An indispensable role of secretin in mediating the osmoregulatory functions of angiotensin II

Abstract

Fluid balance is critical to life and hence is tightly controlled in the body. Angiotensin II (ANGII), one of the most important components of this regulatory system, is recognized as a dipsogenic hormone that stimulates vasopressin (VP) expression and release. However, detailed mechanisms regarding how ANGII brings about these changes are not fully understood. In the present study, we show initially that the osmoregulatory functions of secretin (SCT) in the brain are similar to those of ANGII in mice and, more important, we discovered the role of SCT as the link between ANGII and its downstream effects. This was substantiated by the use of two knockout mice, SCTR(-/-) and SCT(-/-), in which we show the absence of an intact SCT/secretin receptor (SCTR) axis resulted in an abolishment or much reduced ANGII osmoregulatory functions. By immunohistochemical staining and in situ hybridization, the proteins and transcripts of SCT and its receptor are found in the paraventricular nucleus (PVN) and lamina terminalis. We propose that SCT produced in the circumventricular organs is transported and released in the PVN to stimulate vasopressin expression and release. In summary, our findings identify SCT and SCTR as novel elements of the ANGII osmoregulatory pathway in maintaining fluid balance in the body.

Figure 1.

Dipsogenic effects of central SCT or ANGII injection in mice. A, C) Time-course study on the effects of single-dose i.c.v. SCT (250 and 500 ng; A), ANGII (50 and 100 ng; C) or control ACSF injection into lateral ventricle on cumulative water intake at various times (0.5, 1, 2, 4, 6, and 8 h) in WT, SCTR^−/−, and SCT^−/− mice (n=10/group). B, D) Effects of continuous i.c.v. SCT (B), ANGII (D), or ACSF infusion on daily water intake in WT, SCTR^−/−, and SCT^−/− mice (n=6/group). Data are expressed as means ± se. *P < 0.05, **P < 0.001 vs. respective control; ^#P < 0.05, ^##P < 0.001 vs. WT control.

Figure 2.

Effects of central SCT or ANGII injection on VP and SCT expression and release. A–F) Effects of i.c.v. SCT (500 ng; A, B), ANGII (100 ng; C–F), or ACSF on mouse VP (A, C) and SCT expression levels (E) in the PVN, and plasma VP (B, D) and SCT levels (F) postinjection (15 and 30 min, 1 and 4 h) in WT, SCTR^−/−, and SCT^−/− mice. mRNA levels of target gene were normalized with GAPDH levels. Data are expressed as means ± se (n=6–8/group). *P < 0.05, **P < 0.001 vs. respective control; ^#P < 0.05, ^##P < 0.001 vs. WT control. G) Double immunofluorescence staining shows colocalization of SCT and AT₁ receptors in the PVN of mouse brain. White arrows indicate cell where SCT and AT₁ receptor are expressed. Scale bars = 40 μm.

Figure 3.

Effects of hyperosmolality on SCT and SCTR expression and SCT release. Effects of salt loading (2%; 1 or 5 d) and water deprivation (1 d) on SCT and SCTR (A) and VP mRNA levels (C) in the PVN; and plasma SCT (B) and VP levels (D) of WT mice. mRNA levels of target genes are normalized with GAPDH levels. Data are expressed as means ± se (n=10/group). *P < 0.05, **P < 0.001 vs. respective control.

Figure 4.

Changes in Fos immunoreactivity in the lamina terminalis and PVN on i.c.v. ANGII. Immunohistochemical staining shows Fos immunoreactivity in the SFO, MnPO, OVLT, and PVN on i.c.v. ANGII injection in WT, SCTR^−/−, and SCT^−/− mice. Scale bars = 6 μm.

Figure 5.

SCT and SCTR expression in lamina terminalis on hyperosmolality and i.c.v. ANGII. Effects of (a–b) salt loading (2%; 5 d), water deprivation (1 d), and (c–d) i.c.v. ANGII or ACSF injection (30 min, 1 and 4 h) on mSCT and mSCTR expressions in the SFO, OVLT, MnPO, and cortex of WT brains. Data are expressed as means ± se (n = 6/group). *P < 0.05, **P < 0.001 vs. respective control.