An Excitatory Adrenergic Alpha Receptor Mechanism of Terminal Guinea-Pig Ileum

Summary Experiments were performed on segments of terminal guinea-pig ileum utilizing differential alpha and beta adrenergic receptor site blocking techniques. Phenyl-ephrine caused a contraction of this segment of guinea-pig intestine which resembled a pure alpha receptor site response. Both epinephrine and noreinephrine produced biphasic responses which follow a pattern of alpha excitation followed by beta inhibition while isoproterenol elicited a pure beta inhibitory action. The use of phenoxybenzamine as an alpha blocking agent abolished all excitatory responses while augmenting beta actions. Propranolol, through beta adrenergic receptor site blockade, potentiated alpha excitatory actions. The results are discussed in terms of the guinea-pig terminal ileum possessing an unique alpha adrenergic receptor mechanism which is excitatory in nature. The authors thank Dennis Townsend and Sarah Irvine for technical assistance.