Medullary collecting duct acidification. Effects of potassium, HCO3 concentration, and pCO2.

Abstract

Abstract. The medullary collecting duct (MCD) from renal outer medulla possesses significant HCO3 absorptive capacity. In vitro microperfusion studies have shown that HCO3 absorption in this segment is carbonic anhydrase dependent, affected by peritubular and luminal chloride concentrations, is independent of the presence of Na or the presence of Na transport, and is stimulated by mineralocorticoid hormone. The present in vitro microperfusion studies defined regulatory influences on MCD acidification as assessed by acute changes in the extracellular K and HCO3 concentrations and pCO2. These studies showed that acute changes in the peritubular K concentration to either 0 mM K or 50 mM K have no significant effect on HCO3 absorption in MCD. Intracellular voltage recordings showed that elevation of peritubular K concentration from 5 to 50 mM produced only a 2.8 mV depolarization of the basolateral cell membrane of MCD cells. In addition, acute reduction of peritubular K from 5 to 0 mM had no significant effect on intracellular voltage. Studies that were designed to assess the effects of HCO3 concentration and pCO2 on acidification showed that acute reduction of peritubular HCO3 concentration from 25 to 5 mM (pH change from 7.4 to 6.8) increased lumen-positive voltage from 30.2±3.8 to 40.0±4.4 mV, and simultaneously increased net HCO3 absorption from 15.6±1.9 to 22.9±2.9 pmol* mm-*min-'. Elevation of peritubular HCO3 concentration from 25 to 50 mM (pH change from 7.4 to 7.8) significantly decreased lumenpositive voltage from 33.8±2.4 to 26.7± 1.5 mV and simultaneously decreased net HCO3 absorption from 17.9±1.2 to 12.8±1.3 pmol *mm-' *min-'. In addition, acute reduction of peritubular pCO2 from 40 to <14 mmHg (final pH 7.8-7.9) significantly decreased lumenpositive voltage from 31±4.4 to 15.7±1.0 mV. Coincidentally, HCO3 absorption decreased significantly from 11.0±3.7 to 5.3±0.7 pmol * mm' * min-'. We conclude that: (a) alteration of peritubular K concentration from 0 to 50 mM in vitro does not affect HCO3 absorption in the MCD, and that this lack of effect appears to be related to a low basolateral cell membrane K conductance; (b) net HCO3 absorption and the associated lumen-positive voltage can be modulated by in vitro changes in peritubular HCO3 and pCO2 (or pH); and (c) the MCD perfused in vitro appears to be a good model for studying the mechanisms and regulation of distal nephron acidification. IntroductionWe, and more recently others, have demonstrated via in vitro microperfusion studies that the medullary collecting duct (MCD)' from renal outer medulla possesses significant HCO3 absorptive capacity (1-4). Indeed, when the rabbit MCD from inner stripe of outer medulla is perfused and bathed in vitro with symmetrical Ringer's HCO3 solutions, transepithelial

Mineralocorticoid modulation of apical and basolateral membrane H+/OH-/HCO3- transport processes in the rabbit inner stripe of outer medullary collecting duct.

“…The difference may be a result ofthe nominal presence of ambient CO2/ HCO in our studies that was removed to decrease any contribution to pHi by the Cl/HCO-exchanger (9). Jacobsen (25) has found that decreasing peritubular CO2 from 40 mmHg to 10-14 mmHg results in a 52% reduction in net transepithelial H' secretion. Thus, in our experiments JH may have been less than predicted from flux studies due to the nominal presence of CO2/HCO3.…”

“…Tubules were again bathed and luminally perfused in a Cl--free solution containing 25 Basolateral membrane Na+/H+ antiporter activity. Basolateral membrane Na+/H' antiporter activity was assessed as the Na'-dependent pHi recovery from an acid load.…”

Regulation of net bicarbonate transport in rabbit cortical collecting tubule by peritubular pH, carbon dioxide tension, and bicarbonate concentration.

“…The freed segment was then transferred to a thermostatically controlled lucite bath chamber on an inverted microscopic stage. The tubule was cannulated and perfused with concentric micropipettes as previously described (4,23,24). The inner perfusion pipette was advanced 50-100 Am into the tubule lumen and served as a bridge into the tubular lumen to measure transepithelial potential difference (PD, in millivolts).…”

“…These results were also averaged. The perfusate remained unchanged in all of these studies except for the assumed rapid equilibration of the bath and luminal Pco2 (25) (24,26,27 Effect ofchanging ambient Pco2. In experiments designed to simulate acute respiratory acid-base disorders, the bath Pco2 and pH were altered.…”

Hormonal regulation of proton secretion in rabbit medullary collecting duct.

“…Several transport characteristics ofthe medullary collecting tubule make it an ideal segment to study the regulation of distal nephron acidification. These include no evidence for spontaneous net sodium or potassium transport (12); no evidence for bicarbonate secretion (8,13); independence of proton secretion from sodium transport or from the presence of sodium in luminal and peritubular fluids (7,14). With respect to active transport, the medullary collecting tubule thus appears to be involved exclusively in proton secretion.…”