B. Nafz et al., BLOOD-PRESSURE VARIABILITY AND URINE FLOW IN THE CONSCIOUS DOG, American journal of physiology. Renal, fluid and electrolyte physiology, 43(4), 1998, pp. 680-686
Pressure-dependent urine production is considered to be a major factor
in long-term blood pressure control. The phenomenon has been well cha
racterized for fixed levels of renal perfusion pressure (RPP), but the
influence of physiological fluctuations in RPP and spontaneous variat
ions in renal blood now (RBF) on short-term urine flow (UV) remain unc
lear. To clarify this issue, we studied the interdependence of RPP, RB
F, and UV in 13 conscious foxhounds during a single-step pressure redu
ction, under normal conditions, and with induced pressure changes. Red
ucing RPP in a single step to similar to 80 mmHg revealed short respon
se times of RBF (0.4 +/- 0.1 s, n = 7) as well as of UV (8.1 +/- 0.8 s
, n = 7). Under control conditions, UV was coupled with spontaneous va
riations of RBF (r = 0.94, P < 0.001), in contrast to RPP, which showe
d no significant correlation with UV (r = 0.09, P = NS). To discern th
e pressure and blood flow dependency of UV at a reduced RPP, we induce
d 0.9-mHz blood pressure oscillations (80 +/- 10 mmHg), which phase sh
ifted RPP and RBF. Conversely, under these conditions, UV was dependen
t on RPP (r = 0.95, P < 0.001). These results suggest that spontaneous
fluctuations in RBF around a normal baseline level lead to concomitan
t changes in urine production, in contrast to physiological short-term
oscillations in RPP, which are not correlated to changes in UV. Howev
er during induced oscillations of perfusion pressure, the blood flow d
ependence was no longer observed and UV was entirely pressure dependen
t.