Alkaloids from <i>Peganum harmala</i> attenuated contractile responses of vascular smooth muscle cells

Purpose: To investigate the contractile responses of vascular smooth muscle cells (VSMCs) to spasmogens after incubation with harmaline, harmine, and harmalol, which are alkaloids obtained from Peganum harmala L., a member of the Zygophyllaceae family. Methods: Contractile responses of VSMCs to norepinephrine (NE; 1 µmol/L) and potassium chloride (KCl; 60 mmol/L) were recorded in rat aortic ring preparations pre-incubated with 0.5, 1, 5 and 10 µmol/L of each alkaloid for 15 min. Responses were expressed as mean values of contractions in incubated preparations, relative to the recorded tension prior to treatment with alkaloids. Results: Pre- incubation with harmaline at concentration of 10 µmol/L significantly reduced contractile responses to NE by 69.0 ± 3.0 % (p < 0.00002), and decreased KCl-induced contraction by 34.0 ± 9.0 % (p < 0.05). Harmalol was the most effective in inhibiting contractions to KCl (48.0 ± 9.0 %, p < 0.01). However, harmalol produced relatively moderate inhibitory effects on NE-induced contractions (46.0 ± 4.0 %, p < 0.005), followed by harmine (52.0 ± 8.0 %, p < 0.02), but it did not significantly affect contractile responses to KCl. Conclusion: These results highlight the differential effects of pre-incubation with alkaloids from P. harmala and their potential effects on the prevention of VSMC spasms induced by either chemicals or stimuli that change the membrane potential.