Applicability of recent methods used to estimate spontaneous baroreflex sensitivity to resting mice

Abstract

Laude D, Baudrie V, Elghozi J-L. Applicability of recent methods used to estimate spontaneous baroreflex sensitivity to resting mice. Am J Physiol Regul Integr Comp Physiol 294: R142-R150, 2008. First published November 7, 2007 doi:10.1152/ajpregu.00319.2007.-Shortterm blood pressure (BP) variability is limited by the arterial baroreflex. Methods for measuring the spontaneous baroreflex sensitivity (BRS) aim to quantify the gain of the transfer function between BP and pulse interval (PI) or the slope of the linear relationship between parallel BP and PI changes. These frequency-domain (spectral) and time-domain (sequence) techniques were tested in conscious mice equipped with telemetric devices. The autonomic relevance of these indexes was evaluated using pharmacological blockades. The significant changes of the spectral bandwidths resulting from the autonomic blockades were used to identify the low-frequency (LF) and highfrequency (HF) zones of interest. The LF gain was 1.45 Ϯ 0.14 ms/mmHg, with a PI delay of 0.5 s. For the HF gain, the average values were 2.0 Ϯ 0.19 ms/mmHg, with a null phase. LF and HF bands were markedly affected by atropine. On the same 51.2-s segments used for cross-spectral analysis, an average number of 26.4 Ϯ 2.2 slopes were detected, and the average slope in resting mice was 4.4 Ϯ 0.5 ms/mmHg. Atropine significantly reduced the slopes of the sequence method. BRS measurements obtained using the sequence technique were highly correlated to the spectral estimates. This study demonstrates the applicability of the recent methods used to estimate spontaneous BRS in mice. There was a vagal predominance in the baroreflex control of heart rate in conscious mice in the present conditions. baroreceptors; heart rate; sympathetic; vagus nerve THE IMPORTANCE OF THE BAROREFLEX in blood pressure (BP) regulation in mice can be appreciated by the marked increase in BP variability that occurs after sinoaortic deafferentation of baroreceptors that are located in the carotid sinuses and aortic arch (9, 21). The ability of the baroreflex to buffer BP fluctuations varies depending on the frequency of the BP fluctuations (19). BP and heart rate (HR) fluctuate at regular frequencies, the magnitude of which can be accurately quantified using power spectral analysis (25, 30). The absolute frequency bands at which these oscillations occur in mice were analyzed in a previous report (3). One recent approach to estimating the spontaneous baroreflex sensitivity (BRS) over a stationary period is the calculation of the gain of the transfer function between BP and R-R interval in the low-frequency (LF) and high-frequency (HF) bands (20,28). This spectral analysis approach has been recently applied to conscious mice (2, 7, 9 -12, 16, 18, 32). Another recent approach is the dynamic evaluation of the spontaneous BRS called the sequence technique (4). This method is based on the computerized scanning of beat-to-beat series of systolic BP and R-R interval values in search of spontaneous sequences of consecutive increases o...

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“…The low value of BEI observed in the present study is in accordance with previous findings in humans (6) and mice (20), indicating that the baroreflex induces changes in PI only in a minor fraction of systolic AP ramps. The baroreceptor itself is known to be affected at variable degrees by central inhibitory influences (6), and the cardiac rhythm is also controlled by other nonbaroreflex mechanisms (direct central neural control, respiratory activity, humoral substances, etc).…”

“…Baroreflex sensitivity was evaluated using the sequence method described by Bertinieri et al (3) and Martinka et al (27). The sequence analysis is a widely employed method to investigate spontaneous BRS over the years (6), providing information on the dynamic aspects of baroreflex control of HR during spontaneous behavior (20). This approach estimates baroreflex function under conditions in which the baroreflex is not far from its baseline operation, since spontaneous AP changes are of small amplitude (Ͻ10 mmHg).…”

“…The slope of the regression line between spontaneous changes in AP and PI values is taken as an index of BRS, as is done when AP and PI changes are induced by vasoactive drug injections (37). The baroreflex nature of the PI/AP sequences has been demonstrated in cats (3), rats (38), and mice (20). This technique is particularly suitable to mice because these animals are not amenable to receive multiple intravenous injections.…”

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“…With regard to spectral analysis, LF and HF oscillations of AP and HR have been thoroughly examined in conscious mice, validating the role played by the autonomic nervous system in their genesis (3,14,20,28,29,33,69,72). In fact, studies have indicated that all frequencies of HR variability in mice are dominated by parasympathetic modulation, providing a more reliable index of parasympathetic modulation, rather than sympathovagal balance (29,33).…”

“…In fact, studies have indicated that all frequencies of HR variability in mice are dominated by parasympathetic modulation, providing a more reliable index of parasympathetic modulation, rather than sympathovagal balance (29,33).…”