A functional role for the tubuloglomerular feedback mechanism.

There is increasing physiologic evidence that the tubuloglomerular feedback mechanism exerts an important, functional role in the renal control of volume homeostasis. At this juncture, the evidence is not complete but must be considered strongly circumstantial, derived from a variety of recent studies. The tubuloglomerular feedback system functions to prevent renal volume losses apparently through a fine regulatory mechanism that acts to preserve a constancy of tubular flow rates [1–3]. Flow rates appear to be controlled around a range of normal values for either the late proximal or early distal tubular flow rate by a combined vasoconstrictor-vasodilator capacity which influences nephron filtration rate [4]. Investigations now extend beyond the single nephron microperfusion studies conducted during the past several years which established the existence of a relationship between late proximal microperfusion rate, distal tubular delivery and tubular reabsorption and the resulting nephron filtration rate [4–7]. Several examples, potentially pertinent to clinical medicine, exist which provide reasonable evidence that activation or suppression of tubuloglomerular feedback mechanisms contribute significantly to alterations in normal renal physiology. It is doubtful whether most nephrologists and renal physiologists outside this specific field of investigation currently readily appreciate the impact of the tubuloglomerular feedback mechanism on normal and altered renal function. We will attempt to summarize some of the recent examples in which data support the hypothesis of a functional role for feedback mechanisms.