Antihypertensive and vasorelaxant activities of Laelia autumnalis are mainly through calcium channel blockade

Abstract

The aim of the present study was to evaluate the possible mechanism of the vasorelaxant action of methanol extract from Laelia autumnalis (ME La) in isolated rat aortic rings, and to establish its antihypertensive activity in vivo. ME La (0.15→50 µg/mL) induced relaxation in aortic rings pre-contracted with KCl (80 mM), showing an IC 50 value of 34.61 ± 1.41 µg/mL and E max value of 85.0 ± 4.38% (in endothelium-intact rings) and an IC 50 value of 45.11 ± 4.17 µg/mL and E max value of 80.0 ± 12.1% (in endothelium-denuded rings). Serotonin (5-HT, 1 × 10 − 4 M) provoked sustained contraction, which was markedly inhibited by ME La (0.15→50 µg/mL) in a concentration-dependent and endothelium-independent manner. Pretreatment with ME La (15, 46, 150, 300 and 1500 µg/mL) also inhibited contractile responses to norepinephrine (NE 1 × 10 − 11 M to 1 × 10 − 5.5 M). In endothelium-denuded rings, the vasorelaxant effect of ME La was reduced partially by ODQ (1 µM), but not by tetraethylammonium (5 µM), glibenclamide (10 µM), and 2-aminopyridine (100 µM). The extract also reduced NE-induced transient contraction in Ca 2+-free solution, and inhibited contraction induced by increasing external calcium in Ca 2+-free medium plus high KCl (80 mM). The antihypertensive effect of ME La was determined in spontaneously hypertensive rats (SHR). A single oral administration of the extract (100 mg/kg) exhibited a significant decrease in systolic and diastolic blood pressure and heart rate ( p < 0.05) in SHR rats. Our results suggest that ME La induces relaxation in rat aortic rings through an endothelium-independent pathway, involving blockade of Ca 2+ channels and a possible cGMP enhanced concentrations and also causes an antihypertensive effect.