Attenuated mean arterial pressure in interleukin-6 knockout mice during angiotensin II hypertension and a high-salt diet

Plasma levels of IL-6 correlate with high blood pressure under many circumstances, and ANG II has been shown to stimulate IL-6 production from various cell types. This study tested the role of IL-6 in mediating the hypertension caused by high-dose ANG II and a high-salt diet. Male C57BL6 and IL-6 knockout (IL-6 KO) mice were implanted with biotelemetry devices and placed in metabolic cages to measure mean arterial pressure (MAP), heart rate (HR), sodium balance, and urinary albumin excretion. Baseline MAP during the control period averaged 114 ± 1 and 109 ± 1 mmHg for wild-type (WT) and IL-6 KO mice, respectively, and did not change significantly when the mice were placed on a high-salt diet (HS; 4% NaCl). ANG II (90 ng/min sc) caused a rapid increase in MAP in both groups, to 141 ± 9 and 141 ± 4 in WT and KO mice, respectively, on day 2. MAP plateaued at this level in KO mice (134 ± 2 mmHg on day 14 of ANG II) but began to increase further in WT mice by day 4, reaching an average of 160 ± 4 mmHg from days 10 to 14 of ANG II. Urinary albumin excretion on day 4 of ANG II was not different between groups (9.18 ± 4.34 and 8.53 ± 2.85 μg/2 days for WT and KO mice). By day 14, albumin excretion was nearly fourfold greater in WT mice, but MAP dropped rapidly back to control levels in both groups when the ANG II was stopped after 14 days. Thus the ∼30 mmHg greater ANG II hypertension in the WT mice suggests that IL-6 contributes significantly to ANG II-salt hypertension. In addition, the early separation in MAP, the albumin excretion data, and the rapid, post-ANG II recovery of MAP suggest an IL-6-dependent mechanism that is independent of renal injury.

interleukin-6 (IL-6) is a proinflammatory cytokine that is released from numerous cell types, including endothelial cells (12, 21), vascular smooth muscle cells (14, 16), and macrophages (32). The sympathetic nervous system (12, 20, 22) stimulates the release of IL-6 and other proinflammatory cytokines, and sympathetic nerves also are a source of IL-6. Previous results indicate that high IL-6 levels correlate with increased blood pressure and may be an independent risk factor for hypertension (5, 28). However, it has been difficult to establish a cause-and-effect link between IL-6 and chronic hypertension.

Short-term (5 day) infusion studies have begun to provide support for such a link. Alexander et al. (1) reported that a twofold elevation in the plasma levels of the closely related proinflammatory cytokine tumor necrosis factor α (TNF-α) caused a significant increase in arterial pressure in pregnant rats, and Orshal and Khalil (25) followed with similar findings infusing IL-6 in pregnant rats. However, although those studies demonstrated that sustained increases in IL-6 can cause hypertension, they did not test the role of IL-6 in mediating hypertension. Thus the question is whether the correlative relationship between IL-6 and blood pressure means that IL-6 is required for, or plays a role in, the hypertension, and that question must be addressed by blocking IL-6.

Our laboratory used mice with knockout of IL-6 to demonstrate recently that IL-6 is important in mediating the acute hypertensive response to psychosocial stress (18), and we also have shown that the hypertension in that model depends significantly on ANG II (19). ANG II stimulates the release of IL-6 (10, 14, 16, 29), and recent work has shown that an ANG II type 1 receptor antagonist lowers blood pressure, aortic mRNA expression of proinflammatory cytokines IL-1β, TNF-α, and IL-6, as well as plasma levels of IL-6 and IL-1β in spontaneously hypertensive rats (30). The goal of this study was to test more directly the role of IL-6 in long-term blood pressure control by testing the hypothesis that the hypertensive response to chronic (14 days) ANG II infusion would be attenuated significantly in IL-6 knockout (KO) mice compared with wild-type (WT; C57BL/6) mice. Because IL-6 has been linked to hypertensive glomerular injury (3, 17, 29) in addition to effects on vascular contractility (24, 25), we used a high dose of ANG II (90 ng/min) and a 4% high-salt diet to enhance our ability to detect a significant effect of IL-6 on ANG II hypertension.