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Laboratory of Epithelial Transport, Department of Internal Medicine, University of Iowa and Department of Veterans Affairs Medical Center, Iowa City, Iowa 52242
Rats on a low-NaCl diet have a high
Na+ channel activity in colon and
kidney. To address the mechanism of this increased activity, we
measured mRNA levels of three Na+
channel subunits in epithelial tissue (rENaC) from rats having been fed
either a low (0.13%)- or high (8%)-NaCl diet for 2-3 wk. The
size of the mRNA for each of the rENaC subunits as determined by
Northern blot was unaffected by diet. RNase protection assay showed
heterogeneity of response by organs and subunit. In lung, there was no
effect of diet on any of the three subunits. In descending colon, the
low-NaCl diet increased
- and
-rENaC mRNA, with no effect on
-rENaC mRNA. In the kidney, the response to dietary NaCl was
dependent on the region. In cortex and outer medulla, diet had no
effect on any of the subunits. Rats fed the low-NaCl diet had greater
-rENaC in inner medulla but not
- or
-rENaC mRNA. We next
asked whether acute administration of pure glucocorticoid (GC) or
mineralocorticoid (MC) hormones to adrenalectomized rats reproduced the
effects of a low-NaCl diet. Six hours after administration of GC or MC,
a somewhat different heterogeneity occurred. In lung,
-rENaC mRNA
was increased but only in response to GC. In colon, either GC or MC
increased
- or
-rENaC, and there was no effect on
-rENaC. In
kidney, either GC or MC increased
-rENaC, without an effect on
-
or
-rENaC. In contrast to the response to a low-NaCl diet, all three
regions were similarly affected by acute steroids. These results
demonstrate a striking heterogeneity in response to physiological
stimuli that regulate ENaC function. The mRNA levels of each of the
rENaC subunits can be determined by the type of steroid and by factors
unique to the organ and even to the specific region of the kidney.
kidney; kidney medulla; colon; lung; sodium channel; aldosterone; RU-28362; dietary salt
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