Estudo das ações cardiovasculares da fração alcaloídica obtida da espécie nativa amazônica geissospermum argenteum woodson

Abstract

Geissospermum argenteum Woodson (Apocynaceae), known as acariquara-branca is popularly used to treat malaria, gastrointestinal disorders and as curare. Considering that there are no published data reporting the pharmacological activities of this species, the aim of this work was to study the cardiovascular effects of Amazonian G. argenteum in rodents. The bark was extracted with ethanol in Soxhlet apparatus generating the hydroalcoholic extract (EHA). The EHA was partitioned with chloroform in different pH, yielding the chloroform fraction (FCHCl3); further partition yielded the total alkaloids fraction (TAF). FCHCl3 purification in silica gel column chromatography (CC) yielded thirteen fractions. FAT standardization in HPLC showed nine major peaks with retention times from 3.5 to 23.5 min. FAT oral administration did not indicate a central nervous system action, however, curare-like effect was observed after i.p. and i.v. injections. This action was confirmed in vitro. FAT (10, 30 and 100 μg/mL) blocked rat diaphragm contractions induced by phrenic nerve electrical stimulation. The neuromuscular blockade was proportional to the concentrations and was reverted by washing, but not by neostigmine (10-5M). On the same concentrations, contractions induced by direct electrical stimulation were potentiated in rat diaphragm. In anesthetized rats under mechanical respiration, FAT intravenous injection (0.3; 1.0 and 3.0 mg/kg) produced fast and reversible hypotension, not blocked by atropine (1.0 mg/kg). In anesthetized animals treated with the nicotinic ganglion blocker hexamethonium, hypertension induced by FAT was not changed by previous prazosin (1.0 mg/kg) injection, indicating that FAT hypertensive effect is not caused by an increase in noradrenaline (Nor) release by ganglion stimulation. FAT did not alter Nor or acetylcholine (ACh) effects on the blood pressure, indicating that there was not an apparent interaction among the alkaloids and α1, M2 or M3 receptors. On another protocol, FAT inhibited the hypertension induced by the ganglion stimulant DMPP; this effect could due to ganglion blockade and decreased Nor release at sympathetic nerve terminals which could also explain bradycardia and hypotension induced by FAT. In rat isolated atria, FAT (10, 30 and 100 μg/mL), caused negative chronotropism and positive inotropism. In rat papillary muscle, the effective refractory period was not altered, indicating that FAT did not interfere with the cardiac action potential. In rat isolated aorta, FAT (1.0; 3.0 and 10 μg/mL) produced endothelium- independent vasodilatation and shifted Nor cumulative concentration-response curve to the right, without inhibiting the agonist maximal response. This effect indicated α1-adrenergic receptors competitive blockade. Blockade of L-type calcium channel by FAT was not confirmed in cultured cardiac and uterus cells using the FURA-4 method to measure calcium influx. In rat vas deferens (DDR), FAT (10, 30 and 100 μg/mL) shifted Nor and ACh cumulative concentration-response curves to the right and potentiated the maximal effects. The results indicated many different activities of the total alkaloid fraction of G. argenteum: competitive α1-adrenergic receptors blockade and a non-competitive muscular nicotinic receptor blockade; positive inotropism and diaphragm twitch potentiation unrelated to potassium channel blockade during the muscle fiber repolarization, no inhibition of the muscle Ca+2-ATPase activity; potentiation of the rat vas deferens maximal responses to agonists is probably caused by intracellular second-messengers (IP3, DAG, PKC) activation, although no direct data confirm this possibility.