Role of K + and Ca 2+ Channels in the Vasodilator Effects of Plectranthus barbatus (Brazilian Boldo) in Hypertensive Rats.

Plectranthus barbatus , popularly known as Brazilian boldo, is used in Brazilian folk medicine to treat cardiovascular disorders including hypertension. This study investigated the chemical profile by UFLC-DAD-MS and the relaxant effect by using an isolated organ bath of the hydroethanolic extract of P. barbatus (HEPB) leaves on the aorta of spontaneously hypertensive rats (SHR). A total of nineteen compounds were annotated from HEPB, and the main metabolite classes found were flavonoids, diterpenoids, cinnamic acid derivatives, and organic acids. The HEPB promoted an endothelium-dependent vasodilator effect (~100%; EC50 ~347.10  μ g/mL). Incubation of L-NAME (a nonselective nitric oxide synthase inhibitor; EC50 ~417.20  μ g/mL), ODQ (a selective inhibitor of the soluble guanylate cyclase enzyme; EC50 ~426.00  μ g/mL), propranolol (a nonselective α -adrenergic receptor antagonist; EC50 ~448.90  μ g/mL), or indomethacin (a nonselective cyclooxygenase enzyme inhibitor; EC50 ~398.70  μ g/mL) could not significantly affect the relaxation evoked by HEPB. However, in the presence of atropine (a nonselective muscarinic receptor antagonist), there was a slight reduction in its vasorelaxant effect (EC50 ~476.40  μ g/mL). The addition of tetraethylammonium (a blocker of Ca2+ -activated K+ channels; EC50 ~611.60  μ g/mL) or 4-aminopyridine (a voltage-dependent K+ channel blocker; EC50 ~380.50  μ g/mL) significantly reduced the relaxation effect of the extract without the interference of glibenclamide (an ATP-sensitive K+ channel blocker; EC50 ~344.60  μ g/mL) or barium chloride (an influx rectifying K+ channel blocker; EC50 ~360.80  μ g/mL). The extract inhibited the contractile response against phenylephrine, CaCl2 , KCl, or caffeine, similar to the results obtained with nifedipine (voltage-dependent calcium channel blocker). Together, the HEPB showed a vasorelaxant effect on the thoracic aorta of SHR, exclusively dependent on the endothelium with the participation of muscarinic receptors and K+ and Ca2+ channels.