Analysis of nonstationarity in renal autoregulation mechanisms using time-varying transfer and coherence functions

Abstract

Chon KH, Zhong Y, Moore LC, Holstein-Rathlou NH, Cupples WA. Analysis of nonstationarity in renal autoregulation mechanisms using time-varying transfer and coherence functions. Am J Physiol Regul Integr Comp Physiol 295: R821-R828, 2008. First published May 21, 2008 doi:10.1152/ajpregu.00582.2007.-The extent to which renal blood flow dynamics vary in time and whether such variation contributes substantively to dynamic complexity have emerged as important questions. Data from Sprague-Dawley rats (SDR) and spontaneously hypertensive rats (SHR) were analyzed by time-varying transfer functions (TVTF) and time-varying coherence functions (TVCF). Both TVTF and TVCF allow quantification of nonstationarity in the frequency ranges associated with the autoregulatory mechanisms. TVTF analysis shows that autoregulatory gain in SDR and SHR varies in time and that SHR exhibit significantly more nonstationarity than SDR. TVTF gain in the frequency range associated with the myogenic mechanism was significantly higher in SDR than in SHR, but no statistical difference was found with tubuloglomerular (TGF) gain. Furthermore, TVCF analysis revealed that the coherence in both strains is significantly nonstationary and that low-frequency coherence was negatively correlated with autoregulatory gain. TVCF in the frequency range from 0.1 to 0.3 Hz was significantly higher in SDR (7 out of 7, Ͼ0.5) than in SHR (5 out of 6, Ͻ0.5), and consistent for all time points. For TGF frequency range (0.03-0.05 Hz), coherence exhibited substantial nonstationarity in both strains. Five of six SHR had mean coherence (Ͻ0.5), while four of seven SDR exhibited coherence (Ͻ0.5). Together, these results demonstrate substantial nonstationarity in autoregulatory dynamics in both SHR and SDR. Furthermore, they indicate that the nonstationarity accounts for most of the dynamic complexity in SDR, but that it accounts for only a part of the dynamic complexity in SHR.hemodynamics; myogenic; tubuloglomerular feedback; hypertension AUTOREGULATION OF RENAL BLOOD flow is mediated by the myogenic responses of the preglomerular vasculature (MYO) and the TGF feedback system. These two mechanisms exhibit characteristic resonant frequencies (MYO, 0.1-0.3 Hz; TGF, 0.02-0.05 Hz in rats). The marked difference in characteristic frequencies (and, hence, in time constants) between the TGF and MYO mechanisms permit separation and quantification of contributions of the two mechanisms by transfer function (1, 4, 29) and coherence analyses (2,12).A transfer function is the input-output relationship between an independent variable (blood pressure) and a dependent variable [renal blood flow (RBF)]. Its output is given in terms of gain, which reports fluctuation of the output with respect to the input and phase that contains the temporal relationship between the two signals. Thus, normalized gain Ͻ0 dB indicates that RBF is stabilized with respect to blood pressure and hence that autoregulation is effective. The coherence function assesses the degree to which the data are related ...

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