Original Article

K Depletion Enhances the Extracellular Ca²+-induced Inhibition of the Apical K Channels in the mTAL of Rat Kidney

Correspondence to: Wen-Hui Wang, Department of Pharmacology, New York Medical College, Valhalla, NY 10595. Fax:(914) 347-4956 E-mail:wenhui_wang{at}nymc.edu.

We have shown previously that raising extracellular Ca²+ inhibited the apical 70-pS K channel in the thick ascending limb (TAL; Wang, W.H., M. Lu, and S.C. Hebert. 1996. Am. J. Physiol. 270:C103–C111). We now used the patch-clamp technique to study the effect of increasing the extracellular Ca²+ on the 70-pS K channel in the mTAL from rats on a different K diet. Increasing the extracellular Ca²+ from 10 µM to 0.5, 1, and to 1.5 mM in the mTAL from rats on a K-deficient (KD) diet inhibited the channel activity by 30, 65, and 90%, respectively. In contrast, raising the extracellular Ca²+ to 1.5 mM had no significant effect on channel activity in the mTAL from animals on a high K (HK) diet and further increasing the extracellular Ca²+ to 2.5, 3.5, and 5.5 mM decreased the channel activity by 29, 55, and 90%, respectively. Inhibition of the cytochrome P450 monooxygenase completely abolished the effect of the extracellular Ca²+ on channel activity in the mTAL from rats on a different K diet. In contrast, blocking cyclooxygenase did not significantly alter the responsiveness of the 70-pS K channel to the extracellular Ca²+. Moreover, addition of sodium nitropruside, a nitric oxide (NO) donor, not only increased the channel activity, but also blunted the inhibitory effect of the extracellular Ca²+ on the 70-pS K channel and decreased 20-hydroxyeicosatetraenoic acid (20-HETE) concentration in the mTAL from rats on a KD diet. In contrast, inhibiting NOS with L-NAME enhanced the inhibitory effect of the extracellular Ca²+ on the channel activity and increased 20-HETE concentration in the mTAL from rats on a high K diet. Western blot has further shown that the expression of inducible NO synthase (iNOS) is significantly higher in the renal medulla from rats on an HK diet than that on a KD diet. Also, addition of S-nitroso-N-acetylpenicillamine abolished the inhibitory effect of arachidonic acid on channel activity in the mTAL, whereas it did not block the inhibitory effect of 20-HETE. We conclude that a low dietary K intake increases the sensitivity of the 70-pS K channel to the extracellular Ca²+, and that a decrease in NOS activity is involved in enhancing the inhibitory effect of the extracellular Ca²+ on channel activity in the mTAL during K depletion.

Key Words: 20-hydroxyeicosatetraenoic acid, iNOS, cytochrome P450, calcium-sensing receptor, K recycling

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