ABSTRACT
1. Bradykinin and histamine reduced the blood pressure in normotensive anaesthetized rabbits. They produced a pressor or biphasic response when the initial arterial pressure was lowered by acute haemorrhagic shock or mecamylamine blockade. When blood pressure was lowered by pretreatment with reserpine, bradykinin remained depressor and histamine produced a biphasic response.2. Phenoxybenzamine abolished the pressor responses to bradykinin and histamine, but potentiated and prolonged the depressor response to bradykinin.3. Phenoxybenzamine-OH and SY28-OH modified neither the pressor nor depressor responses to histamine, nor the pressor response to bradykinin. However, they greatly potentiated and prolonged the hypotensive effect of bradykinin.4. In the isolated rabbit ear preparation, the initial vascular tone influenced the responses to bradykinin and histamine. In preparations having low vascular tone they were vasoconstrictors, but when the tone was raised by angiotensin or noradrenaline, they were vasodilators.5. Phenoxybenzamine blocked the vasconstrictor effect of bradykinin in a rabbit ear preparation having low vascular tone and phenoxybenzamine or its related ethanolamine potentiated the vasodilator response to bradykinin in preparations in which the tone was high.6. The significance of these findings is discussed.
