From the Guidelines
In the nephron, glucose, sodium, and bicarbonate are primarily absorbed in the proximal convoluted tubule (PCT). The absorption of these substances is crucial for maintaining electrolyte balance and acid-base homeostasis in the body.
- Glucose reabsorption occurs through sodium-glucose cotransporters (SGLT1 and SGLT2) in the early PCT, with about 90% of filtered glucose being reabsorbed in this segment 1.
- Sodium reabsorption occurs throughout the nephron, but approximately 65-70% is reabsorbed in the PCT through various mechanisms including the Na+/H+ exchanger, Na+/K+ ATPase, and sodium-glucose cotransport.
- Bicarbonate reabsorption also predominantly occurs in the PCT (about 80-90%), where filtered bicarbonate combines with hydrogen ions secreted by the tubular cells to form carbonic acid, which then dissociates into carbon dioxide and water. The role of SGLT2 inhibitors in the treatment of Type 2 diabetes mellitus (T2DM) highlights the importance of the PCT in glucose and sodium reabsorption, as these inhibitors act on the SGLT2 proteins mainly expressed in the S1 and S2 segments of the proximal convoluted tubule in the kidneys 1. The segmented reabsorption pattern of glucose, sodium, and bicarbonate in the nephron allows for precise regulation of these substances in the blood.
From the Research
Nephron Absorption
The absorption of glucose, sodium, and bicarbonate in the nephron occurs in the following locations:
- Glucose: proximal convoluted tubule 2, 3
- Sodium: proximal tubule, including both proximal convoluted tubule and proximal straight tubule 4, 2, 3, 5
- Bicarbonate: proximal tubule, including both proximal convoluted tubule and proximal straight tubule 4, 6, 2, 3, 5
Mechanisms of Absorption
The mechanisms of absorption for these substances include:
- Sodium-coupled bicarbonate absorption from renal proximal tubules (PTs) 4
- Sodium-dependent cotransporters for glucose and other solutes 2, 3
- Na(+)-independent passive transporters for various other solutes 3
- Paracellular shunt pathway for passive movement of solutes 5
Regulation of Absorption
The regulation of absorption is influenced by various factors, including:
- Insulin, which may stimulate sodium transport in the proximal tubule 4
- Angiotensin II, which has biphasic effects on proximal tubule transport 4
- Acid-base balance, which affects the absorption of bicarbonate and other solutes 6, 3
- Starling forces, which influence the permeability and transport of solutes in the proximal tubule 5