Blood pressure oscillations: Is there an independent antihypertensive effect?

Independent Antihypertensive Effect? To the Editor: In a recent issue of Circulation, Nafz and colleagues1 described the antihypertensive effects of oscillations of renal perfusion pressure (RPP) in conscious dogs. One of the key findings in that article was that plasma renin activity increased in response to a decrease in the mean RPP to 85 mm Hg; this increase was significantly reduced by 0.1-Hz oscillations in RPP around a similar mean level. In their discussion, the authors write that the attenuated plasma renin activity during the RPP oscillations is a “surprise” and attribute this result to the oscillatory pattern of RPP. We propose an alternative explanation to the authors’ findings. Oscillations in RPP with a mean of 85 mm Hg and an amplitude of 10 mm Hg, as performed in the study, cause RPP to be .90 mm Hg (the threshold level for stimulating renin release2) '33% of the time. Thus, the stimulus for renin secretion is only present for 66% of the time in the oscillation experiment compared with the decrease in the mean RPP to 85 mm Hg; therefore, the oscillations result in a diminished renin release. The authors’ assumption that a low RPP during the trough of oscillations (RPP of 75 mm Hg) strongly stimulates renin release, but RPP during peak oscillations (RPP of 95 mm Hg) does not reduce it to the same extent, would be an argument against our simplistic explanation. Although this assumption is correct in the steady-state, one should also consider the time course of the response of renin secretion to hemodynamic changes. Simchon and Chien3 demonstrated that the time constant of the response of renin secretion to changes in renal blood flow in dogs is '80 s, which is .10 times as long as the duration of RPP,90 mm Hg in the oscillation experiment ('7 s). Thus, the trough of oscillations would cause a much smaller increase in renin release compared with the constant low level of RPP. Keeping that in mind, one would expect the plasma renin activity levels in the oscillation experiment to be between the control levels and the levels found at a mean RPP of 85 mm Hg, as was observed in the study.