Fig. 1

Schema of the arterial baroreflex system. The baroreflex system may be divided into neural arc and peripheral arc subsystems. The negative sign in the neural arc indicates signal inversion through the neural arc. Under a baroreflex closed-loop condition (a), a decrease in arterial pressure (AP) due to an exogenous perturbation (P_D) induces reflex activation of sympathetic nerve activity (SNA), which counteracts the effect of P_D. Under this condition, the AP reduction and the reflex sympathetic activation act synergistically to reduce urine output. However, changes in AP and SNA are not always reciprocal. When SNA increases via a central command, an AP elevation ensues. We refer to this latter situation as primary acute sympathetic activation (PASA). This situation may be mimicked under a baroreflex open-loop condition by imposing P_D on the isolated baroreceptor regions (b). During PASA, the kidneys may receive mutually opposing influences from the increased SNA (an anti-diuretic effect through renal innervation) and the increased AP (a diuretic effect through an increase in renal perfusion pressure). If urine output decreases during PASA, the urine output change can be interpreted as a cause for the AP elevation. Conversely, if urine output increases during PASA, the urine output change needs to be interpreted as an effect of the AP elevation