Mineralocorticoids decrease the activity of the apical small-conductance K channel in the cortical collecting duct

Abstract

. Mineralocorticoids decrease the activity of the apical smallconductance K channel in the cortical collecting duct. Am J Physiol Renal Physiol 289: F1065-F1071, 2005; doi:10.1152/ajprenal.00063.2005.-We used the patch-clamp technique to examine the effect of DOCA treatment (2 mg/kg) on the apical small-conductance K (SK) channels, epithelial Na channels (ENaC), and the basolateral 18-pS K channels in the cortical collecting duct (CCD). Treatment of rats with DOCA for 6 days significantly decreased the plasma K from 3.8 to 3.1 meq and reduced the activity of the SK channel, defined as NPo, from 1.3 in the CCD of control rats to 0.6. In contrast, DOCA treatment significantly increased ENaC activity from 0.01 to 0.53 and the basolateral 18-pS K channel activity from 0.67 to 1.63. Moreover, Western blot analysis revealed that DOCA treatment significantly increased the expression of the nonreceptor type of protein tyrosine kinase (PTK), cSrc, and the tyrosine phosphorylation of ROMK in the renal cortex and outer medulla. The possibility that decreases in apical SK channel activity induced by DOCA treatment were the result of stimulation of PTK activity was further supported by experiments in which inhibition of PTK with herbimycin A significantly increased NPo from 0.6 to 2.1 in the CCD from rats receiving DOCA. Also, when rats were fed a high-K (10%) diet, DOCA treatment did not increase the expression of c-Src and decrease the activity of the SK channel in the CCD. We conclude that DOCA treatment decreased the apical SK channel activity in rats on a normal-K diet and that an increase in PTK expression may be responsible for decreased channel activity in the CCD from DOCA-treated rats.epithelial Na channels; K secretion; protein tyrosine kinase; hypokalemia MINERALOCORTICOIDS HAVE BEEN shown to stimulate renal Na absorption and K secretion (2,18,23). The early effect of mineralocorticoids on Na transport is the result of stimulation of existing Na channels and Na-K-ATPase (4, 5, 17, 23), whereas the late effect results from increasing transcription of the epithelial Na channel (ENaC) and Na-K-ATPase (1). The mineralocorticoid-induced increase in K secretion has been shown to depend on Na transport (9). This indicates that mineralocorticoid-induced stimulation of K secretion is largely due to increases in the electrochemical driving force for K secretion. However, several studies have reported that mineralocorticoids stimulate the apical K conductance in the rabbit cortical collecting duct (CCD) (14,15,18). Moreover, the possibility that mineralocorticoids may have a direct stimulatory effect on apical K channels is supported by the observation that serum and glucocorticoid-induced kinase (SGK), which is stimulated by aldosterone (25), facilitates the membrane insertion of ROMK into the plasma membrane in Xenopus laevis oocytes (33). However, infusion of aldosterone did not increase the number of the apical ROMK-like SK channel (16). This suggests that aldosterone-mediated increases in K secretion may result from in...

Effects of mineralocorticoid and K+concentration on K+secretion and ROMK channel expression in a mouse cortical collecting duct cell line

“…Aldosteronetreated cells showed no increase but rather a tendency to a smaller barium-sensitive conductance, although this change was not significant. Similar results have been published by others: patch-clamp studies from split-open CCD tubules from rats on a normal-K ϩ diet treated with mineralocorticoids (DOCA or aldosterone) have shown a 30 -45% decrease in apical ROMK channel activity (27,38). After 4 -6 days of DOCA treatment, the expression levels of the protein tyrosine kinase c-Src were increased along with an increase in tyrosine phosphorylation of ROMK, which could explain the decreased ROMK channel activity seen in the DOCA-treated animals (38).…”

“…Similar results have been published by others: patch-clamp studies from split-open CCD tubules from rats on a normal-K ϩ diet treated with mineralocorticoids (DOCA or aldosterone) have shown a 30 -45% decrease in apical ROMK channel activity (27,38). After 4 -6 days of DOCA treatment, the expression levels of the protein tyrosine kinase c-Src were increased along with an increase in tyrosine phosphorylation of ROMK, which could explain the decreased ROMK channel activity seen in the DOCA-treated animals (38). Significant increases in the apical membrane K ϩ conductance have been observed upon longer term treatment (7-10 days) of the whole animal by DOCA (23,34).…”

The ROMK potassium channel is present in mammalian urinary tract epithelia and muscle

“…Although this hypotension was suggested to be due to polyuria and volume depletion (21), it is possible that loss of smooth muscle cell ROMK may result in reduced vascular tone in resistance blood vessels. Furthermore, whether ROMK expression in muscle cells is regulated by dietary sodium and potassium and by mineralocorticoids, as is the case for renal ROMK (1,36), is unknown.…”

Angiotensin II Inhibits the ROMK-like Small Conductance K Channel in Renal Cortical Collecting Duct during Dietary Potassium Restriction

“…1 Rats were killed by cervical dislocation. Kidneys were immediately removed, and several thin coronal slices were cut with a razor blade and placed in ice-cold Ringer's solution for microdissection of CCDs, as previously described (18). The Ringer's solution contained (in mM) 140 NaCl, 5 KCl, 1.8 CaCl 2 , 1.8 MgCl 2 , and 10 HEPES (pH 7.4).…”

“…Patch Clamp Recording-The basic patch clamp methods have been described previously (18). In brief, each isolated CCD affixed to a coverglass was transferred to a chamber (1000 l total volume) mounted on the stage of a Nikon inverted microscope.…”