Page 106 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Pathophysiology of Renal Transport Processes
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Genetic or toxic causes, drugs, or hormonal ab- Decreased activity of the Na /H exchanger
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normalities can impair tubular transport pro- (→ A7), of the Na -3HCO 3 – cotransporter
cesses. (→ A8), or inhibition of carbonic anhydrase
At least two luminal transporters are re- (CA) results in proximal-tubular acidosis
–
sponsible for the reabsorption of glucose in (→ p. 88ff.). As the reduced HCO 3 reabsorp-
the proximal tubules. A genetic defect of the tion in the proximal tubules cannot be com-
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renal and intestinal Na -glucose/galactose pensated by the (normally small) distal-tubu-
transporter (→ A1) results in glucose–galac- lar transport capacity, bicarbonate is excreted
Kidney, Salt and Water Balance port rate (type A) or the affinity (type B) is im- tubules can reabsorb the bulk of filtered bicar-
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in the urine even when the HCO 3 load is nor-
tose malabsorption. A defect of the second re-
nal glucose transporter leads to classical renal
mal (→ E2). Nevertheless, if the plasma con-
–
centration of HCO 3 is reduced, the proximal
glycosuria in which either the maximal trans-
paired (→ D3). If in type A plasma concentra-
bonate, and the distal tubules will then pro-
tion exceeds the lowered renal threshold, this
duce urine of normal acidity.
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results in the quantitative excretion of the ad-
cotransport is largely depen-
Na -3HCO 3
dent on the membrane potential, and thus on
ditionally filtered glucose; if the plasma con-
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centration is below the renal threshold, all of
K flux via K channels (→ A15), and on extra-
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renal glycosuria, glucose is excreted even at
larizes the cell membrane and inhibits HCO 3
low plasma concentrations.
reabsorption in the proximal tubules, while
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The Na -phosphate cotransporter (→ A2)
HCO 3 reabsorption is increased by hypokale-
5 the glucose is reabsorbed. However, in type B cellular K concentration. Hyperkalemia depo- –
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can be impaired if there is a genetic defect (re- mia. The renal excretion of H and thus the
nal phosphate diabetes) or a deficiency of cal- acid–base metabolism is thus a function of
citriol. The reduced renal phosphate reabsorp- the extracellular K + concentration (→ p. 86ff.).
tion causes demineralization of bone via a defi- Dehydration stimulates the activity of the
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ciency of phosphate (rickets; → p.132). Raised Na /H exchanger (→ A7) and thus proximal
–
renal phosphate reabsorption in PTH deficien- tubular HCO 3 reabsorption. This results in a
cy (hypoparathyroidism) or abnormal PTH ac- volume depletion alkalosis. Inhibition of the
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tion (pseudohypoparathyroidism), for example, Na /H exchanger or of carbonic anhydrase in-
leads to hyperphosphatemia (→ p.130). creases salt excretion (natriuresis). The inhibi-
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A defect of Na cotransport of certain neu- tion of proximal tubular Na reabsorption is,
tral amino acids (→ A3) in kidney and gut re- however, largely compensated by its increased
sults in Hartnup disease, in which increased reabsorption in more distal nephron segments,
amino acid excretion occurs. As tryptophan is especially in the loop of Henle.
necessary for nicotinic acid synthesis, nicotinic In Fanconi’s syndrome, caused by genetic or
acid deficiency, and thus damage to the ner- acquired (e.g., lead poisoning) factors, several
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vous system and the skin may occur. Na -coupled transport processes are impaired
A defect of the amino acid exchanger for (→ A1–7), resulting in glycosuria, aminoacid-
neutral and dibasic amino acids (→ A4) in- uria, phosphaturia, proximal tubular acidosis,
creases the excretion of ornithine, lysine, argi- and hypokalemia (see below).
nine, and cystine (cystinuria). The poorly solu- Increased proximal Na + and water reab-
ble cystine is precipitated and forms urinary sorption concentrates the luminal uric acid
stones (→ p.120). In familial protein intoler- and thus promotes uric acid reabsorption via
ance the reabsorption of dibasic amino acids luminal and basolateral anion exchangers and
is abnormal. channels (→ A9). This causes hyperuricemia
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A defect of the Na cotransporter for acidic with deposition of poorly soluble uric acid in
amino acids (→ A5) leads to harmless acid some joints (gout; → p. 250).
aminoaciduria; a defect of the carrier for cyclic If an energy deficiency occurs (e.g., inade-
96 amino acids such as proline, results in harm- quate perfusion) Na + /K + -ATPase (→ ABC10) is
less iminoglycinuria (→ A6). impaired, electrolyte reabsorption is reduced
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Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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