Proximal Tubular Function Prof K Sivapalan Proximal Convoluted

Proximal Tubular Function. Prof. K. Sivapalan.

Proximal Convoluted Tubules • 180 Liters of filtrate goes into the PCT in one day. • Substances needed for the body- glucose, amino acids etc. are reabsorbed. • Some waste products are secreted into the tubule. • Water is freely permeable and follows osmotic gradient. The tubular fluid is isosmotic. 9/8/2021 PCT 2

PCT • 15 mm long, 55 µm in diameter. • One layer of cells with tight junctions • The luminal surface has brush border - microvilli. 9/8/2021 PCT 3

Tubular Reabsorption • Endocytosis- some peptide hormones and small proteins. • 60 % of the filtered sodium is absorbed in PCT • There are several mechanisms that absorb sodium. • Glucose and amino acids are absorbed along with sodium in the proximal parts of the PCT • Water follows the solutes and the volume of fluid reduces accordingly. • Angeotensin II increases sodium and bicarbonate absorption in proximal tubule. 9/8/2021 PCT 4

Glucose, Amino Acid Absorption • Glucose and sodium bind to SGLT 2, glucose carrier protein in the luminal membrane. • Na+ is pumped out into interstitium by active transport. • This is secondary active transport • Sodium is absorbed indipendantly also. 9/8/2021 PCT 5

Transfer Maximum • If blood glucose is 90 mg/dl, amount filtered = 90 x 120 / 100 = 108 mg/min. • Maximum amount that can be absorbed by tubules is 375 mg / min for males and 300 mg in females [Tm. G]. • This works out to blood level of 300 mg/dl. • Glucose appears in urine when arterial glucose level is 200 mg/dl [= 180 mg/dl in venous blood] because all nephrons are not the same. • The renal threshold for glucose is the lowest blood level when glucose appears in urine. 9/8/2021 PCT 6

Tubular Secretion • Substances normally produced - ethereal sulfates, steroid and other glucuronides, 5 hydroxyindolacetic acid • Substances administered – derivatives of hippuric acid [PAH], phenol red, penicillin and a variety of iodinated dyes. 9/8/2021 PCT 7

Absorption of Substances in PCT • Concentration of various substances indicate the rate of absorption. • Regulation of K+- Most of the filtered K+ is absorbed and excreted in distal tubule. 9/8/2021 PCT 8