Abstract

Contrary to its depressor effect in cat, dog, monkey and man, histamine produced in the anesthetized rabbit a pressor effect qualitatively comparable to that of epinephrine. This effect was favored by general anesthesia. The deeper the anesthesia the more regular was the pressor effect. In unanesthetized animals or under light anesthesia the responses Were erratic, there was even a depressor effect.

In the dose range of 50 to 250 microgm./kgm., the relationship between log dose and pressor effect of histamine was linear. A dose approximately one hundred times that of epinephrine was required to produce an equal pressor effect.

Epinephrine and histamine were mutually antagonistic. Epinephrine given before histamine in a single small dose diminished and, given in a single higher dose or repeated small doses reversed, the pressor effect of histamine. Histamine given prior to epinephrine diminished the epinephrine pressor effect but never reversed it. Adrenergic blocking agents did not modify the pressor response of histamine even in doses strongly blocking the epinephrine effect. By ganglionic blockade the pressor effect of histamine was enhanced and the antagonism between epinephrine and histamine was turned into synergism. Adrenalectomy did not decrease the pressor effect of histamine.

In the discussion of these findings it is pointed out that they are all compatible with the conclusion that the pressor effect of histamine was not due to the mobilization of epinephrine.