Tribesmen in Eastern Africa used an extract from the climbing oleander, Strophanthus preussii, to poison the tips of their arrows. The active substance is ouabain, which is a specific inhibitor of the Na+–K+ ATPase transporter protein. This protein is required to maintain proper sodium and potassium ion concentrations across neuronal cell membranes. Structure–function relationships that govern ion transport by membrane proteins have been elucidated at the molecular level using X-ray crystallography. The effect of ouabain is that the heart muscle contracts but cannot relax in order to produce another contraction. As a poison, this effect is strong enough to kill a hippopotamus. In small, controlled doses, however, ouabain can act as a heart stimulant when the muscle has trouble contracting. The study of how ouabain works has increased our understanding of the functional importance of the Na+–K+ ATPase transporter protein.