Effects of Olax gambecola methanol extract on smooth muscle and rat blood pressure.

A methanol extract of the roots of Olax gambecola induced a biphasic contractile response consisting of a transitory initial rapid contraction (Phase I), followed by a slowly developing sustained increase in basal tone (Phase II) on rat fundus, antrum, guinea pig taenia coli, rabbit jejunum and aorta. The Phase I contraction was abolished by atropine, attenuated significantly by indomethacin and potentiated by physostigmine while the Phase II response was unaffected. Hexamethonium, morphine, serotonin (5-HT) antagonists or desensitization of 5-HT receptors did not alter either the Phase I or the Phase II contractions. Calcium channel blockers and procedures affecting calcium translocation abolished the Phase I contraction while only reducing the Phase II contractions. Transmural electrical stimulation produced contractions of the fundus which were attenuated by the extract. Single bolus injections of extract produced a rapid fall in blood pressure in both normotensive and spontaneously hypertensive rats but intravenous infusion resulted in a sustained fall in blood pressure which was maintained throughout the infusion period. Chronic i.p. administration of extract to spontaneously hypertensive rats reduced blood pressure markedly but did not alter the blood pressure of normotensive animals. The hypotensive response to single bolus injections was abolished by atropine and potentiated by physostigmine. The activity profile of the extract suggests the presence of at least two active principles in the crude extract of Olax gambecola used in this study.