Isopropylnor-Epinephrine Blockade of Epinephrine Reversal

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Abstract
Blood flow was measured in the blood vessels of the hind leg muscles of anesthetized dogs, in response to intra-arterial injections of l-epinephrine and l-norepinephrine. Injections were made before and during intra-arterial infusions of isopropylnor-epinephrine. These pairs of injections were made in a control state and after establishing various levels of adrenergic blockade with either Ilidar or Dibenzyline. At low levels of adrenergic blockade, the l-epinephrine constrictor responses were converted to dilator responses while the constrictor responses to l-norepinephrine were slightly reduced in magnitude but remained constrictor. At these levels of blockade the isopropylnor-epinephrine infusion converted the responses to l-epinephrine back to constrictor responses which closely resembled those of the l-norepinephrine. At higher levels of adrenergic blockade, the constrictor responses to l-norepinephrine was abolished while the l-epinephrine responses remained markedly dilator in character. The infusion of isopropylnor-epinephrine at this level of blockade abolished the dilator responses to l-epinephrine and now neither l-epinephrine nor l-norepinephrine caused any response. It is concluded that even under degrees of adrenergic blockade at which the l-epinephrine response in the vascular bed of skeletal muscle appears to be purely dilator, the epinephrine is still stimulating the constrictor endings as effectively as do comparable doses of l-norepinephrine, and that this constrictor effect is masked by the concurrent intense stimulation of the dilator endings by l-epinephrine.

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