Phytochemical Characterisation and Antihypertensive Effects of Hibiscus sabdariffa Calyx Extract in Spontaneously Hypertensive Rats

Background: Hibiscus sabdariffa (zobo), consumed widely across Nigeria particularly in the North West and North East, has been reported in ethnomedicine to lower blood pressure. Clinical interest in its antihypertensive properties has grown, but mechanistic and in vivo pharmacological validation using spontaneously hypertensive rat (SHR) models remain limited in African research settings. This study evaluated the antihypertensive effects and mechanism of action of H. sabdariffa calyx extract in SHRs. Methods: Aqueous and ethanolic calyx extracts were prepared and phytochemically characterised. Antihypertensive activity was assessed in 36 male SHRs divided into six groups including normotensive controls, untreated SHRs, lisinopril reference, and three extract dose groups (200, 400, 600 mg/kg). Blood pressure was measured weekly by tail-cuff plethysmography over eight weeks. Serum ACE activity, nitric oxide levels, and vascular histopathology were assessed at termination. Results: Both extracts produced significant, dose-dependent blood pressure reductions from week 2 (p less than 0.01). The 600 mg/kg aqueous extract reduced systolic BP by 31 mmHg at week 8. ACE inhibitory activity and elevated NO levels suggested a dual vasodilatory and renin-angiotensin mechanism. Vascular wall hypertrophy was significantly attenuated. Conclusion: H. sabdariffa calyx extract demonstrates significant antihypertensive effects mediated partly via ACE inhibition and NO enhancement, supporting its nutraceutical development for hypertension management. Keywords: Hibiscus sabdariffa, antihypertensive, spontaneously hypertensive rats, ACE inhibition, phytopharmacology