Some alkaloids are active in a and ft-adrenergic receptors (e.g. ephedrine), a receptors (e.g. ergotamine), uterine a2 receptors (e.g. ergometrine) or presynaptic a2 adrenoreceptors (e.g. yohimbine). Ephedrine increases blood pressure by elevating cardiac output. It is also known to have some stimulant activity on the respiratory centre. Ephedrine may be applied in the treatment of edema in insulin-dependent diabetics.
Ergotamine blocks a receptors. It has an inhibitory effect on the cardiovascular system. Ergometrine also has a high affinity to uterine a2 receptors, and it therefore influences uterine muscle activity. Ergometrine is used in the treatment of postpartum or postbortal haemorrhaging. Yohimbine blocks presynaptic a2 adrenoreceptors and increases the release of noradrenaline at sympathetic nerve endings. It may be used to increase heart rate and blood pressure and even in the treatment of severe cases of male impotency.
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