The working mechanism of the distal convolute tubule is different from other parts of the nephron. DCT is mainly divided into two parts based on their functions as early distal convoluted tubule and late distal convoluted tubule. They are as follows:
Early distal convoluted tubule:
The early DCT is impermeable to water and only helps in the transport of ions like sodium, calcium, and chloride. Macula densa is present in the initial region of DCT and helps in tubuloglomerular feedback. This feedback is responsible for controlling the glomerular filtration rate and blood flow within a nephron. The Na+- K+ ATPase transporter is present in the membrane of the cells of this region and secretes Na+ out of the cell and K+ are allowed inside the cell. Thus, this channel helps in the development of a gradient that favors the intracellular movement of Na+ through other channels. The functioning of the Na+- K+ ATPase transporter is energy driven and needs ATP. This region has a sodium chloride cotransporter also that allow the inward movement of Na+ and Cl- and excess Cl- leave the cellular matrix by Cl- uniporter. The basolateral membrane of these cells also contains a sodium-calcium antiporter (NCX channel) that helps in the movement of Ca++ out of the cell and Na+ is allowed inside the cell. The calcium ion uniporter allows the inward movement of Ca++ from the lumen of the tubule inside the cell.
Late distal convoluted tubule:
The Late distal convoluted tubule has two types of cells that have different functions and help in the reabsorption-secretion process of nephrons. Principal Cells: These cells are the major site for Na+, K+, and water transport. These cells make the majority of tubular cells but unlike other cells of this system, they lack an apical cotransport system for Na+. The basic function of these cells is the uptake of Na+ and pumping out K+ and this process leads to the development of a 10-50mV lumen negative electric potential. The basolateral membrane of these cells contains Na+- K+ ATPase that helps in this exchange of ions. The ionic exchange generates an ionic gradient named epithelial Na+ channel that also supports Na+ uptake. The Na+- K+ ATPase results in the accumulation of K+ inside the cell that is extruded by K+ uniporter present on the membrane into the tubular lumen. Along with the absorption of Na+ and secretion of K+, these cells are also involved in water channel regulation.
Intercalated cells work based on an acid-base control mechanism where they help in controlling the ionic concentration of Hydrogen ions (H+) and bicarbonate ions (HCO3-).
Type A intercalated cell does two major functions:
Secretion of H+ in the lumen of tubular cells with the help of hydrogen-ATPase and H+ / K+ ATPase transporter.
Reabsorption of HCO3-.
The formation of bicarbonate is done by carbonic anhydrase inside a cell with the help of carbon dioxide and water. This mechanism is similar to what happens in the proximal convoluted tubule (PCT) with the difference that type A intercalated cells, actively secrete H+ into the lumen and this secretion works against the concentration gradient. The H+ reacts with molecular entities like phosphate or ammonia in the cell lumen and form charged ions NH4+ and H2PO4-. It prevents the re-entry of H+ into the cell matrix. The inability of H+ make them stay out of the cell that finally leads to their excretion. K+/ Cl- symporter is also present over the membrane that helps in the prevention of accumulation of K+ and Cl- within the cell by allowing their leakage toward the extracellular matrix. Type B intercalated cells have H+ and HCO3- channels present on opposite sides of tubular cells and they are responsible for reabsorption of H+ and secretion of HCO3-. Thus, type A cells work in response to acidosis in the body and type B cells for alkalosis in the body.
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