The biological backdrop is an evolutionary trade-off. Chronic malaria exposure has shaped human polymorphisms in the renin-angiotensin system, including variants in angiotensin-converting enzyme (ACE) and ACE2 that raise circulating angiotensin II. According to the hypothesis advanced by the research team and collaborators in earlier work, these variants confer a survival advantage against severe malaria, both by directly inhibiting the parasite and by signaling through angiotensin receptors, but they simultaneously elevate the lifetime risk of hypertension. In other words, the same hormonal elevation that once helped ancestors survive cerebral malaria may predispose their descendants to cardiovascular disease in adulthood. Disentangling the protective dose from the hypertensive dose is therefore the central problem the new study set out to solve.

Angiotensin II acts through two G protein-coupled receptors with opposing physiology. The type 1 receptor (AT1R), a seven-transmembrane protein coupled to Gq/11, G12/13 and Gβγ complexes, triggers phospholipase C signaling, phosphatidylinositol hydrolysis, inositol trisphosphate release and a rise in intracellular calcium that contracts vascular smooth muscle and raises blood pressure. Chronic AT1R activation also drives cardiac myocyte hypertrophy, inflammation, neointimal proliferation and myocardial fibrosis. The type 2 receptor (AT2R), by contrast, couples to Giα and a cyclic GMP and nitric oxide signaling cascade that produces vasodilation, natriuresis, anti-inflammatory effects and reduced fibrosis, effectively counterbalancing AT1R output. Prior work has shown that AT2R activation, particularly when AT1R is blocked, preserves inter-endothelial junctions in brain microvasculature and protects against experimental cerebral malaria, which is why angiotensin II has been proposed as an adjunct therapy candidate.

To define the therapeutic window, the team ran two parallel assay systems. For the parasite side, they cultivated the 3D7 strain of P. falciparum in RPMI 1640 medium supplemented with human serum at 2 percent hematocrit, synchronized cultures with 5 percent sorbitol to yield more than 98 percent ring forms, and matured the parasites to schizonts over roughly 48 hours. Schizonts at 1 to 2 percent parasitemia were then incubated for 16 hours with four concentrations of angiotensin II, spanning 10⁻⁹ to 10⁻⁴ molar, alongside a negative control and two positive controls, chloroquine at 10 micromolar and medical-grade heparin at 30 international units. New ring formation, the hallmark of recent red cell invasion, was scored microscopically every three hours across five contiguous fields of roughly 200 erythrocytes each.

The results were strikingly dose-dependent. Angiotensin II significantly reduced ring-stage parasitemia at 10⁻⁹ M (p = 0.0238) and 10⁻⁸ M (p = 0.0234), but the strongest anti-invasive effect occurred at 10⁻⁶ M, with a p-value of 0.0013. Because the starting cultures were tightly synchronized late-stage schizonts and no dead ring-stage parasites were observed during the 16-hour monitoring, the authors interpret the drop in new rings as an invasion defect rather than direct killing of the parasite. Notably, the inhibition did not scale upward: at the highest concentration tested, 10⁻⁴ M, the effect was not sustained and parasitemia appeared to rebound, accompanied by high variability. This non-linear behavior echoes earlier findings that angiotensin peptides modulate the P. falciparum erythrocytic cycle through receptor-dependent and receptor-independent pathways, and that supra-physiological concentrations may trigger receptor desensitization, alternative signaling or peptide aggregation.

The discrepancy between this study’s optimal dose of 10⁻⁶ M and the 10⁻⁸ M reported by Saraiva and colleagues in 2011 is instructive. The authors attribute it to methodological differences: they used the 3D7 strain rather than the W2 strain employed previously, sorbitol-based synchronization, different incubation times and readout sensitivities between microscopy-based and molecular endpoints. They also flag a subtler confounder. Fetal bovine serum and human blood contain ACE2, which catalyzes the conversion of angiotensin II into angiotensin-(1–7) and other metabolites. Since Ang-(1–7) exerts biological effects that are similar but non-additive to angiotensin II, the observed anti-invasive activity may not be attributable to the parent peptide alone. Serum-free conditions were deliberately avoided because they impair parasite viability and invasion efficiency, which would confound the interpretation of any invasion assay.

On the receptor side, the team used the human brain microvascular endothelial cell line HBEC5i, an immortalized line derived from human cerebral cortex that expresses both AT1R and AT2R and is well suited for modeling the microvascular changes of cerebral malaria. Cells were grown to roughly 80 percent confluency with viability above 90 percent, then exposed for 20 minutes to angiotensin II at 10⁻⁹, 10⁻⁸ and 10⁻⁷ M. AT1R activation was quantified by intracellular calcium measured with an O-cresolphthalein complexone assay at 580 nanometers, while AT2R activation was tracked by cyclic GMP released into the lysate and quantified by competitive ELISA at 415 nanometers. All experiments were run in triplicate and analyzed by one-way ANOVA with post hoc unpaired t-tests, treating p < 0.05 as significant.

The receptor readouts were, by design, quiet. Calcium responses at the three tested concentrations measured 0.56, 0.53 and 0.60 absorbance units against a negative control of 0.50, differences that were statistically non-significant. The cGMP assay showed modest increases at 10⁻⁹ M (1.19 pmol/mL) and 10⁻⁸ M (1.66 pmol/mL) versus 1.00 pmol/mL in controls, but again without statistical significance. Taken together with the parasite data, the results indicate that the inhibitory concentration of angiotensin II against P. falciparum lies below the threshold for AT1R activation, the property that would make the peptide safer to exploit therapeutically. However, the authors are careful to note that the minimum effective concentration for AT1R activation remains unresolved, because no statistically significant receptor response was captured within the tested range.

Several limitations temper the conclusions. The study lacked a positive control for receptor activation, did not employ AT1R or AT2R blockers to isolate receptor-specific signaling, and was conducted in serum-supplemented media where ACE2 activity may degrade the peptide before it reaches its receptors. Receptor heterodimerization with other GPCRs may also mask activation signals. The authors propose that future experiments use serum-free conditions where parasite fitness can be independently controlled, wider concentration ranges from 10⁻¹¹ to 10⁻⁴ M, higher-throughput fluorescence-based calcium assays of the kind used in prior neuronal and endothelial studies, and more biological replicates to detect minute dose responses. They also suggest reciprocal blockade designs, activating AT2R in the presence of an AT1R blocker and vice versa, to eliminate heterodimer-mediated masking.

The broader significance lies in the intersection of two global health burdens. Malaria still kills hundreds of thousands annually, and artemisinin-based combination therapies remain the first-line treatment, yet new agents are needed to sustain elimination efforts. Hypertension, meanwhile, is rising across malaria-endemic regions, and the evolutionary link between the two conditions suggests that any angiotensin-based therapy must thread a needle: enough peptide to block parasite invasion, not enough to perturb blood pressure homeostasis through AT1R. By establishing that 10⁻⁶ M angiotensin II significantly inhibits invasion while 10⁻⁹ to 10⁻⁷ M fails to activate either receptor significantly, this study provides the first quantitative anchor point for that therapeutic window, even as it acknowledges that the precise activation threshold, and the role of angiotensin metabolites in mediating the anti-parasitic effect, await more refined experiments.

Subject of Research: Antiplasmodial activity of angiotensin II and its angiotensin receptor activation thresholds in vitro

Article Title: Investigating angiotensin II concentration in modulating Plasmodium falciparum infection and angiotensin receptor activation

Article References: Investigating angiotensin II concentration in modulating Plasmodium falciparum infection and angiotensin receptor activation. (n.d.). https://doi.org/10.3389/fpara.2026.1852476

Image Credits: AI Generated

DOI: 10.3389/fpara.2026.1852476

Keywords: angiotensin II, Plasmodium falciparum, malaria, AT1 receptor, AT2 receptor, renin-angiotensin system, invasion inhibition, hypertension, cerebral malaria, HBEC5i cell line, calcium signaling, cGMP