Okay, I know that the sodium-hydrogen antiporter or sodium-hydrogen exchanger is a protein found in the nephron of the kidney that is primarily responsible for maintaining the balance of sodium. It brings sodium in and pumps hydrogen out without using ATP energy (therefore, facilitated transport).
The process of bicarbonate reabsorption occur predominantly in the proximal tubule (about 90%). The rest occur in the thick ascending limb and in the collecting tubule. All involve hydrogen ion secretion as shown in the diagram that is uploaded below.
The primary step in proximal hydrogen secretion is the secretion of H+ by the Na+ - H+ antiporter in the luminal membrane. Hydrogen ions are generated by the intracellular breakdown of H20 to OH- and H+. Hydrogen ions secreted combine with filtered HCO3- ions to form carbonic acid and then CO2 + H2O, which are then passively reabsorbed.
Technically, HCO3- ions reabsorbed in this process are not the same as the ones filtered. Note that a new HCO3- ion is generated from the intracellular breakdown of H20 to OH- and H+ and subsequent reaction of OH- with CO2 to form HCO3- . This new bicarbonate then crosses the basolateral membrane via a Na+ - 3HCO3- cotransporter.
Similar processes occur in the thick ascending loop of Henle and intercalating cells of the collecting duct. In contrast to the proximal tubule, hydrogen ion secretion in the collecting tubule is mediated by a H+ ATPase pump in the luminal membrane and a Cl-HCO3- exchanger in the basolateral membrane as shown in the diagram above. The H+ ATPase pump is influenced by aldosterone, which stimulates increased H+ secretion. Hydrogen ion secretion in the collecting tubule is the process primarily responsible for acidification of the urine, particularly during states of acidosis. The urine pH may fall as low as 4.0.
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Use:
E
X+ = (RT/ZF) ln (C
outside/C
inside)
R = gas constant
T = Temp.
o Kelvin
Z = charge on ion (i.e. -1 for Cl-,
+2 for Ca
2+)
F = Faraday’s number, 96 500 coulombs of charge per mol of ion with single charge
Equilibrium potential of Na+ = +72mV
Equilibrium potential of Cl- = - 58 mVNot sure about these two values though (above in blue)
Check your book and carry out the calculations.
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