Physiologic adaptations of the tubuloglomerular feedback system.

The maintenance of volume homeostasis is sufficiently important to mammalian terrestrial life that a large amount of evolutionary energy has been expended in the development of multiple control systems, each involved in regulating the volume and composition of internal body fluids. The kidney, which participates in most of these systems, has evolved physiologic attributes which enhance the efficiency of volume regulation. Perhaps the most fundamental of these attributes is a close coordination between the processes of glomerular filtration and tubular reabsorption. Such coordination is required to prevent the amplification of small fluctuations in glomerular filtration rate into large fluctuations in total body salt and water content. It was first suggested by Homer Smith that reabsorption of fluid from the nephron should increase as the delivery of tubular fluid into that segment increases [1]. When applied to the proximal tubule, this principle of flow-dependent transport has come to be referred to as "glomerulotubular balance" [2, 3]. Glomerulotubular balance depends upon intrinsic properties of the proximal nephron including the affinities and densities of various solute transporters and the differential permeabilities of the nephron to various solutes and water, and upon the trans-epithelial concentration gradients of these solutes [4–6]. By definition, glomerulotubular balance describes the functional dependence of tubular reabsorption on glomerular filtration rate independently of other neuro-humoral effectors of tubular transport. However, since glomerulotubular balance is a substrate-driven process, it cannot accomplish an increment in proximal tubular reabsorption which exceeds an increment in delivered load. Therefore, in the absence of effectors other than glomerulotubular balance the volume of fluid entering the distal nephron must be a monotonically increasing function of GFR [7]. How then, may the kidney avert an unintentional diuresis should the hemodynamic forces favoring glomerular filtration combine to overwhelm the reabsorptive capacity of the nephron? In 1937 Goormaghtigh suggested that the juxtaglomerular apparatus might participate in the maintenance of volume homeostasis by generating some sort of signal in response to changes in the composition of distal tubular fluid [8]. The peculiar anatomic arrangement of the nephron would facilitate transmission of this signal to the upstream glomerulus and lead to alterations in the physiologic determinants of glomerular filtration. This hypothesis has been refined over the past three decades as substantial experimental data have accrued to support the existence of an operational system of tubuloglomerular feedback (TGF) [9]. Contemporary models of the TGF system, by analogy to negative feedback-driven control systems in engineering control theory, divide the system into three component processes [10]. The first of these components is a parameter which the system is designed to regulate, in this case, the rate at which tubular fluid transits the late proximal nephron or V LP . The second component includes the macula densa and surrounding interstitium which serve to detect differences between the current value of V LP and some internal set-point, and translate this information into an output command. The third component, or effector limb, of the TGF system is constituted by the contractile glomerular mesangium and glomerular arterioles which respond to the aforementioned output command by altering nephron filtration rate (SNGFR) to keep V LP in line with the system's internal set-point. When TGF is allowed to function as a closed-loop system [7], as is the case in vivo, its presence is, by nature, undetectable. However, when late proximal flow is uncoupled from nephron filtration by artificial microperfusion of the late proximal tubule, a dependence of SNGFR on V LP can be defined [11]. This relationship is referred to as the "TGF function", or "gain" of the TGF system [7, 10]. This TGF function specifies a continuum of points in the V LP -SNGFR plane at which the nephron may operate. The actual operating point of the system exists at the point in this plane where the TGF and glomerulotubular balance functions intersect (Fig. 1). The TGF function may vary in response to the changing needs of the organism, both with regard to volume homeostasis and renal function. The altering of TGF under conditions of pregnancy, loss of renal mass, and a variety of other pathophysiologic conditions suggests that the juxtaglomerular apparatus is involved in events pertinent not merely to volume regulation but to overall renal growth and function.