Ganong's Review of Medical Physiology, 23rd Edition

650 SECTION VIII Renal Physiology

GLUCOSE REABSORPTION

Glucose, amino acids, and bicarbonate are reabsorbed along
with Na+ in the early portion of the proximal tubule (Figure
38–9). Farther along the tubule, Na+ is reabsorbed with Cl–.
Glucose is typical of substances removed from the urine by
secondary active transport. It is filtered at a rate of approxi-
mately 100 mg/min (80 mg/dL of plasma × 125 mL/min). Es-
sentially all of the glucose is reabsorbed, and no more than a
few milligrams appear in the urine per 24 h. The amount reab-
sorbed is proportional to the amount filtered and hence to the
plasma glucose level (PG) times the GFR up to the transport
maximum (TmG). When the TmG is exceeded, the amount of
glucose in the urine rises (Figure 38–10). The TmG is about
375 mg/min in men and 300 mg/min in women.
The renal threshold for glucose is the plasma level at which
the glucose first appears in the urine in more than the normal
minute amounts. One would predict that the renal threshold
would be about 300 mg/dL, that is, 375 mg/min (TmG)
divided by 125 mL/min (GFR). However, the actual renal
threshold is about 200 mg/dL of arterial plasma, which corre-
sponds to a venous level of about 180 mg/dL. Figure 38–10
shows why the actual renal threshold is less than the predicted
threshold. The "ideal" curve shown in this diagram would be
obtained if the TmG in all the tubules was identical and if all
the glucose were removed from each tubule when the amount
filtered was below the TmG. This is not the case, and in
humans, for example, the actual curve is rounded and devi-

ates considerably from the “ideal” curve. This deviation is

called splay. The magnitude of the splay is inversely propor-

tionate to the avidity with which the transport mechanism

binds the substance it transports.

GLUCOSE TRANSPORT MECHANISM

Glucose reabsorption in the kidneys is similar to glucose reab-

sorption in the intestine (see Chapter 27). Glucose and Na+

bind to the sodium-dependent glucose transporter (SGLT) 2 in

the apical membrane, and glucose is carried into the cell as Na+

moves down its electrical and chemical gradient. The Na+ is

then pumped out of the cell into the interstitium, and the glu-

cose is transported by glucose transporter (GLUT) 2 into the in-

terstitial fluid. At least in the rat, there is some transport by

SGLT 1 and GLUT 1 as well.

SGLT 2 specifically binds the d isomer of glucose, and the

rate of transport of d-glucose is many times greater than that

of l-glucose. Glucose transport in the kidneys is inhibited, as

it is in the intestine, by the plant glucoside phlorhizin, which

competes with d-glucose for binding to the carrier.

ADDITIONAL EXAMPLES OF

SECONDARY ACTIVE TRANSPORT

Like glucose reabsorption, amino acid reabsorption is most

marked in the early portion of the proximal convoluted tu-

bule. Absorption in this location resembles absorption in the

intestine (see Chapter 27). The main carriers in the apical

membrane cotransport Na+, whereas the carriers in the baso-

lateral membranes are not Na+-dependent. Na+ is pumped

FIGURE 38–9 Reabsorption of various solutes in the
proximal tubule. TF/P, tubular fluid:plasma concentration ratio.
(Courtesy of FC Rector Jr.)

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0 25 30 75 100

Glucose

Amino

acids

% Proximal tubule length

HCO 3 −

osm

Inulin

Cl−

K+ Na+

TFP

FIGURE 38–10 Top: Relation between the plasma level (P) and

excretion (UV) of glucose and inulin. Bottom: Relation between the

plasma glucose level (PG) and amount of glucose reabsorbed (TG).

UV

.

Inulin

Glucose

P

Splay

"Ideal"

Actual

Plasma glucose (PG)

TmG

Glucose reabsorbed (T

)G