Gαi2-protein-mediated signal transduction: central nervous system molecular mechanism countering the development of sodium-dependent hypertension

Abstract

Excess dietary salt intake is an established cause of hypertension. At present, our understanding of the neuropathophysiology of salt-sensitive hypertension is limited by a lack of identification of the central nervous system mechanisms that modulate sympathetic outflow and blood pressure in response to dietary salt intake. We hypothesized that impairment of brain Gαi2-protein-gated signal transduction pathways would result in increased sympathetically mediated renal sodium retention, thus promoting the development of salt-sensitive hypertension. To test this hypothesis, naive or renal denervated Dahl salt-resistant and Dahl salt-sensitive (DSS) rats were assigned to receive a continuous intracerebroventricular control scrambled or a targeted Gαi2-oligodeoxynucleotide infusion, and naive Brown Norway and 8-congenic DSS rats were fed a 21-day normal or high-salt diet. High salt intake did not alter blood pressure, suppressed plasma norepinephrine, and evoked a site-specific increase in hypothalamic paraventricular nucleus Gαi2-protein levels in naive Brown Norway, Dahl salt-resistant, and scrambled oligodeoxynucleotide-infused Dahl salt-resistant but not DSS rats. In Dahl salt-resistant rats, Gαi2 downregulation evoked rapid renal nerve-dependent hypertension, sodium retention, and sympathoexcitation. In DSS rats, Gαi2 downregulation exacerbated salt-sensitive hypertension via a renal nerve-dependent mechanism. Congenic-8 DSS rats exhibited sodium-evoked paraventricular nucleus-specific Gαi2-protein upregulation and attenuated hypertension, sodium retention, and global sympathoexcitation compared with DSS rats. These data demonstrate that paraventricular nucleus Gαi2-protein-gated pathways represent a conserved central molecular pathway mediating sympathoinhibitory renal nerve-dependent responses evoked to maintain sodium homeostasis and a salt-resistant phenotype. Impairment of this mechanism contributes to the development of salt-sensitive hypertension.

Keywords: blood pressure regulation; central G-protein–coupled receptors; renal sympathetic nerves; salt-sensitive hypertension; sympathetic nervous system.

Figure 1

(A) Gα-subunit protein expression in the brain cortex, PVN and VLM of naïve male DSR and DSS rats maintained for 21-days on a normal or high salt-intake (mean SEM, N=6/group). *P<0.05 versus respective normal salt-intake group value. τP<0.05 versus DSR high salt-intake group value, (B) Representative immunoblots illustrating brain Gα-subunit protein levels in DSR and DSS rats.

Figure 2

(A) MAP (mmHg), plasma NE content (nmol/L) and plasma renin activity (ng/ml/h), (B) Index of salt-sensitivity and, (C) Gαi₂-subunit protein expression in the brain cortex, PVN and VLM of male DSR and DSS 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 salt-intake (mean SEM, N=6/group). *P<0.05 versus respective normal salt-intake group value. τP<0.05 versus DSR high salt-intake group value, (D) Representative immunoblots illustrating ODN-mediated downregulation of brain Gαi₂-subunit protein levels in DSR and DSS rats.

Figure 3

Effect of elevated dietary sodium intake on (A) daily MAP (mmHg), (B) 24-h sodium balance (meq), (C) Index of salt-sensitivity, (D) Plasma NE (nmol/L), (E) peak ΔMAP (mmHg) post ip chlorisondamine (5 mg/kg), (F) Estimated plasma volume (EPV) (ml) and, (G) Estimated blood volume (EBV) (ml) in male DSR 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 salt-intake post sham or bilateral RDNX surgery (mean SEM, N=6/group). *P<0.05 versus respective normal salt-intake group value. τP<0.05 versus respective i.c.v. Gαi₂ ODN infusion high salt-intake sham RDNX group value.

Figure 4

Effect of elevated dietary sodium intake on (A) daily MAP (mmHg), (B) 24-h sodium balance (meq), (C) Index of salt-sensitivty, (D) Plasma NE (nmol/L), (E) peak ΔMAP (mmHg) post ip chlorisondamine (5 mg/kg), (F) Estimated plasma volume (EPV) (ml) and, (G) Estimated blood volume (EBV) (ml) in male DSS 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 (mean SEM, N=6/group). *P<0.05 versus respective normal salt-intake group value. τP<0.05 versus respective i.c.v. Gαi₂ ODN infusion high salt-intake sham RDNX group value. ΦP<0.05 versus respective DSS SCR ODN infusion high salt-intake group value.

Figure 5

(A) Gαi₂-subunit protein expression in the PVN of naïve Brown Norway, DSS and 8-congenic DSS rats maintained for 21-days on a normal or high salt-intake (mean SEM, N=6/group). *P<0.05 versus respective normal salt-intake group value. τP<0.05 versus DSS high salt-intake group value, and representative immunoblots illustrating PVN Gαi₂-subunit protein levels in (B) Brown Norway and (C) 8-congenic DSS rats. (DSS data reproduced from Fig 1.)

Figure 6

Effect of elevated dietary sodium intake on (A) daily MAP (mmHg), (B) 24-h sodium balance (meq), (C) Plasma NE (nmol/L), (D) peak ΔMAP (mmHg) post ip chlorisondamine (5 mg/kg), (E) Estimated plasma volume (EPV) (ml) and, (F) Estimated blood volume (EBV) (ml) in naive male Brown Norway, DSS and 8-congenic DSS rats maintained for 21-days (mean SEM, N=6/group). *P<0.05 versus respective normal salt-intake group value. τP<0.05 versus respective DSS high salt-intake group value.