Figure 19

Tubuloglomerular feedback (TGF) and myogenic mechanisms. Two mechanisms are responsible for efficient renal autoregulation: the TGF and myogenic mechanisms. The TGF mechanism is explained here. A, Increases in distal tubular flow past the macula densa generate signals from the macula densa cells to the afferent arterioles to elicit vasoconstriction, whereas decreases in flow cause afferent vasodilation [16,18,19]. Blocking flow to the distal tubule or interrupting the feedback loop attenuates the autoregulatory efficiency of the glomerular filtration rate (GFR), glomerular pressure, and renal blood flow. B, Individual tubules can be blocked and perfused downstream, while collections are made or pressure measured in an early tubular segment. C, When the tubule is perfused at increased flows, the glomerular pressure and GFR of that nephron decrease. The shaded area in the normal relationship represents the normal operating level of the TGF mechanism. This mechanism helps stabilize the filtered load and the solute and sodium load to the distal nephron segment. The responsiveness of the TGF mechanism is modulated by changes in sodium intake and in extracellular fluid (ECF) volume status. At high sodium intake and ECF volume expansion the sensitivity of the TGF mechanism is low, thus allowing greater spillover of salt to the distal nephron. During low sodium intake and other conditions associated with ECF volume contraction, the sensitivity of the TGF mechanism is markedly increased to minimize spillover into the distal nephron and maximize sodium retention. The hormonal and paracrine mechanisms responsible for regulating TGF sensitivity are discussed subsequently.

The myogenic mechanism is intrinsic to the vessel wall and responds to changes in wall tension to regulate vascular smooth muscle tone. Preglomerular arteries and afferent arterioles but not efferent arterioles exhibit myogenic responses to changes in wall tension [16,20]. The residual autoregulatory capacity that exists during blockade of the tubuloglomerular feedback mechanism indicates that the myogenic mechanism contributes about half to the autoregulatory efficiency of the renal vasculature. (Figure adapted from Navar [3].)

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