Abstract Body: Introduction: Tryptophan (Trp) diet attenuates hypertension in male Dahl salt sensitive (S) rats with unknown mechanisms. Kynurenine (a vasodilator) and serotonin (mixed effects on blood pressure) are host-derived Trp metabolites while indole is produced exclusively by the gut microbiota. We have previously shown that the gut microbiota of male, but not female, S rats is susceptible to salt, causing a higher indole level that elevated blood pressure. In this study, we reported the sex differences in all Trp metabolites with a focus on the impact of salt and the mechanisms by which female gut microbiota is more resilient.
Methodology: S rats of both sexes (10–13-week-old, n = 4-7/group) were maintained on either a low-salt (0.3 % NaCl, LS) or high-salt (2 % NaCl, HS) diet for 3 weeks. Trp metabolites were analyzed in serum and cecum by LC-MS/MS. To investigate if estrogen shape gut microbiota, gonadectomy was performed on 7-week-old Spontaneously Hypertensive Rats (SHR) of both sexes (n=5/group). β-estradiol (2.8 mg/kg) was administered subcutaneously for 12 weeks before fecal 16S rRNA gene sequencing.
Results: Females had elevated levels of kynurenine (Kyn) compared to male S rats on both LS (5.5 ± 1.4 vs. 2.1 ± 0.5 µM; P<0.05) and HS (7 ± 1.3 vs. 4.2 ± 1 µM; P<0.05). Kyn metabolites including kynurenic acid, 3-hydroxykynurenine, 3-hydroxyanthranilic acid were elevated in the serum of females compared with males, and HS amplified this difference (all P<0.05). Further, HS significantly decreased the serum serotonin levels in both sexes (P <0.01 in females, P<0.0001 in males). Intriguingly, HS increased cecal indole levels in male (P<0.05), but not in female rats. Microbiota analysis of gonadectomized (GDX) SHR rats revealed (1) Significant differences in β-diversity between intact vs. GDX rats (P <0.01), and between GDX male vs. female rats (P=0.01); (2) Estrogen supplementation eliminates the gut microbiota differences between GDX females and males (P=0.18).
Conclusion: We demonstrated sex differences in Trp metabolism in response to high salt. Further, estrogen masked the microbial differences between GDX male and female SHR rats. In summary, salt led to an elevated Trp-Kyn metabolism in female host, and an enhanced Trp-indole metabolism in male gut microbiota, which could be due to the estrogen’s effect on shaping gut microbiota. Thus, the Trp metabolism in salt-sensitive hypertension is regulated in a sex-specific manner.