THE EFFECT OF NOREPINEPHRINE INFUSIONS UPON SOME RESPONSES OF RESERPINE-TREATED SPINAL CATS TO TYRAMINE

¹ Pharmacology Section, Sterling-Winthrop Research Institute, Rensselaer, New York

Responses of the nictitating membrane, blood pressure and heart rate of untreated and reserpine-treated spinal cats to various doses of tyramine and norepinephrine were recorded. The responses to tyramine were reduced, but not abolished, by pretreatment with reserpine while the response of the nictitating membrane to norepinephrine was unaltered and the cardiovascular responses to norepinephrine were either unaltered or enhanced. It was concluded that supersensitivity to norepinephrine cannot be directly related to subsensitivity to tyramine.

The responses of the nictitating membrane, blood pressure and heart rate to tyramine administered after complete recovery of untreated cats from infusions of norepinephrine were not different from the responses to tyramine administered before such infusions. In reserpine-treated cats the responses of the nictitating membrane and blood pressure were enhanced, but not restored, by norepinephrine infusions while the chronotropic responses were fully restored. These data are discussed in relation to the hypothesis that tyramine acts by releasing norepinephrine from tissue stores.

The responses to tyramine of the nictitating membrane and blood pressure of reserpinetreated cats were enhanced when tyramine was administered after termination of norepinephrine infusions but before recovery from such infusions. The response of the heart rate was not enhanced but the pretyramine heart rates were so high that their ability to be enhanced was undoubtedly limited. It was concluded that the action of tyramine may be enhanced in the presence of increased extracellular norepinephrine and that accordingly its action may not be explained solely by an ability to release norepinephrifle from tissue stores.