High salt impairs energy sensing and autophagy to decrease the synthesis of liver‐derived vasodilator, β‐hydroxybutyrate

Abstract

Overconsumption of dietary salt is unequivocally linked to end organ damage. We have recently observed that a high salt (HS) diet induces liver dysfunction in the form of impaired synthesis of ketone body, β-hydroxybutyrate (βHB), in Dahl salt-resistant (R) and salt-sensitive (S) rats, and therefore, independent of high blood pressure. However, the molecular mechanisms as to why βHB is decreased after consuming a HS diet remains unknown. Evolutionarily, βHB is an energy source during times of nutrient scarcity. Therefore, we hypothesized that a HS diet decreases energy sensing and autophagy induction. To test this hypothesis, we treated low salt (LS)-fed S and R rats, with the 5' adenosine monophosphate-activated protein kinase (AMPK) and autophagy inhibitor dorsomorphin hydrochloride (DH; 25mg/kg), prior to a 24 h fast. As hypothesized, fasting increased serum βHB, and treatment with DH blocked this increase in R (mmol/L, non-fasting: 0.38±0.05 vs. fasting: 1.11±0.09 * vs. fasting+DH 0.42±0.05, *p<0.05) and S (mmol/L, non-fasting: 0.38±0.04 vs. fasting: 1.01±0.18* vs. fasting+DH: 0.47±0.09, *p<0.05). To test whether a HS diet was interfering with AMPK signaling, we took liver biopsies from R and S rats that had been on LS and HS diets for 8 weeks and probed for the protein expression of phospho-AMPKThr172 after a 24 h fast. In R rats fed a LS diet, as expected, fasting induced a significant increase in phospho-AMPKThr172 expression (Fig. 1A). However, Dahl R rats that had been on a high salt diet, fasting did not increase phospho-AMPKThr172 (Fig. 1A). In LS-fed S rats, we unexpectedly observed a basal downregulation of phospho-AMPKThr172 expression that was further exacerbated by a 24 h fast (Fig. 1B). However, in support of our results in HS-fed R rats, there was no difference in phospho-AMPKThr172 expression between non-fasting and fasting HS-fed S rats (Fig. 1B). In addition to being an energy source, βHB can also serve as a signaling molecule, primarily through activation and inhibition of the G-protein coupled receptors Gpr109a and Gpr41, respectively. Surprisingly, vasodilation to βHB was not different in mesenteric resistance arteries from Gpr109a and Gpr41 KO mice (data not shown). However, when mesenteric resistance arteries were incubated with Gpr43 inhibitor, GLPG 0974 (10 uM), vasodilation was prevented in both R [Emax(%), 48.6±1.7 vs. 26.4±1.1*, *p<0.05] and S [Emax(%), 43.2±3.5 vs. 22.6±1.8*, *p<0.05] rats relative to vehicle control. In summary, these data reveal that a HS diet impairs energy sensing and autophagy induction to reduce the synthesis of βHB. Furthermore, βHB partially exerts its vasodilatory effects through non-canonical Gpr43.