Differences in adrenergic receptor populations in canine dorsal metatarsal veins and intralobar pulmonary arteries.

The contractile responses of the canine intralobar pulmonary arteries (IPA) and dorsal metatarsal veins (DMV) to alpha adrenergic receptor agonists were evaluated in the absence and presence of alpha receptor antagonists to determine the type of post-synaptic alpha receptor predominant in the cutaneous and pulmonary canine vasculature. Rings of IPA, in vitro, contracted in response to the alpha 1 receptor agonist phenylephrine (PE), and the mixed alpha agonist norepinephrine (NE) but did not contract in response to clonidine (C), an alpha 2 receptor agonist. DMV contracted in response to each of the agonists. The contractile responses of the IPA to NE and PE were antagonized by tolazoline, phentolamine, clonidine and prazosin. The contractile responses of the DMV to clonidine were only antagonized by clonidine and phentolamine but not by tolazoline or prazosin. These data suggest that: 1) IPA are selectively endowed with post-synaptic alpha 1 adrenergic receptors; 2) DMV are endowed with both post-synaptic alpha 1 and alpha 2 adrenergic receptors; and 3) clonidine interacts with alpha 1 adrenergic receptors to elicit alpha blockade and with alpha 2 adrenergic receptors to initiate contraction of the DMV. In an attempt to verify this hypothesis, experiments were performed in solutions in which the pH was altered above or below pH 7.4. Under these conditions, the agonist properties of clonidine were selectively inhibited whereas its blocking potency was retained. These data support the conclusion that clonidine is antagonistic at alpha 1 adrenergic receptors and agonistic at alpha 2 adrenergic receptors.