Chaotic characteristics of renal nerve peak interval sequence in normotensive and hypertensive rats

1. The use of non-linear dynamic analysis for the measurement of control processes in low-dimensional signals, for example, blood pressure and heart rate variability, are well established and accepted. However, the application of these analytical techniques to a high-dimensional signal, such as renal sympathetic nerve activity (RSNA), has not been validated. 2. The present study set out to develop an approach whereby the high-dimensional signal of RSNA was reduced to a low-dimensional one by extracting the peak interval sequence (PIS), using Cluster analysis, in order to allow the use of non-linear dynamics analysis. Brachial nerves were electrically stimulated (1.6 Hz, 0.2 ms, 15 V) to elicit a sympatho-excitation in groups of anaesthetized normotensive Wistar and stroke-sprone spontaneously hypertensive rats (SHRSP). 3. It was found that, under basal conditions, the correlation dimension, D2, was stable over a range of embedding dimensions from 12 to 25. Moreover, the largest Lyapunov exponent had a small positive value that was also stable over these embedding dimensions. These values showed that the signal was of low dimensionality and that chaos was present. 4. In Wistar rats, brachial nerve stimulation significantly (P < 0.05-0.001) increased blood pressure (by 25%), heart rate (by 5%) and RSNA (by 200%), which was associated with significant (P < 0.05) reductions in the correlation dimension D2 and the largest Lyapunov exponent of the PIS generated from the renal nerve signal. In contrast, in SHRSP, there were similar increases in blood pressure, heart rate and RSNA in response to brachial nerve stimulation, but neither the correlation dimension nor largest Lyapunov exponent was altered. 5. These findings demonstrate that by extracting the PIS from the renal sympathetic nerve signal, the application of non-linear chaos analysis makes it possible to distinguish differences in the pattern of reflexly induced excitation in sympathetic traffic to the kidney in the pathophysiological state of hypertension. Whether this applies to sympathetic outflow to other organs and tissues remains to be investigated.