The role of secretin receptor in the angiotensin II-induced aldosterone biosynthesis and salt conservation

Abstract

Secretin (SCT) and its receptor (SCTR) play an important role in fluid-regulating through controlling the trafficking and expression of renal aquaporin 2 (AQP2), and mediating the central actions of angiotensin II (ANGII) in inducing water-drinking behavior and vasopressin (Vp) release. The indispensable role of this SCT/SCTR axis in the central ANGII-induced water-drinking behavior is achieved through SCTR/AT1aR heterodimer. However, if this axis is also involved in regulating periphery functions of ANGII, such as ANGII-induced aldosterone biosynthesis/release, remains unknown. In addition, whether SCTR/AT1aR complex is functional in mediating other actions of ANGII has not been explored. The homeostasis of water and sodium is usually closely associated, and ANGII is a crucial regulator ii in maintenance of the both homeostasis. Therefore, in the present study, systemic studies were carried out to investigate the potential role of SCT/SCTR axis and SCTR/AT1aR heterodimeric complex in ANGII-conducted regulations on sodium homeostasis. As ANGII-controlled aldosterone dominates renal epithelial sodium channels (ENaCs), therefore in this study it was firstly tested if SCT/SCTR can regulate sodium homeostasis via the renin-angiotensinaldosterone system (RAAS). SCTR knockout (SCTR-/-) mice showed impaired aldosterone synthase (CYP11B2) expression and consequent aldosterone release upon intraperitoneal (IP) injected ANGII. Endogenous ANGII production induced by dietary sodium restriction was higher in SCTR-/- than in C57BL/6N (WT), but CYP11B2 and aldosterone synthesis were not increased accordingly. Cholesteryl ester providing the precursor of aldosterone was not properly accumulated in the adrenal glands of SCTR-/- fed low Na+ diet. Absence of SCTR resulted in elevated adrenal basal CYP11B2 and renal basal ENaCs transcripts. Although the transcript and protein levels of ENaCs were similar in WT/ and SCTR-/-/low Na+, ENaCs in SCTR-/- were less sensitive to amiloride hydrochloride. Next, the hypothesis that SCTR/AT1R mediates the ANGII-inducible aldosterone synthesis/release was extensively examined. Primary zona glomerulosa (ZG) cells were obtained from WT and SCTR-/- mice. Aldosterone release from primary ZG cells exposed to ANGII (10 nM) was determined by enzyme immunoassay (EIA). The primary SCTR-/- adrenal cortical cells showed impaired ANGIIinduced aldosterone secretion, compared with WT. The Fluo-4 AM-loaded primary WT adrenal ZG cells displayed a dose-dependently increased intracellular calcium ([Ca2+]i) responding to ANGII, but not in SCTR-/-, indicating the absence of SCTR has attenuated the ANGII-induced Ca2+ which is EGTA-chelated. Using synthetic transmembrane (TM) peptides, it was found that the SCTR/AT1R heteromers mediated the majority of ANGII-induced Ca2+ influx. Consistently, SCTR TM-II pre-incubated primary ZG cells failed to secrete aldosterone upon ANGII, while the mutated STM-II peptide (STM-IIm) did not show this inhibitory effects. In summary, the data from this study have collectively showed that the knockout of SCTR resulted in defective aldosterone biosynthesis/release and less amiloride-sensitive ENaCs, and the SCT/SCTR axis is required for aldosterone precursor uptake, expression of CYP11B2. Besides, the results here clearly provide evidence that the SCTR/AT1R heterodimer is required for fully functioned ANGII to induce aldosterone biosynthesis/release through mediating the ANGII-triggered Ca2+ influx in the ZG cells.