Introduction
We have previously observed that the duration of the long-lasting hypertensive effect of angiotensin II (AII) is dramatically reduced by inhalation anaesthesia (Eriksson et al. 1995). We tested here whether spectral analysis of blood pressure and heart rate responses to AII in, respectively, conscious and anaesthetized sheep would provide indications of differences in the reflex response pattern. Also the responses to a consecutive i.v. injection of the AII receptor antagonist losartan (LOS) were studied
Methods
AII in isotonic saline was infused bilaterally into the carotid arteries (6 mg min-1) for 20 min in adult ewes. 30 min after cessation of the AII infusion, LOS (10 mg kg-1) was injected i.v. Experiments were performed in both conscious and anaesthetized (isoflurane 2.6% end-tidal concentration) animals. Monitored/measured parameters: Renal (RBF) and femoral (FBF) blood flow; cardiac output, central and systemic blood pressures. Power spectral analysis was performed by using fast Fourier transformation from the arterial pressure curve (Kuo & Chan 1993). Power spectrum frequency bands: 0,02-0,08 Hz (very low frequency, VLF), 0,08-0,15 Hz (low frequency, LF), 0,15-0,5 Hz (high frequency, HF). Mean arterial pressure (MAP) and heart rate (HR) signals were processed in a personal computer, with spectral analyses performed every 32 second.
Results
The MAP response to AII was two times higher in conscious than in anaesthetized animals, and MAP stayed clearly above basal levels during the observation period. LOS lowered MAP in anaesthetized animals only. The HR was higher in anaesthetized animals, and was slightly increased by AII, whereas in conscious animals an initial relative bradycardia was followed by a slow increase. The FBF increased in response to AII in both conscious and anaesthetized animals (about 70% in both cases), returned towards baseline after cessation of infusion in anesthetized animals only, but decreased after LOS in both situations. The RBF was reduced by 50% by the anaesthesia as such. Still the RBF was reduced by 50% in response to AII in both conscious and anaesthetized animals, with an immediate recovery after cessation of infusion. LOS did not affect RBF in conscious animals, but increased above basal levels during anaesthesia. VLF spectral power of MAP increased in parallel to the elevated MAP during AII infusion in conscious animals. The much lower VLF power during anaesthesia showed a small but significant change in the opposite direction. HF spectral power in conscious animals increased during AII infusion, but in contrast to VLF power, there was no sustained activity after AII. Also the HF power was much lower during anaesthesia, but changed in the same direction as in conscious animals
Conclusions
The magnitude and duration of changes in spectral power of VLF and HF in the conscious state support the notion that the long-lasting hypertensive effect of AII is depending on sympathetic activity.
References
Kuo T. & Chan S. Am. J. Physiol. 264: H2208-H2213, 1993.
Eriksson S. Andersson B & Rundgren M. Acta Physiol Scand 155: 427-434, 1995.