PS-B11-9: LOW COLONIC PH AND PROTON-SENSOR GPR65: A MISSING BRIDGE LINKING DIET, GUT MICROBIOTA AND CARDIOVASCULAR HEALTH
Authors: Xie Liang , Jones Dakota Rhys , Muralitharan Rikeish , Dinakis Evany , Nakai Michael , Donnell Joanne O , Jama Hamdi , Salimova Ekaterina , Ang Caroline , Robert Remy , Yao CK , Head Geoffrey A , Muir Jane , Mackay Charles R , Marques Francine Z 
Published: January 2023

Objective:
Dietary fibre regulates blood pressure (BP) through gut microbial production of acidic metabolites called short-chain fatty acids (SCFAs). The acidic environment in the large intestine activates a proton-sensing G-protein coupled receptor, GPR65. We aimed to characterize the luminal pH in the intestine of hypertensive patients and normotensive individuals, and to validate in a murine model that dietary fibre intake is a critical determinant for intestinal pH. Furthermore, we aimed to determine whether pH-sensing by GPR65 mediates the cardiovascular protection conferred by dietary fibre.

Design and method:
15 normotensive and 40 hypertensive (16 treated) participants had ambulatory BP profiles recorded using a Mobil-O-Graph device (CardioScan). Their intestinal luminal pH was measured in real-time by a SmartPillTM Motility Testing System (Medtronic). Analyses were performed on SPSS. Intestinal pH of C57BL/6J wild-type (WT) mice was measured after a 7-day dietary intervention of high-fibre (HF) or low-fibre (LF) diets (n = 5/group). Male WT and Gpr65-/- mice fed with HF or LF diets were implanted with minipumps containing angiotensin II (Ang-II, 0.5 mg/kg/day, 28 days, n = 11–12/group). BP was measured by tail-cuff, cardiac function by ultrasound, and fibrosis by Massons trichrome staining.

Results:
Compared to normotensive individuals, hypertensive participants had higher colonic minimum pH (mean ± SEM: 5.03 ± 0.08 vs 5.22 ± 0.34, the beta coefficient = 2.98, P = 0.024 adjusted for age, sex, BMI), driven by changes in the transverse colon where dietary fibre is fermented. In our murine model, HF fed mice exhibited lower colonic pH compared to LF fed mice (HF vs LF, 6.03 ± 0.42 vs 7.58 ± 0.31, P < 0.001). This was abolished when mice were treated with antibiotics. Under HF diet, Ang-II treated Gpr65-/- mice had significantly higher BP (mean arterial pressure, WT vs Gpr65-/-, 90.7 ± 2.22 vs 101.3 ± 1.82mmHg, P = 0.022), heart to tibia length ratio (0.0096 ± 0.0004 vs 0.0112 ± 0.0018 g/mm, P = 0.036), left ventricle posterior wall in the diastole (0.831 ± 0.016 vs 0.966 ± 0.020 mm, P = 0.010), cardiac fibrosis (1.44 ± 0.15% vs 2.30 ± 0.09%, P = 0.026) and lower ejection fraction (65.65 ± 1.94% vs 54.65 ± 1.60%, P < 0.001) and fractional shortening (22.70 ± 1.52% vs 15.63 ± 0.69%, P < 0.001) than Ang-II treated WT mice. The cardiovascular impact of Gpr65 genotype when Ang-II mice were fed with LF diet was negligible (All P > 0.05).

Conclusions:
This is the first evidence hypertensive participants have a higher luminal pH in the colon. This likely results from the difference in dietary fibre intake, which in a murine model profoundly determined the colonic pH. Low pH activates GPR65 signalling, which underlies the cardiovascular protection by dietary fibre. Our findings demonstrate a novel mechanism for BP regulation.