Characteristics of glomerulotubular balance

Abstract

Glomerulotubular balance (GTB) is defined as the ability of each successive segment of the proximal tubule to reabsorb a constant fraction of glomerular filtrate and solutes delivered to it. For maintenance of GTB the coupling of peritubular blood flow and intratubular fluid flow to the process of glomerular filtration seems to be of functional importance since tubular fluid reabsorption is significantly altered when either or both parameters are experimentally changed. In the case of peritubular blood flow, variations of tubular fluid reabsorption have been ascribed to variations of the mean net colloid osmotic pressure in the peritubular blood and its effects on the paracellular backleak of tubular resorbate. This relationship has, however, been clearly demonstrated only in volume expansion, when GTB is impaired. Under nondiuretic conditions, in which GTB typically occurs, the importance of the peritubular colloid osmotic pressure in control of tubular fluid reabsorption is less clear since variations of peritubular colloid osmotic pressure within a physiologic range exert only a negligible influence on tubular fluid transport. Pharmacologically induced alterations of peritubular hemodynamics or mean net colloid osmotic pressure can affect tubular fluid reabsorption without consistently altering the net interstitial pressure. In the case of intratubular flow rate, variations of tubular fluid reabsorption are comparable to changes seen with GTB. Such an interrelation is found only in tubules perfused by natural tubular fluid. In individual nephrons flow-dependent reabsorption cannot always be observed even when it appears that GTB is preserved in that kidney. Flow dependency of tubular fluid reabsorption might be attributed to some properties or constituents in tubular fluid rather than to some intrinsic characteristics of the tubular epithelium. Because flow dependence and tubular fluid transport are homogeneous along the tubule, fluid reabsorption might be controlled by a mechanism akin to a flow reactor. As yet it is not possible to explain GTB exclusively by peritubular or luminal control alone.

Glomerular and tubular adaptive responses to acute nephron loss in the rat. Effect of prostaglandin synthesis inhibition.

“…This documented ability of the proximal tubule segments to adjust their transepithelial fluid transport in direct proportion to variations in SNGFR supports the contention (25) that glomerulotubular balance is closely maintained in remnant nephrons. Although in general the mechanisms governing glomerulotubular balance remain debatable, based on considerable investigative efforts it appears that peritubular (i.e., Starling forces) and luminal (i.e., intratubular volume flow and solute load) processes, which are in themselves usually linked to the filtered load, exert controlling influences on the maintenance ofglomerulotubular balance (26,27). Through the analysis ofthe relationship between APR and the reabsorptive determinants operating in the peritubular capillary environment of remnant nephrons, it is evident that LpAr could not contribute to increases in peritubular capillary fluid uptake.…”

The effect of intraluminal flow rate on glomerulotubular balance in the proximal tubule of the rat kidney

“…However, the behaviour of the absolute rate of reabsorption in the two groups was opposite: it fell in parallel to SNGFR and inversely to PR in the sixty responders, while it rose slightly, albeit significantly, independent of SNGFR and parallel to PR in the non-responders. DISCUSSION Glomerulotubular balance represents the capacity of proximal volume absorption to follow closely the changes in filtration, so that the fraction of filtrate reclaimed proximally remains relatively constant (Bartoli, 1981;Haberle & von Baeyer, 1983). This property is important since it avoids an 'all-or-none' response to changes in filtration, that would end in anuria when GFR fell below the normal reabsorption rate.…”

Proximal reabsorption with changing tubular fluid inflow in rat nephrons

“…Thus, these data indicate a peritubular mediation of GTB. In contrast, the perfusion of proximal tubular segments in vivo with plasma ultrafiltrate and with native tubular fluid (Haberle et al 1981;Haberle & Von Baeyer, 1983;Tucker & Blantz, 1978) suggested an additional mechanism for modulating proximal reabsorption by intraluminal flow rate per se. These studies were considered necessary after it was demonstrated that in vivo perfusion with artificial perfusates may depress reabsorption .…”