Renal blood flow regulation and arterial pressure fluctuations: a case study in nonlinear dynamics

The arterial blood pressure, a physiological variable on which all renal excretory processes depend, fluctuates over a wide range of amplitudes and frequencies. Much of this variation originates in nonrenal vascular beds to support nonrenal tasks, and the fluctuations provide a noisy environment in which the kidney is obliged to operate. Were it not for renal blood flow autoregulation, it would be difficult to regulate renal excretory processes so as to maintain whole body variables within narrow bounds. Autoregulation is the noise filter on which other renal processes depend for maintaining a relatively noise-free environment in which to work. Because of the time-varying nature of the blood pressure, we have concentrated in this review on the now substantial body of work on the dynamics of renal blood flow regulation and the underlying mechanisms. Renal vascular control mechanisms are not simply reactive but have their own spontaneous dynamics. Both TGF and the myogenic mechanism oscillate autonomously. The TGF oscillation is the better understood of the two. There is an oscillation of tubular pressure, proximal tubular flow, early distal Cl- concentration, and efferent arteriolar blood flow at approximately 35 mHz; all these variables are synchronized when the measurements are made in a single tubule. The autonomous nature of the oscillation is supported by simulations of the nephron and its vasculature, which show that for a reasonable representation of the dynamics of these structures and of the parameters that govern their behavior, the solutions of the equation set are periodic at the frequency of the observed oscillation, and with the same phase relationships among its variables. The simulations also show that the critical variables for the development of the oscillation are the open-loop gain of the feedback system, and the various delays in the system of which convective transport in the axis of the thick ascending limb and signal transmission between the macula densa and the afferent arteriole are the most important. The oscillation in TGF is an example of nonlinear dynamical behavior and is yet another in a long list of oscillations and related dynamics arising in the inherently nonlinear properties of living systems. Some nonlinear systems can bifurcate to states known collectively as deterministic chaos, and TGF is a clear example of such a system. Rats with two different and unrelated forms of experimental hypertension provide tubular pressure records that pass statistical tests for ordered structure and sensitive dependence on initial conditions in the reconstructed state space, two of the hallmarks of deterministic chaos. These records also pass recent more stringent tests for chaos. The significance of deterministic chaos in the context of renal blood flow regulation is that the system regulating blood flow undergoes a physical change to a different dynamical state, and because the change is deterministic, there is every expectation that the critical change will yield itself to experimental discovery.(ABSTRACT TRUNCATED AT 400 WORDS)