Hypotensive and spasmolytic effects of normacusine B from Strychnos atlantica root.

Normacusine B, a tertiary indole alkaloid, was isolated in pure form from the root bark of Strychnos atlantica Krukoff & Barneby. In conscious unrestrained rats, normacusine B (1 mg/kg) decreased the mean arterial blood pressure (27.6 ± 8.4 mmHg, n = 6), followed by a significant increase in heart rate (115.0 ± 12.7 bpm, n = 6). The alkaloid failed to induce tachycardia directly in isolated perfused rat heart. In isolated rat aortic rings, normacusine B antagonized phenylephrine and serotonin-induced contractions. Schild plot analysis of individual cumulative concentration-response curves was compatible with a competitive type of antagonism against phenylephrine (pA(2) = 7.05 ± 0.11) and of a non-competitive nature against 5-hydroxytryptamine (apparent pA(2) = 7.02 ± 0.08). Normacusine B was found inactive against KCl and PGF(2α) induced contractions.