Inhibitory effects of catecholamines in the paravertebral sympathetic ganglia of the anesthetized dog

alpha-Adrenoceptor agonists decreased mean arterial pressure when injected into the arterial blood supply of the paraspinal sympathetic ganglia of pentobarbital-anesthetized open-chest dogs. The hypotensive response occurred concomitantly with selective decreases of vascular resistance in the vessels innervated by neurons arising from these ganglia, and both of these responses were blocked by the ganglionic blocking agent, hexamethonium. The hypotensive response to phenylephrine was selectively blocked by terazosin; alpha 1 selective agonist, and antagonist, respectively, while the hypotension produced by intra-arterial clonidine was blocked by rauwolscine; alpha 2 selective agonist and antagonist, respectively. Either terazosin or rauwolscine reduced the hypotension produced by noradrenaline or dopamine. These results demonstrated the presence of both alpha 1- and alpha 2-adrenoceptors in the paraspinal sympathetic ganglia. Activation of either alpha-adrenoceptor subtype inhibited ganglionic transmission.