Renal Hemodynamic and Urinary Response to Baroreflex-Mediated Sympathetic Modulation: Insights from High-Resolution Arterial Impedance Analysis in Rats

This study investigated the short-term regulatory mechanisms of systemic and renal hemodynamics at the whole-organism level by independently controlling sympathetic nerve activity (SNA) and arterial pressure. In anesthetized male rats (N = 8), SNA was altered via carotid sinus pressure ( P _cs ) control, while mean arterial pressure was regulated at 100 mmHg using an arterial pressure control system (APC-On) or allowed to vary with SNA (APC-Off). Renal blood flow remained stable across P _cs levels or APC status. In contrast, urine flow decreased with increasing P _cs under APC-Off but increased under APC-On (32.2 ± 6.7 vs. 71.3 ± 3.7 µL·min ⁻¹ ·kg ⁻¹ at a P _cs of 140 mmHg, P = 0.012). A three-element Windkessel model was applied to describe systemic ( Z _sys ) and renal arterial impedance ( Z _renal ). Characteristic impedance ( R _c ) showed opposite trends in Z _sys and Z _renal with increasing P _cs under APC-Off, whereas such differential responses were not observed under APC-On. Distal resistance ( R _d ) decreased with increasing P _cs in both Z _sys and Z _renal under APC-Off, reflecting a vasodilatory effect of SNA suppression. Under APC-On, R _d in Z _sys showed a decreasing trend with increasing P _cs , whereas R _d in Z _renal increased, indicating that maintaining arterial pressure elicited a renal vasoconstrictive effect counteracting SNA suppression. Compliance ( C) in Z _sys remained relatively stable across P _cs under both APC conditions. In contrast, C in Z _renal markedly increased with Pcs reaching 230.0 ± 45.5%, under APC-Off, whereas it remained stable under APC-On. In conclusion, systemic vascular tone is primarily governed by SNA, whereas renal hemodynamics is stabilized by local autoregulatory adjustments.