Influencing the heart rate of rats with weak external mechanical stimulation

EFFECTS OF AORTA BENDING AND RENAL LIGATION ON THE BLOOD PRESSURE: A SIMULATION ON ARTERIAL STIFFENING

Motivation: Arterial stiffening is thought to be the principal cause of hypertension [1-5]. In modern hemodynamic theories, it amplifies the pulse pressure by affecting wave reflection and reducing the cushion function. Methods: In this study, we performed aorta bending experiment on rats to simulate the arterial stiffening in vivo. Results: The blood pressure (both the steady and the pulsatile component) dropped rather than rose, which is quite different from deduction from models of "flow theories". Conclusion: It is a strong support that the artery is a transmission system for the pressure wave and the coupled resonance mechanism [6-14] accounts for the improving transmission efficiency. It may help us to understand the mechanism of hypertension.

The possible role of arterial radial vibration in heart rate and blood pressure matching

Motivation: This study tests if the ‘oscillating spring’ analogy for the radial vibration of the arterial wall can help to describe the relationship between the heart rate (HR), the blood pressure (BP), and properties of the arterial wall when different types of stimulation are applied on the cardiovascular system. It may help to suggest a possible role for arterial radial vibration in the association between the arterial stiffening and hypertension. Methods: Either mechanical stimulation was applied (0.5-mmHg pressure variation) to Wistar rats by at near-HR frequency (group A) or administered Propranolol (2mg/kg i.p.; group B), and measured HR and BP simultaneously. Results: In both groups, HR and BP were noted to change in the same direction ( r2 = 0.72 and 0.62, respectively; p both < 0.05 by F-test). ANCOVA was performed on these two regression lines, and it was found that there was no significant difference between them ( p>0.3). Conclusion: In both groups, changes in haemodynamic parameters can be explained by the ‘oscillating spring’ analogy for the radial vibration of the arterial wall. This illustrates that, when facing various stimulations, it may be an important regulatory mechanism for the heart and the arteries to restore their frequency-matching condition in order to improve the arterial transmission efficiency. Paying more attention to the radial movement of the wall may therefore help to suggest a novel explanation of the mechanism that underlies the bi-directional relation between hypertension and arterial stiffening.

Effects of whole-body mechanical stimulation at double the heart rate on the blood pressure waveform in rats

The effects of mechanical stimulation on hemodynamics, such as due to mechanotransduction in vascular endothelial cells, have been widely discussed recently. We previously proposed a resonance model in which the arterial system is treated as a pressure-transmitting system, and suggested that the application of external mechanical stimulation with frequencies near the heart rate (HR) or harmonics thereof can be sensed by the arterial system and induce hemodynamic changes. In this study, we monitored the effects of external mechanical stimulation at a frequency of double the HR on BPW (blood pressure waveform), HRV (HR variability) and BPHV (blood-pressure-harmonics variability) in rats. A motor beating a waterbed mattress was used to generate pressure variations of 0.5 mmHg to apply onto the rats. The experiments were performed on three groups of rats with different beating frequencies: (A) double the HR, (B) 5% deviation from double the HR and (C) 1.5 times the HR. The experimental procedure was a 15 min control period followed by application of the mechanical stimulation for 15 min and further recording for 15 min (OFF period). During the OFF period, the amplitude of the second harmonic in the BPW significantly increased by >5% in group A with decreased HRV and BPHV. The second harmonic increased less in group B, and decreased in group C. The increase in the second-harmonic amplitude in group A may be due to the filtering properties of the renal arterial structure. This mechanism could be used to improve the local blood supply into the kidneys, and hence provide a new treatment modality for some important diseases, such as renal hypertension or nephrosis.