysis) domains; 2) AP response to 
1-adrenergic and ganglionic blockade and 3) intrinsic HR after ganglionic blockade. Pulsatile AP was recorded (5 kHz) for measurement of AP and PI and respective variability. Steady state AP responses to prazosin (1 µg/g, i.p.) and hexamethonium (30 µg/g, i.p.) were also measured. AP was lower in AT1a -/- vs AT1a +/+ while HR was not changed. Prazosin and hexamethonium produced greater decreases in MAP in AT1a -/- than in AT1a +/+. The BP difference was marked after ganglionic blockade ( 
MAP:- 44±10 vs - 18±2, mmHg, -/- vs +/+). Intrinsic HR was also lower in AT1a-/- mice (431 ± 32 vs 524 ± 22 bpm; -/- vs +/+). Beat-by-beat series of systolic AP (SAP) and PI were submitted to autoregressive spectral estimation with variability quantified in low (LF: 0.1-1 Hz) and high (HF: 1-5 Hz) frequency ranges. AT1a -/- mice showed a reduction in SAP LF variability (4.3±0.8 vs 9.8±1.3 mmHg2) with no change in HF (3.3±0.6 vs 2.7±0.2 mmHg2). There was a reduction in PI variability of AT1a -/- in both LF (18.7±3.7 vs 32.1±4.2 ms2) and HF (17.7±1.9 vs 40.3±7.3 ms2) ranges. The association of lower AP and PI variability in AT1a -/- mice with enhanced AP response to 1-adrenergic and ganglionic blockade suggests that removal of the Ang AT1a receptor produces autonomic imbalance. This is seen as enhanced sympathetic drive to compensate for the lack of Ang signaling.
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