Active H + secretion: in the interstitial cells of the distal tubule and the manifold
Although H + excretion at the end of the distal tubule and the manifold accounts for only about 5% of the total H + excretion. But this mechanism is very important in maximizing urine acidification.
Starting at the end of the distal tubule and continuing to the end of the remainder of the tubular system, the tubular epithelial cells secrete H + through the first activation channel. The nature of this transport process differs from the aforementioned mechanism of the glide near a part of the Henle loop and the distal tip of the distal tubule described above.
Figure. Active excretion of H + through alternating type A epithelial cell membranes of the distal tubule and the manifold. Type A cells contain hydro-adenosine triphosphatase (ATPase) and hydro-potassium-ATPase in the cell membrane and secrete hydrogen ions while reabsorption of bicarbonate and potassium ions under acidosis. Note that one HCO3− is absorbed for each H + secreted, and one chloride ion is passively secreted along with the H +.
The mechanism of the first activation of H +. It occurs in the tubular membrane when H + is transported directly by a specific protein, an H + ATPase pump and an H + -K + ATPase pump. The energy required for these proteins is obtained from the breakdown of ATP into ADP.
The first activation of H + occurs in a special cell called a type A cell of the distal tubule and the manifold. The H + ions in these cells are excreted in 2 steps: (1) CO2 in the cell reacts with water to produce H2CO3, and (2) H2CO3 dissociates into HCO3-, then reabsorbed into the blood, H + ions. is pumped out by one of the two proteins above. For each H + excreted, one HCO3 А is reabsorbed, similar to the proximal tubule process. The main difference is that H + moves across the membrane by an active H + pump instead of a two-way protein channel like in the anterior nephron.
Although H + excretion at the end of the distal tubule and the manifold accounts for only about 5% of the total H + excretion. But this mechanism is very important in maximizing urine acidification. In the proximal tubule, the H + concentration can be increased by 3-4 times and the pH of the dialysis solution can be reduced to 6.7, although a large amount of H + is excreted by this segment of the Nephron, the H + concentration is possible. is increased up to 900 times in the manifold. This mechanism reduces the PH of the dialysis solution to 4.5, pH lower than the lower limit of the renal parenchymal pH can be achieved.