Question

define renal threshold and glucose transport maximum and state how they would be affected if there are more glucose carriers/transporters in the nephron

Answer 1

Renal threshold: Renal threshold is the concentration above which the substance appears in urine. Below the renal threshold, the substance can be filtered and reabsorbed by the kidneys. Glucose In blood above the renal threshold cannot be filtered and reabsorbed and hence, will appear in urine. This is seen in diabetes mellitus.

Glucose Transport maxima (Tm): Glucose transport maxima is the maximum capacity of the sodium glucose transporter (SGLUT) transporters to transport glucose from filtrate into blood. This value in humans is 375 mg/min. Above the glucose transport maxima; glucose is not reabsorbed but is excreted in urine. SGLUT 2 is the glucose transporter present in the apical membrane in proximal convoluted tubule S1 segment and transport glucose from filtrate to the epithelial cytoplasm. GLUT2 is present on basolateral membrane of these epithelial cells and transport glucose into the blood. SGLUT 1 is present in apical surface in S3 segment of proximal tubule while GLUT1 is present in basolateral membrane of S3 segment. All these glucose transporters help in reabsorption of glucose from urine.

Higher the number of SGLUTs and GLUTs in the kidneys, higher will be the reabsorption capacity for glucose. Hence, the renal threshold as well as glucose transport maxima should increase. The increased transport of glucose across the epithelium into blood in proximal tubules will cause more reabsorption of glucose (beyond 375 mg/min). Hence, more glucose is filtered out. Therefore there is less likelihood of glucose appearing in urine. Hence, renal threshold will increase.

Glucose transport maxima will also depend upon the protein expression for SGLUTs and GLUTs. If there are more glucose transporters present, Tm will increase as there are more transporters available for reabsorption. In case of diabetics, this will increase glucose levels in blood. It is known that inhibiting SGLUTs will cause glucose to appear in urine.