Central nervous system Gαi2-subunit proteins maintain salt resistance via a renal nerve-dependent sympathoinhibitory pathway

Abstract

In salt-resistant phenotypes, chronic elevated dietary sodium intake evokes suppression of renal sodium-retaining mechanisms to maintain sodium homeostasis and normotension. We have recently shown that brain Gαi(2) protein pathways are required to suppress renal sympathetic nerve activity and facilitate maximal sodium excretion during acute intravenous volume expansion in Sprague-Dawley rats. Here, we studied the role of brain Gαi(2) proteins in the endogenous central neural mechanisms acting to maintain fluid and electrolyte homeostasis and normotension during a chronic elevation in dietary salt intake. Naive or bilaterally renal denervated adult male Sprague-Dawley rats were randomly assigned to receive an intracerebroventricular scrambled or Gαi(2) oligodeoxynucleotide infusion and then subjected to either a normal salt (0.4%) or high-salt (8.0%) diet for 21 days. In scrambled oligodeoxynucleotide-infused rats, salt loading, which did not alter blood pressure, evoked a site-specific increase in hypothalamic paraventricular nucleus Gαi(2) protein levels and suppression of circulating norepinephrine content and plasma renin activity. In salt-loaded rats continuously infused intracerebroventricularly with a Gαi(2) oligodeoxynucleotide, animals exhibited sodium and water retention, elevated plasma norepinephrine levels, and hypertension, despite suppression of plasma renin activity. Furthermore, in salt-loaded bilaterally renal denervated rats, Gαi(2) oligodeoxynucleotide infusion failed to evoke salt-sensitive hypertension. Therefore, in salt-resistant rats subjected to a chronic high-salt diet, brain Gαi(2) proteins are required to inhibit central sympathetic outflow to the kidneys and maintain sodium balance and normotension. In conclusion, these data demonstrate a central role of endogenous brain, likely paraventricular nucleus-specific, Gαi(2)-subunit protein-gated signal transduction pathways in maintaining a salt-resistant phenotype.

Figure 1

(A) Effect of elevated dietary sodium intake on brain Gα-subunit expression in naïve male Sprague-Dawley rats maintained on either a normal or high sodium diet for 21-days for which physiological data is presented in Table 1. Gα-subunit protein expression was normalized to GAPDH and expressed as optical density units/mm² in the brain cortex, hypothalamic paraventricular nucleus (PVN) and ventrolateral medulla (VLM). The values are means ± SEM (N=6/group) and, (B) representative immunoblots illustrating GAPDH and Gα-subunit protein levels in the hypothalamic PVN from male Sprague-Dawley rats for which group data is presented in Fig 2A. *P<0.05 compared with respective normal sodium intake group value.

Figure 2

(A) Effect of elevated dietary sodium intake on brain Gα-subunit expression in male Sprague-Dawley rats receiving an i.c.v. SCR ODN infusion (25µg/6µl/day) maintained on a normal or high sodium diet for 21-days for which physiological data is presented in Fig. 3. Gα-subunit protein expression was normalized to GAPDH and expressed as optical density units/mm² (mean ± SEM) in the supraoptic nucleus (SON), hypothalamic paraventricular nucleus (PVN), posterior hypothalamus (PH) and ventrolateral medulla (VLM). The values are means ± SEM (N=6/group) and, (B) representative immunoblots illustrating GAPDH and Gα-subunit protein levels in the hypothalamic PVN from male Sprague-Dawley rats for which group data is presented in Fig 4A. ^*P<0.05 compared with respective i.c.v. SCR ODN group value.

Figure 3

(A) Effect an i.c.v. of Gαi₂ ODN infusion (25µg/6µl/day) on brain Gα-subunit expression in male Sprague-Dawley rats maintained on a normal or high sodium diet for 21-days for which physiological data is presented in Fig 3. Gα-subunit protein expression was normalized to GAPDH and expressed as optical density units/mm² in the brain (BC), PVN and VLM. The values are means ± SEM (N=6/group) and, (B) representative immunoblots illustrating GAPDH and Gα-subunit protein levels in the hypothalamic PVN from male Sprague-Dawley rats for which group data is presented in Fig 5A. *P<0.05 compared with i.c.v. SCR ODN group value.

Figure 4

(A) peak ΔHR (bpm) post iv atropine (1 mg/kg), (B) peak ΔHR (bpm) post iv propranolol (1mg/kg), and (C) peak ΔMAP (mmHg) post iv hexamethonium (30 mg/kg) in male Sprague-Dawley rats receiving an i.c.v. infusion of a SCR or Gαi₂ ODN (25µg/6µl/day) measured on day-21 of a normal or high sodium diet. The values are means ± SEM (N=6/group). *P<0.05 compared with respective normal sodium intake group value. ^τP<0.05 compared with respective i.c.v. SCR ODN group value.

Figure 5

Index of salt-sensitivity in male Sprague-Dawley rats receiving an i.c.v. infusion of a SCR or Gαi₂ ODN (25µg/6µl/day) maintained for 21-days on a normal or high sodium diet post sham or bilateral RDNX surgery. The values are means ± SEM (N=6/group).