Decreased susceptibility to renovascular hypertension in mice lacking the prostaglandin I2 receptor IP

Abstract

Persistent reduction of renal perfusion pressure induces renovascular hypertension by activating the renin-angiotensin-aldosterone system; however, the sensing mechanism remains elusive. Here we investigated the role of PGI2 in renovascular hypertension in vivo, employing mice lacking the PGI2 receptor (IP-/- mice). In WT mice with a two-kidney, one-clip model of renovascular hypertension, the BP was significantly elevated. The increase in BP in IP-/- mice, however, was significantly lower than that in WT mice. Similarly, the increases in plasma renin activity, renal renin mRNA, and plasma aldosterone in response to renal artery stenosis were all significantly lower in IP-/- mice than in WT mice. All these parameters were measured in mice lacking the four PGE2 receptor subtypes individually, and we found that these mice had similar responses to WT mice. PGI2 is produced by COX-2 and a selective inhibitor of this enzyme, SC-58125, also significantly reduced the increases in plasma renin activity and renin mRNA expression in WT mice with renal artery stenosis, but these effects were absent in IP-/- mice. When the renin-angiotensin-aldosterone system was activated by salt depletion, SC-58125 blunted the response in WT mice but not in IP-/- mice. These results indicate that PGI2 derived from COX-2 plays a critical role in regulating the release of renin and consequently renovascular hypertension in vivo.

Figure 1

Morphological changes of the kidneys and renovascular hypertension in 2K1C model. (A) Representative photographs of the kidneys from sham-operated WT mice (left) and from mice subjected to 2K1C (right). At day 14 of 2K1C, the left kidney became atrophic and the right kidney showed compensatory hypertrophy. (B and C) Time course of sBP in WT, sham-operated WT, EP1^–/–, EP2^–/–, EP3^–/–, and IP^–/– mice (B), and in F2-WT, sham-operated F2-WT, and EP4^–/– mice (C) in 2K1C model. Each point represents mean ± SEM of 8_10 mice per group. *P < 0.05 versus WT mice.

Figure 2

Activation of the RAA system in WT and IP^–/– mice in 2K1C model. (A and B) Increases in PRA (A) and renin mRNA expression in the kidney (B) at day 7 of 2K1C. Each column represents mean ± SEM of 5_15 mice per group. In B values were expressed as a ratio of renin/β-actin mRNA of 2K1C mice to that of sham-operated mice. *P < 0.05 versus WT mice. (C) Immunohistochemical analysis of renin expression in the WT kidney in 2K1C model. The immunoreactivities of renin in the kidney from the sham-operated WT mouse were detected only in the cells of JGA (left). The immunoreactivities of renin in the kidney at day 7 of 2K1C were detected in the afferent arterioles adjacent to JGA in addition to in JG cells (right). Arrows indicate the renin immunoreactivities. Original magnification, ×400. (D) Increase in PAC at day 7 of 2K1C. Each column represents mean ± SEM of 5_15 mice per group. *P < 0.05 versus WT mice. Pre-ope, preoperation.

Figure 3

Expression of COX-2 mRNA in the WT kidney in 2K1C model. The expression of COX-2 mRNA increased significantly after the operation and reached a peak level at day 4 of 2K1C. Each column represents mean ± SEM of 5_12 mice per group. Values were expressed as a ratio of COX-2/β-actin mRNA of 2K1C mice to that of sham-operated mice.

Figure 4

PRA and renin mRNA expression in the kidney in a salt-deficiency model. Mice were subjected to a low-salt diet (0.12% NaCl) for 7 days and injected with furosemide (25 mg/kg, intraperitoneally) every day. PRA (A) and renin mRNA expression (B) increased significantly more than in control mice at day 7 of salt depletion. These increases were blunted significantly in IP^–/– mice compared with those in WT mice. Each column represents mean ± SEM of 5_12 mice per group. Values in B were expressed as a ratio of renin/β-actin mRNA of salt-depleted mice to that of control mice. *P < 0.05 versus WT mice.

Figure 5

Effects of cicaprost, an IP agonist, and PGE2 on renin mRNA expression in cultured cells rich in JG cells. (A) A representative photograph showing stimulatory effects of cicaprost on renin mRNA expression in cultured cells prepared from WT mice. (B) Cicaprost significantly increased the expression level of renin mRNA in a concentration-dependent manner in the cells prepared from WT mice, although the effects disappeared completely from the cells prepared from IP^–/– mice. In contrast, PGE2 had no effect on renin mRNA expression in the cells prepared from WT mice. Each point represents mean ± SEM of 4 cell groups. *P < 0.05 versus WT mice.

Figure 6

Effects of cicaprost on renin activity and cAMP contents in cultured cells rich in JG cells. Cicaprost (1 μM) significantly increased the renin activity (A) and cAMP contents (B) in the cells prepared from WT mice, while the effects disappeared completely in the cells prepared from IP^–/– mice. Each point represents mean ± SEM of 4 cell groups. *P < 0.05 versus control.