Question

Fanconi syndrome can be diagnosed with acidosis (high blood acid levels) and frequent urination with glycosuria (high urine glucose levels) but with normal blood glucose levels. It can because by a number of things, but one way is Proximal renal tubular acidosis. In this case, normal reabsorption in the proximal convoluted tubule (PCT) is impaired. ​

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a. Explain the normal way bicarbonate and glucose get reabsorbed in the PCT from ​lumen to blood, including transport proteins if applicable.

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b. Explain how acidosis would occur and why glycosuria might cause frequent urination.

Answer 1

Ans:-(a)Reabsorption of glucose by the renal tubule is an example of secondary active transport.

secondary active transport of glucose and amino acids in the proximal tubule. In both instances, specific carrier proteins in the brush border combine with a sodium ion and an amino acid or a glucose molecule at the same time. These transport mechanisms are so efficient that they remove virtually all the glucose and amino acids from the tubular lumen. After entry into the cell, glucose and amino acids exit across the basolateral membranes by diffusion, driven by the high glucose and amino acid concentrations in the cell facilitated by specific transport proteins.

Sodium glucose co-transporters (SGLT2 and SGLT1) are located on the brush border of proximal tubular cells and carry glucose into the cell cytoplasm against a concentration gradient,

glucose diffuses out of the cell into the interstitial spaces with the help of glucose transporters—GLUT2 in the S1 segment and GLUT1 in the latter part (S3 segment) of the proximal tubule.

the secretion of hydrogen ions into the tubular lumen is an important step in the removal of bicarbonate ions from the tubule (by combining H+ with the HCO3− to form H2CO3, which then dissociates into H2O and CO2).

(b) the mechanism by which the kidneys secrete H+ and reabsorb HCO3−, we can explain how the kidneys readjust the pH of the extracelullar fluid when it becomes abnormal, we can see that acidosis occurs when the ratio of HCO3− to CO2 in the extracellular fluid decreases, thereby decreasing pH. If this ratio decreases because of a fall in HCO3−, the acidosis is referred to as metabolic acidosis. If the pH falls because of an increase in PCO2, the acidosis is referred to as respiratory acidosis.

Renal Glycosuria—Failure of the Kidneys to Reabsorb Glucose. In renal glycosuria, the blood glucose concentration may be normal, but the transport mechanism for tubular reabsorption of glucose is greatly limited or absent, due to which excretion of large amount of glucose, 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 the diagnosis of diabetes mellitus.