interactions between histamine and bradykinin assessed by continuous recording of increased vascular permeability.

1. The responses of the cutaneous blood vessels of the guinea-pig to histamine and bradykinin have been studied in order to determine whether these factors mediate vascular changes by acting on different blood vessel receptors.2. In animals injected intravenously with bovine serum albumin labelled with phosphorus (32)P the reactions to histamine and bradykinin have been studied by continuously recording the local radioactive emission from the skin.3. Histamine increased the intensity of radioactive emission for 7-15 min. The level of radioactivity then remained almost constant for at least 1 hr. Bradykinin increased the rate of emission for 5-7 min but the radioactivity decreased by approx. 30% in 1 hr.4. The intensity of the response to histamine was markedly diminished in sites previously treated with either histamine or bradykinin, compared with the reaction in normal skin. However, a response of similar intensity to that in normal skin was evoked by histamine earlier in sites previously treated with bradykinin than in sites treated with histamine. By contrast, bradykinin evoked a similar reaction to that in normal skin, in sites previously treated with either histamine or bradykinin.5. The dissimilarity of the responses evoked by histamine and bradykinin suggests that there are two types of blood vessel receptors. One type may be activated by both histamine and bradykinin but becomes refractory to further stimulation with histamine. The other type of receptor is activated only by bradykinin and does not become refractory.