Evidence for Alpha-adrenergic Innervation of the Isolated Canine Thoracic Duct

The excitatory innervation of isolated thoracic duct segments was studied using tissue bath techniques. No spontaneous activity was present in longitudinal or helical strips obtained from a portion of the thoracic duct cephalad to the hilum of the lung. Norepinephrine (10(-8) to 10(-5) M) and tyramine (3 x 10(-5) M) produced contractions that were antagonized by phentolamine (2 x 10(-5) M) and phenoxybenzamine (10(-7) M). Acetylcholine (10(-7) to 10(-4) M) produced contractions that were antagonized by atropine (5 x 10(-9) M). Thoracic duct strips also contracted in response to field electrical stimulation, and maximal responses were obtained with a stimulus of 15 V, 15 Hz, and 1-ms pulse duration. These electrically induced contractions were abolished by tetrodotoxin (5 x 10(-7) M), phentolamine (2 x 10(-5) M), phenoxybenzamine (10(-7) M), and guanethidine (3 x 10(-6) M), but not by atropine (10(-6) M). We conclude that smooth muscle of the canine thoracic duct contains alpha-adrenergic and acetylcholine receptors, both of which cause contraction when stimulated. However, only the alpha-receptors appear to be innervated.