DYNAMIC, SHORT-TERM COUPLING BETWEEN CHANGES IN ARTERIAL-PRESSURE AND URINE 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, pressure 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. There are no systematic studies, however, on the rapidity with which changes in AP induce changes in UF in vivo. Therefore, we measured the delay between induced changes in AP and the subsequent change in UF. Nine anesthetized rats were instrumented with arterial, venous, and ureteral catheters. AP and UF were measured every 2 s, while acute changes in AP were induced by 1) occlusion of the aorta above or below the renal vessels; 2) brief tail pinch; or 3) intravenous administration of acetylcholine (1 mug), phenylephrine (1 mug), or angiotensin II (0.1 mug). The rapidity of the urinary response to induced changes in AP was determined by calculating the delay between a significant change in AP (+/-2 SD from baseline) and a significant change in UF. The delay averaged 6.0 +/- 0.5 s for all conditions. Also, examining the relationship 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 with 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 pressure diuresis may operate at a high frequency and with great efficiency to effect the regulation of blood volume and AP.