DYNAMIC, SHORT-TERM COUPLING BETWEEN CHANGES IN ARTERIAL-PRESSURE ANDURINE FLOW

Pressure diuresis refers to the direct effect of arterial pressure (AP ) on the rate of urine flow (UF). On the basis of computer modeling, p ressure diuresis has been viewed as a long-term mechanism that acts to set the level of the blood volume and, thus, the steady-state AP. The re are no systematic studies, however, on the rapidity with which chan ges in AP induce changes in UF in vivo. Therefore, we measured the del ay between induced changes in AP and the subsequent change in UF. Nine anesthetized rats were instrumented with arterial, venous, and ureter al catheters. AP and UF were measured every 2 s, while acute changes i n AP were induced by 1) occlusion of the aorta above or below the rena l vessels; 2) brief tail pinch; or 3) intravenous administration of ac etylcholine (1 mug), phenylephrine (1 mug), or angiotensin II (0.1 mug ). The rapidity of the urinary response to induced changes in AP was d etermined by calculating the delay between a significant change in AP (+/-2 SD from baseline) and a significant change in UF. The delay aver aged 6.0 +/- 0.5 s for all conditions. Also, examining the relationshi p between the magnitude of the induced changes in AP and the magnitude of the responses in UF revealed an exponential influence of AP on UF. That is, there were proportionately larger changes in UF compared wit h AP (less-than-or-equal-to 10 times greater magnitude) in response to the experimental interventions. These results demonstrate that UF can be influenced by moment-to-moment changes in AP and suggest that pres sure diuresis may operate at a high frequency and with great efficienc y to effect the regulation of blood volume and AP.