Abstract
Betanin is a natural pigment belonging to the group of betalains with vasoconstrictive properties. This study investigated the underlying mechanism involved in the vasoconstrictive effect of betanin in rat aorta. Betanin enhanced the contractile response in aortic rings precontracted with phenylephrine. In aortic preparations under resting tonus, betanin induced potent and sustained contractions. The contractile effect of betanin was increased in endothelium-denuded aortic rings, while treatment of endothelium-intact preparations with the alpha-1-adrenergic receptor antagonist prazosin abolished the contractile effects of betanin. In the presence of verapamil, the contractile effects of betanin were decreased, and they were abolished under Ca 2+ -free solution. Vasoconstriction was not affected by treatment with yohimbine and guanethidine. Betanin partially restored the endothelium-dependent vasorelaxant effect induced by acetylcholine, a response that was blunted in aortic rings treated with baicalin, an inhibitor of the nitric oxide (NO) - cyclic guanosine monophosphate (cGMP) - protein kinase G (PKG) pathway. In conclusion, the vasoconstrictive effects of betanin likely involve alpha-1 adrenoceptors and are dependent on endothelial modulation.
Keywords:
Betanin; Contractile effect; Endothelium function; Vascular smooth muscle
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Panels A and C show representative experiments of the effects caused by cumulative addition of betanin (1–3000 µM) on aortic rings. Acetylcholine (ACh; 1 µM) relaxed an initial contraction induced by phenylephrine (Phe; 0.1 µM), confirming the presence of intact endothelium. Aortic rings were then washed and betanin was added: in Panel A, during a sustained Phe (1 µM) contraction; in Panel C, under resting tone. Panel B depicts a graph demonstrating that cumulative betanin (1–3000 µM; n = 5) caused a slight but significant increase in the contraction of pre-contracted aortic rings with Phe. Under resting conditions (Panel D), betanin (1–3000 µM; n = 8) concentration-dependently contracted both endothelium-intact (E+) and denuded (E-) aortic rings. Data are expressed as a percentage of the initial contraction induced by Phe (1 µM) in Panel A or KCl (60 mM) in Panel D. Data represent mean ± standard error of the mean (SEM). *p < 0.05, Holm-Sidak test, compared to values before betanin addition (Panel B) or E+ contractions (Panel D).
Concentration-response curves were generated for betanin (1–3000 μM) in endothelium-intact aortic rings under various conditions: control (no antagonist, n = 8), guanethidine (1 μM, n = 5), yohimbine (30 nM, n = 5), or prazosin (10 μM, n = 5). Data are expressed as a percentage of the reference contraction induced by KCl (60 mM) and represent mean ± standard error of the mean (SEM). *p < 0.05, Holm-Sidak test, compared to the control curve (betanin alone).
Panel A: Concentration-response curves for betanin (1–3000 μM) were generated in endothelium-intact aortic rings maintained in calcium-containing medium: control (no verapamil, n = 8) and with verapamil (1 μM, n = 5). Panel B: Concentration-response curves for betanin (1–3000 μM) were generated in endothelium-intact aortic rings, either with calcium-containing medium (control) or calcium-free medium. Removing extracellular calcium abolished betanin's contractile effect. Data expressed as a percentage of the reference contraction induced by KCl (60 mM) and represent mean ± standard error of the mean (SEM). #, , p < 0.05, Holm-Sidak test, compared to the control curve
Panels A-C depict representative experiments of endothelium-dependent relaxation by increasing acetylcholine (ACh; 0.001 – 100 μM) in endothelium-intact aortic rings pre-contracted with phenylephrine (0.3 μM). Aortic rings were maintained under different conditions: no additional treatment (control), baicalin (100 μM, a nitric oxide-cGMP-protein kinase G pathway inhibitor), or a combination of baicalin and betanin (1000 μM). Panel D shows vasorelaxant responses expressed as a percentage of the phenylephrine-induced contraction before ACh addition. Data represent mean ± standard error of the mean (SEM). Statistical analysis (two-way ANOVA followed by Holm-Sidak test) compared each treatment group to the control curve, with # and Ö indicating significance (p < 0.05).