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. 1/16/2019 PCT

PCT 15 mm long, 55 µm in diameter. One layer of cells with tight junctions The luminal surface has brush border - microvilli. 1/16/2019 PCT

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. 1/16/2019 PCT

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. 1/16/2019 PCT

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 [TmG]. 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. 1/16/2019 PCT

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. 1/16/2019 PCT

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. 1/16/2019 PCT