Sodium reabsorption in thick ascending limb of Henle's loop: effect of potassium channel blockade in vivo

Abstract

1. Based on previous in vitro studies, inhibition of K(+) recycling in thick ascending limb (TAL) is expected to lower Na(+) reabsorption through (i) reducing the luminal availability of K(+) to reload the Na(+)-2Cl(-)-K(+) cotransporter and (ii) diminishing the lumen positive transepithelial potential difference which drives paracellular cation transport. 2. This issue was investigated in anaesthetized rats employing microperfusion of Henle's loop downstream from late proximal tubular site with K(+)-free artificial tubular fluid in nephrons with superficial glomeruli. 3. The unselective K(+) channel blocker Cs(+) (5 - 40 mM) dose-dependently increased early distal tubular delivery of fluid and Na(+) with a maximum increase of approximately 20 and 185%, respectively, indicating predominant effects on water-impermeable TAL. 4. The modest inhibition of Na(+) reabsorption in response to the 15 mM of Cs(+) but not the enhanced inhibition by 20 mM Cs(+) was prevented by luminal K(+) supplementation. Furthermore, pretreatment with 20 mM Cs(+) did not attenuate the inhibitory effect of furosemide (100 microM) on Na(+)-2Cl(-)-K(+) cotransport. 5. Neither inhibitors of large (charybdotoxin 1 microM) nor low (glibenclamide 250 microM; U37883A 100 microM) conductance K(+) channels altered loop of Henle fluid or Na(+) reabsorption. 6. The intermediate conductance K(+) channel blockers verapamil and quinine (100 microM) modestly increased early distal tubular Na(+) but not fluid delivery, indicating a role for this K(+) channel in Na(+) reabsorption in TAL. As observed for equieffective concentrations of Cs(+) (15 mM), Na(+) reabsorption was preserved by K(+) supplementation. 7. The results indicate that modest inhibition of K(+) channels lowers the luminal availability of K(+) and thus transcellular Na(+) reabsorption in TAL. More complete inhibition lowers paracellular Na(+) transport probably by reducing or even abolishing the lumen positive transepithelial potential difference. Under the latter conditions, transcellular Na(+) transport may be restored by paracellular K(+) backleak.

Figure 1

Effect of Cs⁺ on loop of Henle reabsorption of fluid, Na⁺, and Cl⁻. Depicted are the changes in fractional early distal tubular delivery during perfusion of Henle's loop with vehicle or Cs⁺ as compared to basal measurements. n=5–9 nephrons each group.

Figure 2

Influence of K⁺ supplementation on the effect of 15 mM Cs⁺ on loop of Henle reabsorption of fluid and Na⁺. Depicted are values for early distal tubular flow rate and Na⁺ concentration (V_ED, [Na⁺]_ED-A) as well as the changes in early distal tubular flow rate and Na⁺ delivery as compared to basal measurements (B). n=7–8.

Figure 3

Influence of K⁺ supplementation on the effect of 20 mM Cs⁺ on loop of Henle reabsorption of fluid and Na⁺. Depicted are values for early distal tubular flow rate and Na⁺ concentration (V_ED, [Na⁺]_ED-A) as well as the changes in early distal tubular flow rate and Na⁺ delivery as compared to basal measurements (B). n=5–7.

Figure 4

Influence of Cs⁺ pretreatment on the effect of furosemide on loop of Henle reabsorption of fluid and Na⁺. Depicted are values for early distal tubular flow rate and Na⁺ concentration (V_ED, [Na⁺]_ED-A) as well as changes in early distal tubular flow rate and Na⁺ delivery as compared to basal measurements (B). Basal: perfusion with artificial tubular fluid (if not otherwise indicated); exp, experimental period. n=5–8.

Figure 5

Effect of verapamil on loop of Henle reabsorption of fluid, Na⁺, and K⁺. Depicted are changes in early distal tubular delivery of fluid, Na⁺, and K⁺ as compared to basal measurements. n=5–7.

Figure 6

Effect of quinine on loop of Henle reabsorption of fluid, Na⁺, and K⁺. Depicted are changes in early distal tubular delivery of fluid, Na⁺, and K⁺ as compared to basal measurements. n=5–7.

Figure 7

Influence of K⁺ supplementation on the effect of verapamil or quinine on loop of Henle reabsorption of fluid and Na⁺. Depicted are values for early distal tubular flow rate and Na⁺ concentration (V_ED, [Na⁺]_ED-A) as well as the changes in early distal tubular flow rate and Na⁺ delivery as compared to basal measurements (B). n=5–6.

Figure 8

Proposed model for a dependency of the mechanism of Na⁺ transport inhibition in TAL on the extent of K⁺ channel blockade. (A) Under basal conditions, luminal K⁺ channels conduct K⁺ recycling back to the lumen to (i) reload the Na⁺-2Cl⁻-K⁺ cotransporter in the apical membrane and (ii) set up the lumen positive transepithelial potential difference which drives paracellular Na⁺ transport. (B) Partial blockade of luminal K⁺ channels (e.g., by 15 mM Cs⁺ or 100 μM verapamil or quinine) attenuates K⁺ recycling back to the lumen and reduces the luminal availability of K⁺ for reloading the Na⁺-2Cl⁻-K⁺ cotransporter and thus to a certain extent the transcellular Na⁺ transport. The partial blockade of K⁺ channels is insufficient, however, to reduce the transepithelial lumen positive potential difference to an extent which would significantly inhibit paracellular Na⁺ transport. (C) More complete blockade of luminal K⁺ channels (e.g., by 20 mM of Cs⁺ or Ba²⁺) lowers the lumen positive transepithelial potential difference to an extent which inhibits paracellular reabsorption or even allows for paracellular back leak of Na⁺. Paracellular back leak of K⁺ reloads the Na⁺-2Cl⁻-K⁺ cotransporter and maintains transcellular Na⁺ reabsorption.