Despite increasing aldosterone, elevated potassium is not necessary for activating aldosterone-sensitive HSD2 neurons or sodium appetite

Abstract

Restricting dietary sodium promotes sodium appetite in rats. Prolonged sodium restriction increases plasma potassium (pK), and elevated pK is largely responsible for a concurrent increase in aldosterone, which helps promote sodium appetite. In addition to increasing aldosterone, we hypothesized that elevated potassium directly influences the brain to promote sodium appetite. To test this, we restricted dietary potassium in sodium-deprived rats. Potassium restriction reduced pK and blunted the increase in aldosterone caused by sodium deprivation, but did not prevent sodium appetite or the activation of aldosterone-sensitive HSD2 neurons. Conversely, supplementing potassium in sodium-deprived rats increased pK and aldosterone, but did not increase sodium appetite or the activation of HSD2 neurons relative to potassium restriction. Supplementing potassium without sodium deprivation did not significantly increase aldosterone and HSD2 neuronal activation and only modestly increased saline intake. Overall, restricting dietary sodium activated the HSD2 neurons and promoted sodium appetite across a wide range of pK and aldosterone, and saline consumption inactivated the HSD2 neurons despite persistent hyperaldosteronism. In conclusion, elevated potassium is important for increasing aldosterone, but it is neither necessary nor sufficient for activating HSD2 neurons and increasing sodium appetite.

Keywords: aldosterone; dietary sodium; mineralocorticoid; potassium; salt appetite; salt hunger.

FIGURE 1

Experimental protocol. Primary endpoints were (a) activation of HSD2 neurons, estimated using nuclear immunoreactivity for c‐Fos, and (b) sodium appetite, estimated from 3% NaCl consumption in rats given 2 h access to saline. Figure S2 shows the experimental protocol for a subsequent pair of comparison groups given uninterrupted access to 3% NaCl

FIGURE 2

Blood plasma values of (a) potassium, (b) aldosterone, (c) sodium, and (d) total protein. Asterisks indicate a significant difference from controls. Hash symbols indicate a significant difference from the sodium‐deprived (NaD) group

FIGURE 3

Effects of dietary manipulations on sodium appetite. Consumption of (a) concentrated saline (3% NaCl) and (b) water (or 3% KCl, in potassium‐supplemented groups, indicated by green asterisks) over a 2 h sodium appetite test. Potassium supplementation slightly elevated 3% NaCl intake, but potassium deprivation did not reduce 3% NaCl intake in sodium‐deprived rats. (c, d) Total fluid volumes for each group. Asterisks indicate a significant difference from controls. Hash symbols indicate a significant difference from the sodium‐deprived (NaD) group

FIGURE 4

Activation of aldosterone‐sensitive HSD2 neurons. (a, c) Dietary sodium deprivation increases c‐Fos (red) nuclear immunoreactivity in neurons in the nucleus of the solitary tract (NTS), which is immunoreactive for 11‐beta‐hydroxysteroid dehydrogenase type 2 (HSD2, green). (b, d) Saline ingestion greatly increases c‐Fos nuclear immunoreactivity in the medial NTS, while reducing c‐Fos in HSD2 neurons. Examples in the top row are from sodium‐deprived rats (NaD), with (a) or without (b) 2‐h access to a saline drinking tube. Examples in the second row are from sodium‐ and potassium‐deprived rats (NaD+KD), with (a) or without (b) 2 h access to saline. There were no qualitative differences in c‐Fos activation produced by sodium deprivation with or without concurrent potassium deprivation. (e) The large increase in HSD2 neuronal activation produced by NaD, as measured by nuclear immunoreactivity for c‐Fos, was not significantly different in groups that were simultaneously potassium‐supplemented (NaD+K‐supp) or potassium‐deprived (NaD+KD). In all NaD groups, 3% NaCl consumption resulted in a significant decrease in HSD2 neuronal activity. (f) Shown, for reference, are the total intake volumes of 3% NaCl of the subgroups given a 2 h sodium appetite test before perfusion. Asterisks indicate a significant difference from controls. Hash symbols in panel (e) indicate a significant difference from the same group before 3% NaCl. Hash symbols in panel (f) indicate a significant difference from the sodium‐deprived (NaD) group

FIGURE 5

(a) Relationship between plasma values of potassium and aldosterone across groups. Despite an apparently positive association with aldosterone (a), plasma potassium had no apparent correlation with c‐Fos activation of the HSD2 neurons in that HSD2 neuronal activation was similarly elevated in rats that were deprived of dietary sodium across a wide range of potassium values (d). (b) Relationship of plasma aldosterone and c‐Fos activation of HSD2 neurons across groups. (c) In one group (K‐supp +NaD), we measured plasma aldosterone both with and without 2 h access to 3% NaCl, and found that HSD2 neuronal activation reduced sharply despite persistently elevated plasma aldosterone