Specific Tubular Disorders
We point out that several mechanisms are responsible for transporting different individual sub-stances across the tubular epithelial membranes. We also point out that each cellular enzyme and each carrier protein is formed in response to a respective gene in the nucleus. If any required gene happens to be absent or abnormal, the tubules may be deficient in one of the appropriate carrier proteins or one of the enzymes needed for solute transport by the renal tubular epithelial cells. For this reason, many hereditary tubular disorders occur because of the trans-port of individual substances or groups of substances through the tubular membrane. In addition, damage to the tubular epithelial membrane by toxins or ischemia can cause important renal tubular disorders.
Renal Glycosuria—Failure of the Kidneys to Reabsorb Glucose. In this condition, the blood glucose concentration may be normal, but the transport mechanism for tubular reabsorption of glucose is greatly limited or absent. Consequently, despite a normal blood glucose level, large amounts of glucose pass into the urine each day. Because diabetes mellitus is also associated with the presence of glucose in the urine, renal glycosuria, which is a relatively benign condition, must be ruled out before making a diagnosis of diabetes mellitus.
Aminoaciduria—Failure of the Kidneys to Reabsorb Amino Acids. Some amino acids share mutual transport systems for reabsorption, whereas other amino acids have their own distinct transport systems. Rarely, a condition called generalized aminoaciduria results from deficient reab-sorption of all amino acids; more frequently, deficiencies of specific carrier systems may result in (1) essentialcystinuria, in which large amounts of cystine fail to bereabsorbed and often crystallize in the urine to form renal stones; (2) simple glycinuria, in which glycine fails to be reabsorbed; or (3) beta-aminoisobutyricaciduria, which occurs in about 5 per cent of all people but appar-ently has no major clinical significance.
Renal Hypophosphatemia—Failure of the Kidneys to Reabsorb Phosphate. In renal hypophosphatemia, the renaltubules fail to reabsorb large enough quantities of phos-phate ions when the phosphate concentration of the body fluids falls very low. This condition usually does not cause serious immediate abnormalities, because the phosphate concentration of the extracellular fluid can vary widely without causing major cellular dys-function. Over a long period, a low phosphate level causes diminished calcification of the bones, causing the person to develop rickets. This type of rickets is refrac-tory to vitamin D therapy, in contrast to the rapid response of the usual type of rickets.
Renal Tubular Acidosis—Failure of the Tubules to Secrete Hydro- gen Ions. In this condition, the renal tubules are unableto secrete adequate amounts of hydrogen ions. As a result, large amounts of sodium bicarbonate are contin-ually lost in the urine. This causes a continued state of metabolic acidosis. This type of renal abnormality can be caused by hereditary dis-orders, or it can occur as a result of widespread injury to the renal tubules.
Nephrogenic Diabetes Insipidus—Failure of the Kidneys to Respond to Antidiuretic Hormone. Occasionally, the renaltubules do not respond to antidiuretic hormone, causing large quantities of dilute urine to be excreted. As long as the person is supplied with plenty of water, this con-dition seldom causes severe difficulty. However, when adequate quantities of water are not available, the person rapidly becomes dehydrated.
Fanconi’s Syndrome—A Generalized Reabsorptive Defect of the Renal Tubules. Fanconi’s syndrome is usually associatedwith increased urinary excretion of virtually all amino acids, glucose, and phosphate. In severe cases, other manifestations are also observed, such as (1) failure to reabsorb sodium bicarbonate, which results in metabolic acidosis; (2) increased excretion of potassium and sometimes calcium; and (3) nephrogenic diabetes insipidus.
There are multiple causes of Fanconi’s syndrome, which results from a generalized inability of the renal tubular cells to transport various substances. Some of these causes include (1) hereditary defects in cell trans-port mechanisms, (2) toxins or drugs that injure the renal tubular epithelial cells, and (3) injury to the renal tubular cells as a result of ischemia. The proximal tubular cells are especially affected in Fanconi’s syn-drome caused by tubular injury, because these cells reabsorb and secrete many of the drugs and toxins that can cause damage.