Mechanism Of Renal Transport Of Sodium & Glucose

Sodium transport via the Na-K-ATPase at the basolateral membrane. This is the only transport step for sodium in the proximal tubule that involves the direct input of energy to move sodium against its electrochemical gradient (in this case from the inside of the cell to the outside).

Na+/H+ exchanger (NHE3) is located in the luminal membrane of the proximal tubule epithelial cell.

This transport system allows Na+ to enter the cell from the tubular lumen in exchange for a proton (H+) from inside the cell. 

As in all portions of the nephron, Na+/K+-ATPase in the basolateral membrane pumps the reabsorbed Na+ into the interstitium so as to maintain a low intracellular Na+ concentration. 

The H+ secreted into the lumen combines with bicarbonate (HCO3–) to form H2CO3 (carbonic acid).

Ref: Ives H.E. (2012). Chapter 15. Diuretic Agents. In B.G. Katzung, S.B. Masters, A.J. Trevor (Eds), Basic & Clinical Pharmacology, 12e.

• Approximately 66% of filtered sodium ions (Na+), 85% of the NaHCO3, 65% of the K+, 60% of the water, and virtually all of the filtered glucose and amino acids are reabsorbed in the proximal tubule.
• About half of filtered urea is reabsorbed in the proximal tubule. 
• An amount equal to that reabsorbed is then secreted back into the loop of Henle. 
• Finally, about half is reabsorbed again in the medullary collecting duct. The net result is that about half the filtered load is excreted.

• Glucose, amino acids, and bicarbonate are reabsorbed along with Na+ in the early portion of the proximal tubule. 
• Glucose is typical of substances removed from the urine by secondary active transport. 
• It is filtered at a rate of approximately 100 mg/min (80 mg/dL of plasma x 125 mL/min). 
• Essentially all of the glucose is reabsorbed, and no more than a few milligrams appear in the urine per 24 h. 

Under all circumstances most of the filtrate is reabsorbed in the proximal tubule. Normally, this reabsorption amounts to about two-thirds of the sodium and water filtered. Normally 60% of Na+ is reabsorbed in the proximal tubule, primarily by Na-H exchange.

Ref: Ganong’s review of Medical Physiology, 23rd Ed, page 648-650

B i.e. Proximal tubule

Proximal convoluted tubule (early part) reabsorbs almost all of glucose, amino acids, protein, lactate & inorganic phosphate and most (65%) of bicorbonate,Na+,11)0, K+ and CI Q. PCT is also an important site for secretion of organic acids and bases such as oxalate, urate, bile salts, H+ and catecholamines, along with toxins, drugs eg penicillin, salicylates and PAH. So its’ a work horse of nephronQ.

B i.e. Aldosterone in collecting duct

Ans. is ‘a’ i.e., PCT