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Am J Physiol Renal Physiol 279: F919-F926, 2000;
0363-6127/00 $5.00
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Vol. 279, Issue 5, F919-F926, November 2000

PKA site mutations of ROMK2 channels shift the pH dependence to more alkaline values

Jens Leipziger1, Gordon G. MacGregor1, Gordon J. Cooper1, Jason Xu2, Steven C. Hebert2, and Gerhard Giebisch1

1 Department of Cellular and Molecular Physiology, Yale University, New Haven, Connecticut 06520; and 2 Division of Nephrology, Vanderbilt University, Nashville, Tennessee 37322 - 2372

Close similarity between the rat native low-conductance K+ channel in the apical membrane of renal cortical collecting duct principal cells and the cloned rat ROMK channel strongly suggest that the two are identical. Prominent features of ROMK regulation are a steep pH dependence and activation by protein kinase A (PKA)-dependent phosphorylation. In this study, we investigated the pH dependence of cloned renal K+ channel (ROMK2), wild-type (R2-WT), and PKA site mutant channels (R2-S25A, R2-S200A, and R2-S294A). Ba2+-sensitive outward whole cell currents (holding voltage -50 mV) were measured in two-electrode voltage-clamp experiments in Xenopus laevis oocytes expressing either R2-WT or mutant channels. Intracellular pH (pHi) was measured with pH-sensitive microelectrodes in a different group of oocytes from the same batch on the same day. Resting pHi of R2-WT and PKA site mutants was the same: 7.32 ± 0.02 (n = 22). The oocytes were acidified by adding 3 mM Na butyrate with external pH (pHo) adjusted to 7.4, 6.9, 6.4, or 5.4. At pHo 7.4, butyrate led to a rapid (tau : 163 ± 14 s, where tau  means time constant, n = 4) and stable acidification of the oocytes (Delta pHi 0.13 ± 0.02 pH units, where Delta  means change, n = 12). Intracellular acidification reversibly inhibited ROMK2-dependent whole cell current. The effective acidic dissociation constant (pKa) value of R2-WT was 6.92 ± 0.03 (n = 8). Similarly, the effective pKa value of the N-terminal PKA site mutant R2-S25A was 6.99 ± 0.02 (n = 6). The effective pKa values of the two COOH-terminal PKA site mutant channels, however, were significantly shifted to alkaline values; i.e., 7.15 ± 0.06 (n = 5) for R2-S200A and 7.16 ± 0.03 (n = 8) for R2-S294A. The apparent Delta pH shift between the R2-WT and the R2-S294A mutant was 0.24 pH units. In excised inside-out patches, alkaline pH 8.5 activated R2-S294A channel current by 32 ± 6.7%, whereas in R2-WT channel patches alkalinzation only marginally increased current by 6.5 ± 1% (n = 5). These results suggest that channel phosphorylation may substantially influence the pH sensitivity of ROMK2 channel. Our data are consistent with the hypothesis that in the native channel PKA activation involves a shift of the pKa value of ROMK channels to more acidic values, thus relieving a H+-mediated inhibition of ROMK channels.

phosphorylation; potassium ion channel; inward-rectifier; potassium ion secretion; intracellular pH


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