Dynamic, short-term coupling between changes in arterial pressure and urine flow

Abstract

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 microgram), phenylephrine (1 microgram), or angiotensin II (0.1 microgram). 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 (< or = 10 times greater magnitude) in response to the experimental interventions.(ABSTRACT TRUNCATED AT 250 WORDS)

Moment-to-Moment Characteristics of the Relationship Between Arterial Pressure and Renal Interstitial Hydrostatic Pressure

“…That is, moment-to-moment changes in RIHP may impact on urine formation, and the cumulative effect of these acute changes over time may account for the long-term aspect of pressure-natriuresis by inducing homeostatically appropriate changes in blood volume and thereby returning arterial pressure toward its operating point. 8,18 Similar to previous studies using conventional pressurenatriuresis methodology, 9 -17 the overall acute RAP-RIHP relationship was found to correlate linearly and to have a similar slope over a wide range of changes in RAP. However, despite the numerous studies describing the RAP-RIHP relationship as linear, none assessed whether there are differential regulations above and below the arterial pressure operating point.…”

“…These results are consistent with a previous study assessing the onset of changes in urine flow, a down- stream step from RIHP in the pressure-natriuresis mechanism, where a 6-second delay after various arterial pressure changes was documented. 18 Furthermore, the time difference between achieving a steady-state RAP and RIHP was found to depend, in part, on the magnitude of arterial pressure change ranging from 6 to 12 seconds for pressure changes of Ϯ5 to 30 mm Hg. These findings suggest that the consequences of the oscillations in RIHP (ie, on sodium excretion) after spontaneous changes in arterial pressure could occur at a frequency of 5 to 10 changes per minute.…”

“…Previously, it was shown that acute increases in arterial pressure above the operating point are dealt with more efficiently than decreases with respect to urine flow on a moment-to-moment basis. 18 Because RIHP is an intermediate step to urine flow, it may be hypothesized that small, moment-to-moment increases in RIHP produce greater effects on sodium excretion than do decreases. This concept and its teleological significance will need to be determined in several models.…”

“…8 This same group also found that urine flow, a variable downstream from RIHP, responded to changes in arterial pressure within Ϸ6 seconds. 18 Because changes in RIHP are an intermediate step in pressure-natriuresis, there must be a shorter delay between arterial pressure and RIHP; however, the exact time course is yet to be elucidated.…”

“…In fact, Steele et al 18 suggest that acute increases above the arterial pressure operating point are dealt with more effectively than decreases with respect to urine flow. Whether such a differential response to pressor and depressor stimuli exists with regard to RIHP has yet to be determined.…”