Efeitos dos alcalóides de Geissospermum urceolatum a. H. Gentry (acariquara-branca) na pressão arterial e na contração do músculo liso de ratos

Abstract

Geissospermum urceolatum is an Apocynaceae popularly known as acariquara-branca, acariquara, quinarana, acariubarana, acarirana, pereira, pau-pereira and pau-forquilha. Besides the quality of its wood, the bitter bark infusion is commonly used in the folk medicine of the Amazonian region to treat stomachache, fever and malaria. There are no scientific reports of this species confirming these actions. This work shows the effects of Geissospermum urceolatum alkaloids in the arterial blood pressure and in rats smooth mucle contraction. The dried and powdered bark was extracted obtaining the aqueous extract (AE) by infusion (2,5%), the ethanolic extract (EETOH) by maceration with ethanol 50% at room temperature, the hydro-alcoholic extract (HAE) by extraction in the Soxhlet apparatus with ethanol 80%; the extracts were filtrated and concentrated. The HAE was re-suspended in chloroform and partitioned with acidic water generating the total alkaloids fraction (TAF). The importance of the popular indication as anti-malarial guided the TAF preliminary test in rodents infected with Plasmodium chabaudi. The TAF purification by HPLC yielded 14 fractions referent to the main peaks (>1% of chromatogram total area), possible chemical markers of the extract after identification and pharmacological evaluation. In the immediate chemical standardization performed to the pharmacological studies, 6 major components were chosen, they are still in identification process. The studies with the standardized extracts were planned for the experimental detection of a wide spectrum of the pharmacological actions, from general tests in vivo with oral administration of the extracts until the study of the purified fractions in isolated preparations in vitro. The pharmacological screening was initiated with the observation test, or Hippocratic test. The oral administration of EETOH (1,0 g/kg) did not produce evident signs of pharmacological activity in the animals. However, the systemic administration by intra-peritoneal pathway reduced the motility, the respiratory rate and produced cyanosis with occurrence and intensity proportional to the doses. These effects were faster and more intense with TAF, confirming the higher concentration of the compounds in this fraction. In anesthetized rats blood pressure, TAF induced fast and temporary hypotension without affecting the hypertensive effect of noradrenalin or the hypotension induced by acetylcholine. Higher doses produced irregular responses with late reversion to the basal level. In rats atrium and papillary muscle, TAF inhibited the pacemaker excitability, but facilitated the excitation-contraction coupling, potentiating the papillary muscle contraction. In rat aortic rings with intact endothelium, TAF relaxed the muscular tonus induced by noradrenalin, but with NOS blockade by the previous incubation of L-NAME, the effect was only 40%, indicating a NO independent mechanism in the TAF relaxant activity. This possibility was confirmed when TAF also relaxed the endothelium deprived aortic rings preparations. In depolarized rat portal vein, TAF reduced the maximum contraction produced by CaCl2 addition, indicating the calcium channel blockade by the alkaloids. In rat vas deferens, TAF potentiated the maximum contraction induced by noradrenalin and reduced the drug affinity to the α1-adrenergic receptors; the acethylcholine action was not changed. These results showed that TAF potentiated the second-messengers of the adrenergic pathway, but did not potentiate the contraction when the second-messengers were activated by the muscarinic receptors stimulation. The study of the purified alkaloids will be essential to understand the mechanisms of action of the effects until now observed. The present work show results with vasorelaxation that may be explained by two mechanisms: indirect, via endothelial NOS activation and direct in the smooth muscle, apparently by blocking the calcium ion influx. This action, however, is not so clear because in the vas deferens, at the same time that the noradrenalin EC50 was increased in the presence of TAF, the maximum contractile effect of the agonist was potentiated, as if sensitization of the contractile myofibrils had been produced. The purification and isolation of the TAF constituents may permit the dissociation of the effects observed and the determination of the mechanisms of action, mainly of the poorly known effect that potentiated the maximum contraction of the total agonist noradrenalin.