Axial heterogeneity of bicarbonate, chloride, and water transport in the rat proximal convoluted tubule. Effects of change in luminal flow rate and of alkalemia.

Abstract

These studies examined regulation of superficial proximal convoluted tubule (PCIT) transport as a function of length. When single nephron glomerular filtration rate (SNGFR) increased from 28.7±0.7 nl/min in hydropenia to 41.5±0.4 nl/min in euvolemia, bicarbonate, chloride, and water reabsorption in the early (1st mm) PCI' increased proportionally: from 354±21 peaJ mm * min, 206±55 peq/mni * min, and 5.9±0.4 nl/mm * min to 520±12 peq/mm. min, 585±21 peq/mm * min, and 10.1 ±0.4 nl/ mm* min, respectively. These high transport rates did not increase further, however, when SNGFR went to 51.2±0.7 or 50.7±0.6 nl/min after atrial natriuretic factor or glucagon administration. Anion and water transport rates in the late PCI were lower and exhibited less flow dependence. During chronic metabolic alkalosis, acidification was inhibited in the late but not early PCT. In conclusion, the early PCI is distinguished from the late PCI by having high-capacity, flow-responsive but saturable, anion-and water-reabsorptive processes relatively unaffected by alkalemia.

Mechanism of apical and basolateral Na(+)-independent Cl-/base exchange in the rabbit superficial proximal straight tubule.

“…OH-secretion into the tubule lumen will diminish net proximal tubule HCO -absorption. In the rat, transepithelial HCO-absorption is greatest in the first millimeter after the glomerulus (51). Whether the Cl-/base exchange rate is lower in the superficial S1 proximal tubule compared with the S2 segment has not been determined.…”

Angiotensin II stimulates early proximal bicarbonate absorption in the rat by decreasing cyclic adenosine monophosphate.

“…There were 11 two-period protocols, each with five animals (except No. 4, which had a total of six animals): (1) …”

“…The early, SI proximal convoluted tubule (PCT)' is the site for reabsorption of 50% of the normal filtered load of bicarbonate (1). Recently, we found that SI PCT cells had angiotensin II receptors in high density and angiotensin II was a potent regulator of bicarbonate reabsorption in this nephron segment (2,3).…”

Angiotensin II stimulation of hydrogen ion secretion in the rat early proximal tubule. Modes of action, mechanism, and kinetics.

“…To further examine the transport kinetic changes induced by angiotensin II, we compared the response to angiotensin II when the basal rate of proximal bicarbonate absorption was submaximal (using a 30-n/min microperfusion rate as described above) to that when the rate was initially at a maximal level (15,17,18), achieved by increasing the microperfusion rate to 45 nl/min (18). As shown in Table IV, when perfusion rate was 45 nl/min and bicarbonate absorption was maximal, angiotensin II caused little further change in net bicarbonate absorption in the early PCT (from 548±9 to 557±18 peq/mm * min, NS) (Fig.…”

“…However, receptor-mediated stimulation of epithelial cell hydrogen ion secretion by angiotensin II has not been previously described. Direct hormonal control of SI cell function could potentially have great physiologic importance because these cells are normally responsible for fully half of renal bicarbonate reabsorption (15).…”