BLOOD-PRESSURE VARIABILITY AND URINE FLOW IN THE CONSCIOUS DOG

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.