Exploring Open-Loop Baroreflex Dynamics: A Comparison Between Female and Male Rats

Objective: Sex differences in baroreflex responses have been reported in both humans and rats. Recent studies have identified the presence of female-specific myelinated fibers in rats, which might contribute to these differences in baroreflex function. Although the dynamic characteristics are essential for the rapid and stable arterial pressure (AP) regulation mediated by the arterial baroreflex, to the best of our knowledge, the baroreflex dynamic characteristics have been identified mainly in male rats. In this study, we quantified the baroreflex dynamic characteristics in female rats using open-loop analysis, and compared them to male rats. Methods: In female and male Sprague-Dawley rats (n = 8 each), the bilateral carotid sinus regions were isolated from the systemic arterial system, and the aortic depressor nerves and vagal nerves were severed to establish an open-loop condition regarding the carotid sinus baroreflex. Carotid sinus pressure (CSP) was perturbed using Gaussian white noise with a mean of 120 mmHg and a standard deviation of 20 mmHg at a switching interval of 500 ms. The sympathetic nerve activity (SNA) was recorded from the left splanchnic sympathetic nerve and expressed in arbitrary units (AU). Transfer functions were calculated for the following input-output pairs: CSP to SNA (neural arc), SNA to AP (peripheral arc), and CSP to AP (total arc). Step responses were obtained from time integrals of the inverse Fourier transforms of the transfer functions. Results: The neural arc transfer function from CSP to SNA exhibited derivative characteristics with a high-cut feature in both female and male rats [the asymptotic dynamic gain: 0.93 ± 0.03 vs 0.98 ± 0.04 AU/mmHg in female and male, respectively (p = 0.438), the derivative corner frequency: 0.08 ± 0.01 vs 0.10 ± 0.01 Hz (p = 0.131), the high-cut corner frequency: 1.00 ± 0.14 vs 1.20 ± 0.12 Hz (p = 0.290), and the pure delay: 0.08 ± 0.01 vs 0.11 ± 0.01 s (p = 0.059)]. The peripheral arc transfer function from SNA to AP exhibited a second-order low-pass filter in both female and male rats [the asymptotic dynamic gain: 0.40 ± 0.08 vs 0.47 ± 0.06 mmHg/AU (p = 0.503), the natural frequency: 0.07 ± 0.01 vs 0.07 ± 0.01 Hz (p = 0.817), the damping ratio: 1.31 ± 0.09 vs 1.08 ± 0.09 (p = 0.088), and the pure delay: 0.41 ± 0.02 vs 0.43 ± 0.02 s (p = 0.498)]. The total arc transfer function from CSP to AP approximated a first-order low-pass filter [the asymptotic dynamic gain: −0.49 ± 0.10 vs −0.57 ± 0.06 mmHg/mmHg (p = 0.540), the cut-off frequency: 0.05 ± 0.01 vs 0.05 ± 0.01 Hz (p = 0.983), and the pure delay: 0.75 ± 0.03 vs 0.81 ± 0.04 s (p = 0.281)]. There were no significant differences in step response parameters between female and male rats. Conclusion: The fast neural arc compensates for the slow peripheral arc to accelerate the baroreflex-mediated AP regulation. These fundamental properties did not show significant differences between female and male rats. NTT Research, Inc. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.